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INFO-DIR-SECTION GNU Ada tools
START-INFO-DIR-ENTRY
* GNAT Reference Manual: (gnat_rm).  Reference Manual for GNU Ada tools.
END-INFO-DIR-ENTRY

Copyright (C) 1995-2001, Free Software Foundation

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU Free Documentation License", with the
Front-Cover Texts being "GNAT Reference Manual", and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".

File: gnat_rm.info,  Node: Top,  Next: About This Guide,  Prev: (dir),  Up: (dir)

GNAT Reference Manual
*********************

   GNAT Reference Manual

   GNAT, The GNU Ada 95 Compiler

   GNAT Version for GCC 3.3.6

   Ada Core Technologies, Inc.

Copyright (C) 1995-2001, Free Software Foundation

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU Free Documentation License", with the
Front-Cover Texts being "GNAT Reference Manual", and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".
* Menu:

* About This Guide::
* Implementation Defined Pragmas::
* Implementation Defined Attributes::
* Implementation Advice::
* Implementation Defined Characteristics::
* Intrinsic Subprograms::
* Representation Clauses and Pragmas::
* Standard Library Routines::
* The Implementation of Standard I/O::
* The GNAT Library::
* Interfacing to Other Languages::
* Machine Code Insertions::
* GNAT Implementation of Tasking::
* Code generation for array aggregates::
* Specialized Needs Annexes::
* Compatibility Guide::
* GNU Free Documentation License::
* Index::

 --- The Detailed Node Listing ---

About This Guide

* What This Reference Manual Contains::
* Related Information::

The Implementation of Standard I/O

* Standard I/O Packages::
* FORM Strings::
* Direct_IO::
* Sequential_IO::
* Text_IO::
* Wide_Text_IO::
* Stream_IO::
* Shared Files::
* Open Modes::
* Operations on C Streams::
* Interfacing to C Streams::

The GNAT Library

* Ada.Characters.Latin_9 (a-chlat9.ads)::
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads)::
* Ada.Characters.Wide_Latin_9 (a-cwila9.ads)::
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* Ada.Direct_IO.C_Streams (a-diocst.ads)::
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)::
* Ada.Sequential_IO.C_Streams (a-siocst.ads)::
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)::
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads)::
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)::
* Ada.Text_IO.C_Streams (a-tiocst.ads)::
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* GNAT.AWK (g-awk.ads)::
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* GNAT.Bubble_Sort_G (g-busorg.ads)::
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* GNAT.Case_Util (g-casuti.ads)::
* GNAT.CGI (g-cgi.ads)::
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* GNAT.Spitbol.Table_VString (g-sptavs.ads)::
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* GNAT.Traceback.Symbolic (g-trasym.ads)::
* Interfaces.C.Extensions (i-cexten.ads)::
* Interfaces.C.Streams (i-cstrea.ads)::
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* Interfaces.Os2lib.Threads (i-os2thr.ads)::
* Interfaces.Packed_Decimal (i-pacdec.ads)::
* Interfaces.VxWorks (i-vxwork.ads)::
* Interfaces.VxWorks.IO (i-vxwoio.ads)::
* System.Address_Image (s-addima.ads)::
* System.Assertions (s-assert.ads)::
* System.Partition_Interface (s-parint.ads)::
* System.Task_Info (s-tasinf.ads)::
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* System.Wch_Con (s-wchcon.ads)::

Text_IO

* Text_IO Stream Pointer Positioning::
* Text_IO Reading and Writing Non-Regular Files::
* Get_Immediate::
* Treating Text_IO Files as Streams::
* Text_IO Extensions::
* Text_IO Facilities for Unbounded Strings::

Wide_Text_IO

* Wide_Text_IO Stream Pointer Positioning::
* Wide_Text_IO Reading and Writing Non-Regular Files::

Interfacing to Other Languages

* Interfacing to C::
* Interfacing to C++::
* Interfacing to COBOL::
* Interfacing to Fortran::
* Interfacing to non-GNAT Ada code::

GNAT Implementation of Tasking

* Mapping Ada Tasks onto the Underlying Kernel Threads::
* Ensuring Compliance with the Real-Time Annex::


File: gnat_rm.info,  Node: About This Guide,  Next: Implementation Defined Pragmas,  Prev: Top,  Up: Top

About This Guide
****************

This manual contains useful information in writing programs using the
GNAT compiler.  It includes information on implementation dependent
characteristics of GNAT, including all the information required by Annex
M of the standard.

   Ada 95 is designed to be highly portable,and guarantees that, for
most programs, Ada 95 compilers behave in exactly the same manner on
different machines.  However, since Ada 95 is designed to be used in a
wide variety of applications, it also contains a number of system
dependent features to Functbe used in interfacing to the external world.

   Note: Any program that makes use of implementation-dependent features
may be non-portable.  You should follow good programming practice and
isolate and clearly document any sections of your program that make use
of these features in a non-portable manner.

* Menu:

* What This Reference Manual Contains::
* Conventions::
* Related Information::


File: gnat_rm.info,  Node: What This Reference Manual Contains,  Next: Conventions,  Up: About This Guide

What This Reference Manual Contains
===================================

   This reference manual contains the following chapters:

   * *Note Implementation Defined Pragmas:: lists GNAT
     implementation-dependent pragmas, which can be used to extend and
     enhance the functionality of the compiler.

   * *Note Implementation Defined Attributes:: lists GNAT
     implementation-dependent attributes which can be used to extend and
     enhance the functionality of the compiler.

   * *Note Implementation Advice:: provides information on generally
     desirable behavior which are not requirements that all compilers
     must follow since it cannot be provided on all systems, or which
     may be undesirable on some systems.

   * *Note Implementation Defined Characteristics:: provides a guide to
     minimizing implementation dependent features.

   * *Note Intrinsic Subprograms:: describes the intrinsic subprograms
     implemented by GNAT, and how they can be imported into user
     application programs.

   * *Note Representation Clauses and Pragmas:: describes in detail the
     way that GNAT represents data, and in particular the exact set of
     representation clauses and pragmas that is accepted.

   * *Note Standard Library Routines:: provides a listing of packages
     and a brief description of the functionality that is provided by
     Ada's extensive set of standard library routines as implemented by
     GNAT.

   * *Note The Implementation of Standard I/O:: details how the GNAT
     implementation of the input-output facilities.

   * *Note Interfacing to Other Languages:: describes how programs
     written in Ada using GNAT can be interfaced to other programming
     languages.

   * *Note Specialized Needs Annexes:: describes the GNAT
     implementation of all of the special needs annexes.

   * *Note Compatibility Guide:: includes sections on compatibility of
     GNAT with other Ada 83 and Ada 95 compilation systems, to assist
     in porting code from other environments.

   This reference manual assumes that you are familiar with Ada 95
language, as described in the International Standard
ANSI/ISO/IEC-8652:1995, Jan 1995.


File: gnat_rm.info,  Node: Conventions,  Next: Related Information,  Prev: What This Reference Manual Contains,  Up: About This Guide

Conventions
===========

Following are examples of the typographical and graphic conventions used
in this guide:

   * `Functions', `utility program names', `standard names', and
     `classes'.

   * `Option flags'

   * `File Names', `button names', and `field names'.

   * `Variables'.

   * _Emphasis_.

   * [optional information or parameters]

   * Examples are described by text
          and then shown this way.

Commands that are entered by the user are preceded in this manual by the
characters `$ ' (dollar sign followed by space).  If your system uses
this sequence as a prompt, then the commands will appear exactly as you
see them in the manual.  If your system uses some other prompt, then
the command will appear with the `$' replaced by whatever prompt
character you are using.


File: gnat_rm.info,  Node: Related Information,  Prev: Conventions,  Up: About This Guide

Related Information
===================

   See the following documents for further information on GNAT:

   * `GNAT User's Guide', which provides information on how to use the
     GNAT compiler system.

   * `Ada 95 Reference Manual', which contains all reference material
     for the Ada 95 programming language.

   * `Ada 95 Annotated Reference Manual', which is an annotated version
     of the standard reference manual cited above.  The annotations
     describe detailed aspects of the design decision, and in
     particular contain useful sections on Ada 83 compatibility.

   * `DEC Ada, Technical Overview and Comparison on DIGITAL Platforms',
     which contains specific information on compatibility between GNAT
     and DEC Ada 83 systems.

   * `DEC Ada, Language Reference Manual, part number AA-PYZAB-TK' which
     describes in detail the pragmas and attributes provided by the DEC
     Ada 83 compiler system.



File: gnat_rm.info,  Node: Implementation Defined Pragmas,  Next: Implementation Defined Attributes,  Prev: About This Guide,  Up: Top

Implementation Defined Pragmas
******************************

Ada 95 defines a set of pragmas that can be used to supply additional
information to the compiler.  These language defined pragmas are
implemented in GNAT and work as described in the Ada 95 Reference
Manual.

   In addition, Ada 95 allows implementations to define additional
pragmas whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-dependent pragmas which can be used to
extend and enhance the functionality of the compiler.  This section of
the GNAT Reference Manual describes these additional pragmas.

   Note that any program using these pragmas may not be portable to
other compilers (although GNAT implements this set of pragmas on all
platforms).  Therefore if portability to other compilers is an important
consideration, the use of these pragmas should be minimized.

`pragma Abort_Defer'
     Syntax:

          pragma Abort_Defer;

     This pragma must appear at the start of the statement sequence of a
     handled sequence of statements (right after the `begin').  It has
     the effect of deferring aborts for the sequence of statements (but
     not for the declarations or handlers, if any, associated with this
     statement sequence).

`pragma Ada_83'
     Syntax:

          pragma Ada_83;

     A configuration pragma that establishes Ada 83 mode for the unit to
     which it applies, regardless of the mode set by the command line
     switches.  In Ada 83 mode, GNAT attempts to be as compatible with
     the syntax and semantics of Ada 83, as defined in the original Ada
     83 Reference Manual as possible.  In particular, the new Ada 95
     keywords are not recognized, optional package bodies are allowed,
     and generics may name types with unknown discriminants without
     using the `(<>)' notation.  In addition, some but not all of the
     additional restrictions of Ada 83 are enforced.

     Ada 83 mode is intended for two purposes.  Firstly, it allows
     existing legacy Ada 83 code to be compiled and adapted to GNAT
     with less effort.  Secondly, it aids in keeping code backwards
     compatible with Ada 83.  However, there is no guarantee that code
     that is processed correctly by GNAT in Ada 83 mode will in fact
     compile and execute with an Ada 83 compiler, since GNAT does not
     enforce all the additional checks required by Ada 83.

`pragma Ada_95'
     Syntax:

          pragma Ada_95;

     A configuration pragma that establishes Ada 95 mode for the unit
     to which it applies, regardless of the mode set by the command
     line switches.  This mode is set automatically for the `Ada' and
     `System' packages and their children, so you need not specify it
     in these contexts.  This pragma is useful when writing a reusable
     component that itself uses Ada 95 features, but which is intended
     to be usable from either Ada 83 or Ada 95 programs.

`pragma Annotate'
     Syntax:

          pragma Annotate (IDENTIFIER {, ARG});
          
          ARG ::= NAME | EXPRESSION

     This pragma is used to annotate programs.  IDENTIFIER identifies
     the type of annotation.  GNAT verifies this is an identifier, but
     does not otherwise analyze it.  The ARG argument can be either a
     string literal or an expression.  String literals are assumed to
     be of type `Standard.String'.  Names of entities are simply
     analyzed as entity names.  All other expressions are analyzed as
     expressions, and must be unambiguous.

     The analyzed pragma is retained in the tree, but not otherwise
     processed by any part of the GNAT compiler.  This pragma is
     intended for use by external tools, including ASIS.

`pragma Assert'
     Syntax:

          pragma Assert (
            boolean_EXPRESSION
            [, static_string_EXPRESSION])

     The effect of this pragma depends on whether the corresponding
     command line switch is set to activate assertions.  The pragma
     expands into code equivalent to the following:

          if assertions-enabled then
             if not boolean_EXPRESSION then
                System.Assertions.Raise_Assert_Failure
                  (string_EXPRESSION);
             end if;
          end if;

     The string argument, if given, is the message that will be
     associated with the exception occurrence if the exception is
     raised.  If no second argument is given, the default message is
     `FILE:NNN', where FILE is the name of the source file containing
     the assert, and NNN is the line number of the assert.  A pragma is
     not a statement, so if a statement sequence contains nothing but a
     pragma assert, then a null statement is required in addition, as
     in:

          ...
          if J > 3 then
             pragma Assert (K > 3, "Bad value for K");
             null;
          end if;

     Note that, as with the `if' statement to which it is equivalent,
     the type of the expression is either `Standard.Boolean', or any
     type derived from this standard type.

     If assertions are disabled (switch `-gnata' not used), then there
     is no effect (and in particular, any side effects from the
     expression are suppressed).  More precisely it is not quite true
     that the pragma has no effect, since the expression is analyzed,
     and may cause types to be frozen if they are mentioned here for
     the first time.

     If assertions are enabled, then the given expression is tested,
     and if it is `False' then `System.Assertions.Raise_Assert_Failure'
     is called which results in the raising of `Assert_Failure' with
     the given message.

     If the boolean expression has side effects, these side effects
     will turn on and off with the setting of the assertions mode,
     resulting in assertions that have an effect on the program.  You
     should generally avoid side effects in the expression arguments of
     this pragma.  However, the expressions are analyzed for semantic
     correctness whether or not assertions are enabled, so turning
     assertions on and off cannot affect the legality of a program.

`pragma Ast_Entry'
     Syntax:

          pragma AST_Entry (entry_IDENTIFIER);

     This pragma is implemented only in the OpenVMS implementation of
     GNAT.  The argument is the simple name of a single entry; at most
     one `AST_Entry' pragma is allowed for any given entry.  This
     pragma must be used in conjunction with the `AST_Entry' attribute,
     and is only allowed after the entry declaration and in the same
     task type specification or single task as the entry to which it
     applies.  This pragma specifies that the given entry may be used
     to handle an OpenVMS asynchronous system trap (`AST') resulting
     from an OpenVMS system service call.  The pragma does not affect
     normal use of the entry.  For further details on this pragma, see
     the DEC Ada Language Reference Manual, section 9.12a.

`pragma C_Pass_By_Copy'
     Syntax:

          pragma C_Pass_By_Copy
            ([Max_Size =>] static_integer_EXPRESSION);

     Normally the default mechanism for passing C convention records to
     C convention subprograms is to pass them by reference, as
     suggested by RM B.3(69).  Use the configuration pragma
     `C_Pass_By_Copy' to change this default, by requiring that record
     formal parameters be passed by copy if all of the following
     conditions are met:

        * The size of the record type does not exceed
          STATIC_INTEGER_EXPRESSION.

        * The record type has `Convention C'.

        * The formal parameter has this record type, and the subprogram
          has a foreign (non-Ada) convention.

     If these conditions are met the argument is passed by copy, i.e.
     in a manner consistent with what C expects if the corresponding
     formal in the C prototype is a struct (rather than a pointer to a
     struct).

     You can also pass records by copy by specifying the convention
     `C_Pass_By_Copy' for the record type, or by using the extended
     `Import' and `Export' pragmas, which allow specification of
     passing mechanisms on a parameter by parameter basis.

`pragma Comment'
     Syntax:

          pragma Comment (static_string_EXPRESSION);

     This is almost identical in effect to pragma `Ident'.  It allows
     the placement of a comment into the object file and hence into the
     executable file if the operating system permits such usage.  The
     difference is that `Comment', unlike `Ident', has no limit on the
     length of the string argument, and no limitations on placement of
     the pragma (it can be placed anywhere in the main source unit).

`pragma Common_Object'
     Syntax:

          pragma Common_Object (
               [Internal =>] LOCAL_NAME,
            [, [External =>] EXTERNAL_SYMBOL]
            [, [Size     =>] EXTERNAL_SYMBOL] )
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma enables the shared use of variables stored in overlaid
     linker areas corresponding to the use of `COMMON' in Fortran.  The
     single object LOCAL_NAME is assigned to the area designated by the
     EXTERNAL argument.  You may define a record to correspond to a
     series of fields.  The SIZE argument is syntax checked in GNAT,
     but otherwise ignored.

     `Common_Object' is not supported on all platforms.  If no support
     is available, then the code generator will issue a message
     indicating that the necessary attribute for implementation of this
     pragma is not available.

`pragma Complex_Representation'
     Syntax:

          pragma Complex_Representation
                  ([Entity =>] LOCAL_NAME);

     The ENTITY argument must be the name of a record type which has
     two fields of the same floating-point type.  The effect of this
     pragma is to force gcc to use the special internal complex
     representation form for this record, which may be more efficient.
     Note that this may result in the code for this type not conforming
     to standard ABI (application binary interface) requirements for
     the handling of record types.  For example, in some environments,
     there is a requirement for passing records by pointer, and the use
     of this pragma may result in passing this type in floating-point
     registers.

`pragma Component_Alignment'
     Syntax:

          pragma Component_Alignment (
               [Form =>] ALIGNMENT_CHOICE
            [, [Name =>] type_LOCAL_NAME]);
          
          ALIGNMENT_CHOICE ::=
            Component_Size
          | Component_Size_4
          | Storage_Unit
          | Default

     Specifies the alignment of components in array or record types.
     The meaning of the FORM argument is as follows:

    `Component_Size'
          Aligns scalar components and subcomponents of the array or
          record type on boundaries appropriate to their inherent size
          (naturally aligned).  For example, 1-byte components are
          aligned on byte boundaries, 2-byte integer components are
          aligned on 2-byte boundaries, 4-byte integer components are
          aligned on 4-byte boundaries and so on.  These alignment
          rules correspond to the normal rules for C compilers on all
          machines except the VAX.

    `Component_Size_4'
          Naturally aligns components with a size of four or fewer
          bytes.  Components that are larger than 4 bytes are placed on
          the next 4-byte boundary.

    `Storage_Unit'
          Specifies that array or record components are byte aligned,
          i.e.  aligned on boundaries determined by the value of the
          constant `System.Storage_Unit'.

    `Default'
          Specifies that array or record components are aligned on
          default boundaries, appropriate to the underlying hardware or
          operating system or both.  For OpenVMS VAX systems, the
          `Default' choice is the same as the `Storage_Unit' choice
          (byte alignment).  For all other systems, the `Default'
          choice is the same as `Component_Size' (natural alignment).

     If the `Name' parameter is present, TYPE_LOCAL_NAME must refer to
     a local record or array type, and the specified alignment choice
     applies to the specified type.  The use of `Component_Alignment'
     together with a pragma `Pack' causes the `Component_Alignment'
     pragma to be ignored.  The use of `Component_Alignment' together
     with a record representation clause is only effective for fields
     not specified by the representation clause.

     If the `Name' parameter is absent, the pragma can be used as either
     a configuration pragma, in which case it applies to one or more
     units in accordance with the normal rules for configuration
     pragmas, or it can be used within a declarative part, in which
     case it applies to types that are declared within this declarative
     part, or within any nested scope within this declarative part.  In
     either case it specifies the alignment to be applied to any record
     or array type which has otherwise standard representation.

     If the alignment for a record or array type is not specified (using
     pragma `Pack', pragma `Component_Alignment', or a record rep
     clause), the GNAT uses the default alignment as described
     previously.

`pragma Convention_Identifier'
     Syntax:

          pragma Convention_Identifier (
                   [Name =>]       IDENTIFIER,
                   [Convention =>] convention_IDENTIFIER);

     This pragma provides a mechanism for supplying synonyms for
     existing convention identifiers. The `Name' identifier can
     subsequently be used as a synonym for the given convention in
     other pragmas (including for example pragma `Import' or another
     `Convention_Identifier' pragma). As an example of the use of this,
     suppose you had legacy code which used Fortran77 as the identifier
     for Fortran. Then the pragma:

          pragma Convention_Indentifier (Fortran77, Fortran);

     would allow the use of the convention identifier `Fortran77' in
     subsequent code, avoiding the need to modify the sources. As
     another example, you could use this to parametrize convention
     requirements according to systems. Suppose you needed to use
     `Stdcall' on windows systems, and `C' on some other system, then
     you could define a convention identifier `Library' and use a single
     `Convention_Identifier' pragma to specify which convention would
     be used system-wide.

`pragma CPP_Class'
     Syntax:

          pragma CPP_Class ([Entity =>] LOCAL_NAME);

     The argument denotes an entity in the current declarative region
     that is declared as a tagged or untagged record type.  It
     indicates that the type corresponds to an externally declared C++
     class type, and is to be laid out the same way that C++ would lay
     out the type.

     If (and only if) the type is tagged, at least one component in the
     record must be of type `Interfaces.CPP.Vtable_Ptr', corresponding
     to the C++ Vtable (or Vtables in the case of multiple inheritance)
     used for dispatching.

     Types for which `CPP_Class' is specified do not have assignment or
     equality operators defined (such operations can be imported or
     declared as subprograms as required).  Initialization is allowed
     only by constructor functions (see pragma `CPP_Constructor').

     Pragma `CPP_Class' is intended primarily for automatic generation
     using an automatic binding generator tool.  See *Note Interfacing
     to C++:: for related information.

`pragma CPP_Constructor'
     Syntax:

          pragma CPP_Constructor ([Entity =>] LOCAL_NAME);

     This pragma identifies an imported function (imported in the usual
     way with pragma `Import') as corresponding to a C++ constructor.
     The argument is a name that must have been previously mentioned in
     a pragma `Import' with `Convention' = `CPP', and must be of one of
     the following forms:

        * `function FNAME return T'Class'

        * `function FNAME (...) return T'Class'

     where T is a tagged type to which the pragma `CPP_Class' applies.

     The first form is the default constructor, used when an object of
     type T is created on the Ada side with no explicit constructor.
     Other constructors (including the copy constructor, which is
     simply a special case of the second form in which the one and only
     argument is of type T), can only appear in two contexts:

        * On the right side of an initialization of an object of type T.

        * In an extension aggregate for an object of a type derived
          from T.

     Although the constructor is described as a function that returns a
     value on the Ada side, it is typically a procedure with an extra
     implicit argument (the object being initialized) at the
     implementation level.  GNAT issues the appropriate call, whatever
     it is, to get the object properly initialized.

     In the case of derived objects, you may use one of two possible
     forms for declaring and creating an object:

        * `New_Object : Derived_T'

        * `New_Object : Derived_T := (CONSTRUCTOR-FUNCTION-CALL WITH
          ...)'

     In the first case the default constructor is called and extension
     fields if any are initialized according to the default
     initialization expressions in the Ada declaration.  In the second
     case, the given constructor is called and the extension aggregate
     indicates the explicit values of the extension fields.

     If no constructors are imported, it is impossible to create any
     objects on the Ada side.  If no default constructor is imported,
     only the initialization forms using an explicit call to a
     constructor are permitted.

     Pragma `CPP_Constructor' is intended primarily for automatic
     generation using an automatic binding generator tool.  See *Note
     Interfacing to C++:: for more related information.

`pragma CPP_Virtual'
     Syntax:

          pragma CPP_Virtual
               [Entity     =>] ENTITY,
            [, [Vtable_Ptr =>] vtable_ENTITY,]
            [, [Position   =>] static_integer_EXPRESSION])

     This pragma serves the same function as pragma `Import' in that
     case of a virtual function imported from C++.  The ENTITY argument
     must be a primitive subprogram of a tagged type to which pragma
     `CPP_Class' applies.  The VTABLE_PTR argument specifies the
     Vtable_Ptr component which contains the entry for this virtual
     function.  The POSITION argument is the sequential number counting
     virtual functions for this Vtable starting at 1.

     The `Vtable_Ptr' and `Position' arguments may be omitted if there
     is one Vtable_Ptr present (single inheritance case) and all
     virtual functions are imported.  In that case the compiler can
     deduce both these values.

     No `External_Name' or `Link_Name' arguments are required for a
     virtual function, since it is always accessed indirectly via the
     appropriate Vtable entry.

     Pragma `CPP_Virtual' is intended primarily for automatic generation
     using an automatic binding generator tool.  See *Note Interfacing
     to C++:: for related information.

`pragma CPP_Vtable'
     Syntax:

          pragma CPP_Vtable (
            [Entity      =>] ENTITY,
            [Vtable_Ptr  =>] vtable_ENTITY,
            [Entry_Count =>] static_integer_EXPRESSION);

     Given a record to which the pragma `CPP_Class' applies, this
     pragma can be specified for each component of type
     `CPP.Interfaces.Vtable_Ptr'.  ENTITY is the tagged type, VTABLE_PTR
     is the record field of type `Vtable_Ptr', and ENTRY_COUNT is the
     number of virtual functions on the C++ side.  Not all of these
     functions need to be imported on the Ada side.

     You may omit the `CPP_Vtable' pragma if there is only one
     `Vtable_Ptr' component in the record and all virtual functions are
     imported on the Ada side (the default value for the entry count in
     this case is simply the total number of virtual functions).

     Pragma `CPP_Vtable' is intended primarily for automatic generation
     using an automatic binding generator tool.  See *Note Interfacing
     to C++:: for related information.

`pragma Debug'
     Syntax:

          pragma Debug (PROCEDURE_CALL_WITHOUT_SEMICOLON);
          
          PROCEDURE_CALL_WITHOUT_SEMICOLON ::=
            PROCEDURE_NAME
          | PROCEDURE_PREFIX ACTUAL_PARAMETER_PART

     The argument has the syntactic form of an expression, meeting the
     syntactic requirements for pragmas.

     If assertions are not enabled on the command line, this pragma has
     no effect.  If asserts are enabled, the semantics of the pragma is
     exactly equivalent to the procedure call statement corresponding
     to the argument with a terminating semicolon.  Pragmas are
     permitted in sequences of declarations, so you can use pragma
     `Debug' to intersperse calls to debug procedures in the middle of
     declarations.

`pragma Elaboration_Checks'
     Syntax:

          pragma Elaboration_Checks (RM | Static);

     This is a configuration pragma that provides control over the
     elaboration model used by the compilation affected by the pragma.
     If the parameter is RM, then the dynamic elaboration model
     described in the Ada Reference Manual is used, as though the
     `-gnatE' switch had been specified on the command line.  If the
     parameter is Static, then the default GNAT static model is used.
     This configuration pragma overrides the setting of the command
     line.  For full details on the elaboration models used by the GNAT
     compiler, see section "Elaboration Order Handling in GNAT" in the
     `GNAT User's Guide'.

`pragma Eliminate'
     Syntax:

          pragma Eliminate (
              [Unit_Name =>] IDENTIFIER |
                             SELECTED_COMPONENT);
          
          pragma Eliminate (
              [Unit_Name       =>]  IDENTIFIER |
                                    SELECTED_COMPONENT,
              [Entity          =>]  IDENTIFIER |
                                    SELECTED_COMPONENT |
                                    STRING_LITERAL
            [,[Parameter_Types =>]  PARAMETER_TYPES]
            [,[Result_Type     =>]  result_SUBTYPE_NAME]
            [,[Homonym_Number  =>]  INTEGER_LITERAL]);
          
          PARAMETER_TYPES ::= (SUBTYPE_NAME {, SUBTYPE_NAME})
          SUBTYPE_NAME    ::= STRING_LITERAL

     This pragma indicates that the given entity is not used outside the
     compilation unit it is defined in.  The entity may be either a
     subprogram or a variable.

     If the entity to be eliminated is a library level subprogram, then
     the first form of pragma `Eliminate' is used with only a single
     argument.  In this form, the `Unit_Name' argument specifies the
     name of the library  level unit to be eliminated.

     In all other cases, both `Unit_Name' and `Entity' arguments are
     required.  item is an entity of a library package, then the first
     argument specifies the unit name, and the second argument specifies
     the particular entity.  If the second argument is in string form,
     it must correspond to the internal manner in which GNAT stores
     entity names (see compilation unit Namet in the compiler sources
     for details).

     The remaining parameters are optionally used to distinguish
     between overloaded subprograms.  There are two ways of doing this.

     Use `Parameter_Types' and `Result_Type' to specify the profile of
     the subprogram to be eliminated in a manner similar to that used
     for the extended `Import' and `Export' pragmas, except that the
     subtype names are always given as string literals, again
     corresponding to the internal manner in which GNAT stores entity
     names.

     Alternatively, the `Homonym_Number' parameter is used to specify
     which overloaded alternative is to be eliminated.  A value of 1
     indicates the first subprogram (in lexical order), 2 indicates the
     second etc.

     The effect of the pragma is to allow the compiler to eliminate the
     code or data associated with the named entity.  Any reference to
     an eliminated entity outside the compilation unit it is defined in,
     causes a compile time or link time error.

     The parameters of this pragma may be given in any order, as long as
     the usual rules for use of named parameters and position parameters
     are used.

     The intention of pragma `Eliminate' is to allow a program to be
     compiled in a system independent manner, with unused entities
     eliminated, without the requirement of modifying the source text.
     Normally the required set of `Eliminate' pragmas is constructed
     automatically using the gnatelim tool.  Elimination of unused
     entities local to a compilation unit is automatic, without
     requiring the use of pragma `Eliminate'.

     Note that the reason this pragma takes string literals where names
     might be expected is that a pragma `Eliminate' can appear in a
     context where the relevant names are not visible.

`pragma Export_Exception'
     Syntax:

          pragma Export_Exception (
               [Internal =>] LOCAL_NAME,
            [, [External =>] EXTERNAL_SYMBOL,]
            [, [Form     =>] Ada | VMS]
            [, [Code     =>] static_integer_EXPRESSION]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma is implemented only in the OpenVMS implementation of
     GNAT.  It causes the specified exception to be propagated outside
     of the Ada program, so that it can be handled by programs written
     in other OpenVMS languages.  This pragma establishes an external
     name for an Ada exception and makes the name available to the
     OpenVMS Linker as a global symbol.  For further details on this
     pragma, see the DEC Ada Language Reference Manual, section
     13.9a3.2.

`pragma Export_Function ...'
     Syntax:

          pragma Export_Function (
               [Internal         =>] LOCAL_NAME,
            [, [External         =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types  =>] PARAMETER_TYPES]
            [, [Result_Type      =>] result_SUBTYPE_MARK]
            [, [Mechanism        =>] MECHANISM]
            [, [Result_Mechanism =>] MECHANISM_NAME]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     Use this pragma to make a function externally callable and
     optionally provide information on mechanisms to be used for
     passing parameter and result values.  We recommend, for the
     purposes of improving portability, this pragma always be used in
     conjunction with a separate pragma `Export', which must precede
     the pragma `Export_Function'.  GNAT does not require a separate
     pragma `Export', but if none is present, `Convention Ada' is
     assumed, which is usually not what is wanted, so it is usually
     appropriate to use this pragma in conjunction with a `Export' or
     `Convention' pragma that specifies the desired foreign convention.
     Pragma `Export_Function' (and `Export', if present) must appear in
     the same declarative region as the function to which they apply.

     INTERNAL_NAME must uniquely designate the function to which the
     pragma applies.  If more than one function name exists of this
     name in the declarative part you must use the `Parameter_Types' and
     `Result_Type' parameters is mandatory to achieve the required
     unique designation.  SUBTYPE_ MARKs in these parameters must
     exactly match the subtypes in the corresponding function
     specification, using positional notation to match parameters with
     subtype marks.  Passing by descriptor is supported only on the
     OpenVMS ports of GNAT.

`pragma Export_Object ...'
     Syntax:

          pragma Export_Object
                [Internal =>] LOCAL_NAME,
             [, [External =>] EXTERNAL_SYMBOL]
             [, [Size     =>] EXTERNAL_SYMBOL]
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma designates an object as exported, and apart from the
     extended rules for external symbols, is identical in effect to the
     use of the normal `Export' pragma applied to an object.  You may
     use a separate Export pragma (and you probably should from the
     point of view of portability), but it is not required.  SIZE is
     syntax checked, but otherwise ignored by GNAT.

`pragma Export_Procedure ...'
     Syntax:

          pragma Export_Procedure (
               [Internal        =>] LOCAL_NAME
            [, [External        =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types =>] PARAMETER_TYPES]
            [, [Mechanism       =>] MECHANISM]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     This pragma is identical to `Export_Function' except that it
     applies to a procedure rather than a function and the parameters
     `Result_Type' and `Result_Mechanism' are not permitted.  GNAT does
     not require a separate pragma `Export', but if none is present,
     `Convention Ada' is assumed, which is usually not what is wanted,
     so it is usually appropriate to use this pragma in conjunction
     with a `Export' or `Convention' pragma that specifies the desired
     foreign convention.

`pragma Export_Valued_Procedure'
     Syntax:

          pragma Export_Valued_Procedure (
               [Internal        =>] LOCAL_NAME
            [, [External        =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types =>] PARAMETER_TYPES]
            [, [Mechanism       =>] MECHANISM]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     This pragma is identical to `Export_Procedure' except that the
     first parameter of LOCAL_NAME, which must be present, must be of
     mode `OUT', and externally the subprogram is treated as a function
     with this parameter as the result of the function.  GNAT provides
     for this capability to allow the use of `OUT' and `IN OUT'
     parameters in interfacing to external functions (which are not
     permitted in Ada functions).  GNAT does not require a separate
     pragma `Export', but if none is present, `Convention Ada' is
     assumed, which is almost certainly not what is wanted since the
     whole point of this pragma is to interface with foreign language
     functions, so it is usually appropriate to use this pragma in
     conjunction with a `Export' or `Convention' pragma that specifies
     the desired foreign convention.

`pragma Extend_System'
     Syntax:

          pragma Extend_System ([Name =>] IDENTIFIER);

     This pragma is used to provide backwards compatibility with other
     implementations that extend the facilities of package `System'.  In
     GNAT, `System' contains only the definitions that are present in
     the Ada 95 RM.  However, other implementations, notably the DEC
     Ada 83 implementation, provide many extensions to package `System'.

     For each such implementation accommodated by this pragma, GNAT
     provides a package `Aux_XXX', e.g. `Aux_DEC' for the DEC Ada 83
     implementation, which provides the required additional
     definitions.  You can use this package in two ways.  You can
     `with' it in the normal way and access entities either by
     selection or using a `use' clause.  In this case no special
     processing is required.

     However, if existing code contains references such as `System.XXX'
     where XXX is an entity in the extended definitions provided in
     package `System', you may use this pragma to extend visibility in
     `System' in a non-standard way that provides greater compatibility
     with the existing code.  Pragma `Extend_System' is a configuration
     pragma whose single argument is the name of the package containing
     the extended definition (e.g. `Aux_DEC' for the DEC Ada case).  A
     unit compiled under control of this pragma will be processed using
     special visibility processing that looks in package
     `System.Aux_XXX' where `Aux_XXX' is the pragma argument for any
     entity referenced in package `System', but not found in package
     `System'.

     You can use this pragma either to access a predefined `System'
     extension supplied with the compiler, for example `Aux_DEC' or you
     can construct your own extension unit following the above
     definition.  Note that such a package is a child of `System' and
     thus is considered part of the implementation.  To compile it you
     will have to use the appropriate switch for compiling system
     units.  See the GNAT User's Guide for details.

`pragma External'
     Syntax:

          pragma External (
            [   Convention    =>] convention_IDENTIFIER,
            [   Entity        =>] local_NAME
            [, [External_Name =>] static_string_EXPRESSION ]
            [, [Link_Name     =>] static_string_EXPRESSION ]);

     This pragma is identical in syntax and semantics to pragma
     `Export' as defined in the Ada Reference Manual.  It is provided
     for compatibility with some Ada 83 compilers that used this pragma
     for exactly the same purposes as pragma `Export' before the latter
     was standardized.

`pragma External_Name_Casing'
     Syntax:

          pragma External_Name_Casing (
            Uppercase | Lowercase
            [, Uppercase | Lowercase | As_Is]);

     This pragma provides control over the casing of external names
     associated with Import and Export pragmas.  There are two cases to
     consider:

    Implicit external names
          Implicit external names are derived from identifiers.  The
          most common case arises when a standard Ada 95 Import or
          Export pragma is used with only two arguments, as in:

                  pragma Import (C, C_Routine);

          Since Ada is a case insensitive language, the spelling of the
          identifier in the Ada source program does not provide any
          information on the desired casing of the external name, and
          so a convention is needed.  In GNAT the default treatment is
          that such names are converted to all lower case letters.
          This corresponds to the normal C style in many environments.
          The first argument of pragma `External_Name_Casing' can be
          used to control this treatment.  If `Uppercase' is specified,
          then the name will be forced to all uppercase letters.  If
          `Lowercase' is specified, then the normal default of all
          lower case letters will be used.

          This same implicit treatment is also used in the case of
          extended DEC Ada 83 compatible Import and Export pragmas
          where an external name is explicitly specified using an
          identifier rather than a string.

    Explicit external names
          Explicit external names are given as string literals.  The
          most common case arises when a standard Ada 95 Import or
          Export pragma is used with three arguments, as in:

               pragma Import (C, C_Routine, "C_routine");

          In this case, the string literal normally provides the exact
          casing required for the external name.  The second argument
          of pragma `External_Name_Casing' may be used to modify this
          behavior.  If `Uppercase' is specified, then the name will be
          forced to all uppercase letters.  If `Lowercase' is specified,
          then the name will be forced to all lowercase letters.  A
          specification of `As_Is' provides the normal default behavior
          in which the casing is taken from the string provided.

     This pragma may appear anywhere that a pragma is valid.  In
     particular, it can be used as a configuration pragma in the
     `gnat.adc' file, in which case it applies to all subsequent
     compilations, or it can be used as a program unit pragma, in which
     case it only applies to the current unit, or it can be used more
     locally to control individual Import/Export pragmas.

     It is primarily intended for use with OpenVMS systems, where many
     compilers convert all symbols to upper case by default.  For
     interfacing to such compilers (e.g. the DEC C compiler), it may be
     convenient to use the pragma:

          pragma External_Name_Casing (Uppercase, Uppercase);

     to enforce the upper casing of all external symbols.

`pragma Finalize_Storage_Only'
     Syntax:

          pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);

     This pragma allows the compiler not to emit a Finalize call for
     objects defined at the library level.  This is mostly useful for
     types where finalization is only used to deal with storage
     reclamation since in most environments it is not necessary to
     reclaim memory just before terminating execution, hence the name.

`pragma Float_Representation'
     Syntax:

          pragma Float_Representation (FLOAT_REP);
          
          FLOAT_REP ::= VAX_Float | IEEE_Float

     This pragma is implemented only in the OpenVMS implementation of
     GNAT.  It allows control over the internal representation chosen
     for the predefined floating point types declared in the packages
     `Standard' and `System'.  For further details on this pragma, see
     the DEC Ada Language Reference Manual, section 3.5.7a.  Note that
     to use this pragma, the standard runtime libraries must be
     recompiled.  See the description of the `GNAT LIBRARY' command in
     the OpenVMS version of the GNAT Users Guide for details on the use
     of this command.

`pragma Ident'
     Syntax:

          pragma Ident (static_string_EXPRESSION);

     This pragma provides a string identification in the generated
     object file, if the system supports the concept of this kind of
     identification string.  The maximum permitted length of the string
     literal is 31 characters.  This pragma is allowed only in the
     outermost declarative part or declarative items of a compilation
     unit.  On OpenVMS systems, the effect of the pragma is identical
     to the effect of the DEC Ada 83 pragma of the same name.

`pragma Import_Exception'
     Syntax:

          pragma Import_Exception (
               [Internal =>] LOCAL_NAME,
            [, [External =>] EXTERNAL_SYMBOL,]
            [, [Form     =>] Ada | VMS]
            [, [Code     =>] static_integer_EXPRESSION]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma is implemented only in the OpenVMS implementation of
     GNAT.  It allows OpenVMS conditions (for example, from OpenVMS
     system services or other OpenVMS languages) to be propagated to
     Ada programs as Ada exceptions.  The pragma specifies that the
     exception associated with an exception declaration in an Ada
     program be defined externally (in non-Ada code).  For further
     details on this pragma, see the DEC Ada Language Reference Manual,
     section 13.9a.3.1.

`pragma Import_Function ...'
     Syntax:

          pragma Import_Function (
               [Internal                 =>] LOCAL_NAME,
            [, [External                 =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types          =>] PARAMETER_TYPES]
            [, [Result_Type              =>] SUBTYPE_MARK]
            [, [Mechanism                =>] MECHANISM]
            [, [Result_Mechanism         =>] MECHANISM_NAME]
            [, [First_Optional_Parameter =>] IDENTIFIER]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     This pragma is used in conjunction with a pragma `Import' to
     specify additional information for an imported function.  The
     pragma `Import' (or equivalent pragma `Interface') must precede the
     `Import_Function' pragma and both must appear in the same
     declarative part as the function specification.

     The INTERNAL_NAME argument must uniquely designate the function to
     which the pragma applies.  If more than one function name exists
     of this name in the declarative part you must use the
     `Parameter_Types' and RESULT_TYPE parameters to achieve the
     required unique designation.  Subtype marks in these parameters
     must exactly match the subtypes in the corresponding function
     specification, using positional notation to match parameters with
     subtype marks.

     You may optionally use the MECHANISM and RESULT_MECHANISM
     parameters to specify passing mechanisms for the parameters and
     result.  If you specify a single mechanism name, it applies to all
     parameters.  Otherwise you may specify a mechanism on a parameter
     by parameter basis using either positional or named notation.  If
     the mechanism is not specified, the default mechanism is used.

     Passing by descriptor is supported only on the to OpenVMS ports of
     GNAT.

     `First_Optional_Parameter' applies only to OpenVMS ports of GNAT.
     It specifies that the designated parameter and all following
     parameters are optional, meaning that they are not passed at the
     generated code level (this is distinct from the notion of optional
     parameters in Ada where the parameters are passed anyway with the
     designated optional parameters).  All optional parameters must be
     of mode `IN' and have default parameter values that are either
     known at compile time expressions, or uses of the
     `'Null_Parameter' attribute.

`pragma Import_Object'
     Syntax:

          pragma Import_Object
               [Internal =>] LOCAL_NAME,
            [, [External =>] EXTERNAL_SYMBOL],
            [, [Size     =>] EXTERNAL_SYMBOL])
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma designates an object as imported, and apart from the
     extended rules for external symbols, is identical in effect to the
     use of the normal `Import' pragma applied to an object.  Unlike the
     subprogram case, you need not use a separate `Import' pragma,
     although you may do so (and probably should do so from a
     portability point of view).  SIZE is syntax checked, but otherwise
     ignored by GNAT.

`pragma Import_Procedure'
     Syntax:

          pragma Import_Procedure (
               [Internal                 =>] LOCAL_NAME,
            [, [External                 =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types          =>] PARAMETER_TYPES]
            [, [Mechanism                =>] MECHANISM]
            [, [First_Optional_Parameter =>] IDENTIFIER]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     This pragma is identical to `Import_Function' except that it
     applies to a procedure rather than a function and the parameters
     `Result_Type' and `Result_Mechanism' are not permitted.

`pragma Import_Valued_Procedure ...'
     Syntax:

          pragma Import_Valued_Procedure (
               [Internal                 =>] LOCAL_NAME,
            [, [External                 =>] EXTERNAL_SYMBOL]
            [, [Parameter_Types          =>] PARAMETER_TYPES]
            [, [Mechanism                =>] MECHANISM]
            [, [First_Optional_Parameter =>] IDENTIFIER]);
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION
          
          PARAMETER_TYPES ::=
            null
          | SUBTYPE_MARK {, SUBTYPE_MARK}
          
          MECHANISM ::=
            MECHANISM_NAME
          | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
          
          MECHANISM_ASSOCIATION ::=
            [formal_parameter_NAME =>] MECHANISM_NAME
          
          MECHANISM_NAME ::=
            Value
          | Reference
          | Descriptor [([Class =>] CLASS_NAME)]
          
          CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

     This pragma is identical to `Import_Procedure' except that the
     first parameter of LOCAL_NAME, which must be present, must be of
     mode `OUT', and externally the subprogram is treated as a function
     with this parameter as the result of the function.  The purpose of
     this capability is to allow the use of `OUT' and `IN OUT'
     parameters in interfacing to external functions (which are not
     permitted in Ada functions).  You may optionally use the
     `Mechanism' parameters to specify passing mechanisms for the
     parameters.  If you specify a single mechanism name, it applies to
     all parameters.  Otherwise you may specify a mechanism on a
     parameter by parameter basis using either positional or named
     notation.  If the mechanism is not specified, the default
     mechanism is used.

     Note that it is important to use this pragma in conjunction with a
     separate pragma Import that specifies the desired convention,
     since otherwise the default convention is Ada, which is almost
     certainly not what is required.

`pragma Initialize_Scalars'
     Syntax:

          pragma Initialize_Scalars;

     This pragma is similar to `Normalize_Scalars' conceptually but has
     two important differences.  First, there is no requirement for the
     pragma to be used uniformly in all units of a partition, in
     particular, it is fine to use this just for some or all of the
     application units of a partition, without needing to recompile the
     run-time library.

     In the case where some units are compiled with the pragma, and
     some without, then a declaration of a variable where the type is
     defined in package Standard or is locally declared will always be
     subject to initialization, as will any declaration of a scalar
     variable.  For composite variables, whether the variable is
     initialized may also depend on whether the package in which the
     type of the variable is declared is compiled with the pragma.

     The other important difference is that there is control over the
     value used for initializing scalar objects.  At bind time, you can
     select whether to initialize with invalid values (like
     Normalize_Scalars), or with high or low values, or with a
     specified bit pattern.  See the users guide for binder options for
     specifying these cases.

     This means that you can compile a program, and then without having
     to recompile the program, you can run it with different values
     being used for initializing otherwise uninitialized values, to
     test if your program behavior depends on the choice.  Of course
     the behavior should not change, and if it does, then most likely
     you have an erroneous reference to an uninitialized value.

     Note that pragma `Initialize_Scalars' is particularly useful in
     conjunction with the enhanced validity checking that is now
     provided in GNAT, which checks for invalid values under more
     conditions.  Using this feature (see description of the `-gnatv'
     flag in the users guide) in conjunction with pragma
     `Initialize_Scalars' provides a powerful new tool to assist in the
     detection of problems caused by uninitialized variables.

`pragma Inline_Always'
     Syntax:

          pragma Inline_Always (NAME [, NAME]);

     Similar to pragma `Inline' except that inlining is not subject to
     the use of option `-gnatn' for inter-unit inlining.

`pragma Inline_Generic'
     Syntax:

          pragma Inline_Generic (generic_package_NAME)

     This is implemented for compatibility with DEC Ada 83 and is
     recognized, but otherwise ignored, by GNAT.  All generic
     instantiations are inlined by default when using GNAT.

`pragma Interface'
     Syntax:

          pragma Interface (
               [Convention    =>] convention_identifier,
               [Entity =>] local_name
            [, [External_Name =>] static_string_expression],
            [, [Link_Name     =>] static_string_expression]);

     This pragma is identical in syntax and semantics to the standard
     Ada 95 pragma `Import'.  It is provided for compatibility with Ada
     83.  The definition is upwards compatible both with pragma
     `Interface' as defined in the Ada 83 Reference Manual, and also
     with some extended implementations of this pragma in certain Ada 83
     implementations.

`pragma Interface_Name'
     Syntax:

          pragma Interface_Name (
               [Entity        =>] LOCAL_NAME
            [, [External_Name =>] static_string_EXPRESSION]
            [, [Link_Name     =>] static_string_EXPRESSION]);

     This pragma provides an alternative way of specifying the
     interface name for an interfaced subprogram, and is provided for
     compatibility with Ada 83 compilers that use the pragma for this
     purpose.  You must provide at least one of EXTERNAL_NAME or
     LINK_NAME.

`pragma License'
     Syntax:

          pragma License (Unrestricted | GPL | Modified_GPL | Restricted);

     This pragma is provided to allow automated checking for
     appropriate license conditions with respect to the standard and
     modified GPL.  A pragma `License', which is a configuration pragma
     that typically appears at the start of a source file or in a
     separate `gnat.adc' file, specifies the licensing conditions of a
     unit as follows:

        * Unrestricted This is used for a unit that can be freely used
          with no license restrictions.  Examples of such units are
          public domain units, and units from the Ada Reference Manual.

        * GPL This is used for a unit that is licensed under the
          unmodified GPL, and which therefore cannot be `with''ed by a
          restricted unit.

        * Modified_GPL This is used for a unit licensed under the GNAT
          modified GPL that includes a special exception paragraph that
          specifically permits the inclusion of the unit in programs
          without requiring the entire program to be released under the
          GPL.  This is the license used for the GNAT run-time which
          ensures that the run-time can be used freely in any program
          without GPL concerns.

        * Restricted This is used for a unit that is restricted in that
          it is not permitted to depend on units that are licensed
          under the GPL.  Typical examples are proprietary code that is
          to be released under more restrictive license conditions.
          Note that restricted units are permitted to `with' units
          which are licensed under the modified GPL (this is the whole
          point of the modified GPL).


     Normally a unit with no `License' pragma is considered to have an
     unknown license, and no checking is done.  However, standard GNAT
     headers are recognized, and license information is derived from
     them as follows.

          A GNAT license header starts with a line containing 78
          hyphens.  The following comment text is searched for the
          appearence of any of the following strings.

          If the string "GNU General Public License" is found, then the
          unit is assumed to have GPL license, unless the string "As a
          special exception" follows, in which case the license is
          assumed to be modified GPL.

          If one of the strings "This specification is adapated from
          the Ada Semantic Interface" or "This specification is derived
          from the Ada Reference Manual" is found then the unit is
          assumed to be unrestricted.

     These default actions means that a program with a restricted
     license pragma will automatically get warnings if a GPL unit is
     inappropriately `with''ed.  For example, the program:

          with Sem_Ch3;
          with GNAT.Sockets;
          procedure Secret_Stuff is
          ...
          end Secret_Stuff

     if compiled with pragma `License' (`Restricted') in a `gnat.adc'
     file will generate the warning:

          1.  with Sem_Ch3;
                  |
             >>> license of withed unit "Sem_Ch3" is incompatible
          
          2.  with GNAT.Sockets;
          3.  procedure Secret_Stuff is

     Here we get a warning on `Sem_Ch3' since it is part of the GNAT
     compiler and is licensed under the GPL, but no warning for
     `GNAT.Sockets' which is part of the GNAT run time, and is
     therefore licensed under the modified GPL.

`pragma Link_With'
     Syntax:

          pragma Link_With (static_string_EXPRESSION {,static_string_EXPRESSION});

     This pragma is provided for compatibility with certain Ada 83
     compilers.  It has exactly the same effect as pragma
     `Linker_Options' except that spaces occurring within one of the
     string expressions are treated as separators. For example, in the
     following case:

          pragma Link_With ("-labc -ldef");

     results in passing the strings `-labc' and `-ldef' as two separate
     arguments to the linker. In addition pragma Link_With allows
     multiple arguments, with the same effect as successive pragmas.

`pragma Linker_Alias'
     Syntax:

          pragma Linker_Alias (
            [Entity =>] LOCAL_NAME
            [Alias  =>] static_string_EXPRESSION);

     This pragma establishes a linker alias for the given named entity.
     For further details on the exact effect, consult the GCC manual.

`pragma Linker_Section'
     Syntax:

          pragma Linker_Section (
            [Entity  =>] LOCAL_NAME
            [Section =>] static_string_EXPRESSION);

     This pragma specifies the name of the linker section for the given
     entity.  For further details on the exact effect, consult the GCC
     manual.

`pragma No_Run_Time'
     Syntax:

          pragma No_Run_Time;

     This is a configuration pragma that makes sure the user code does
     not use nor need anything from the GNAT run time.  This is mostly
     useful in context where code certification is required.  Please
     consult the `GNAT Pro High-Integrity Edition User's Guide' for
     additional information.

`pragma Normalize_Scalars'
     Syntax:

          pragma Normalize_Scalars;

     This is a language defined pragma which is fully implemented in
     GNAT.  The effect is to cause all scalar objects that are not
     otherwise initialized to be initialized.  The initial values are
     implementation dependent and are as follows:

    `Standard.Character'
          Objects whose root type is Standard.Character are initialized
          to Character'Last.  This will be out of range of the subtype
          only if the subtype range excludes this value.

    `Standard.Wide_Character'
          Objects whose root type is Standard.Wide_Character are
          initialized to Wide_Character'Last.  This will be out of
          range of the subtype only if the subtype range excludes this
          value.

    `Integer types'
          Objects of an integer type are initialized to
          base_type'First, where base_type is the base type of the
          object type.  This will be out of range of the subtype only
          if the subtype range excludes this value.  For example, if
          you declare the subtype:

               subtype Ityp is integer range 1 .. 10;

          then objects of type x will be initialized to Integer'First,
          a negative number that is certainly outside the range of
          subtype `Ityp'.

    `Real types'
          Objects of all real types (fixed and floating) are
          initialized to base_type'First, where base_Type is the base
          type of the object type.  This will be out of range of the
          subtype only if the subtype range excludes this value.

    `Modular types'
          Objects of a modular type are initialized to typ'Last.  This
          will be out of range of the subtype only if the subtype
          excludes this value.

    `Enumeration types'
          Objects of an enumeration type are initialized to all
          one-bits, i.e. to the value `2 ** typ'Size - 1'.  This will
          be out of range of the enumeration subtype in all cases
          except where the subtype contains exactly 2**8, 2**16, or
          2**32 elements.

`pragma Long_Float'
     Syntax:

          pragma Long_Float (FLOAT_FORMAT);
          
          FLOAT_FORMAT ::= D_Float | G_Float

     This pragma is implemented only in the OpenVMS implementation of
     GNAT.  It allows control over the internal representation chosen
     for the predefined type `Long_Float' and for floating point type
     representations with `digits' specified in the range 7 through 15.
     For further details on this pragma, see the `DEC Ada Language
     Reference Manual', section 3.5.7b.  Note that to use this pragma,
     the standard runtime libraries must be recompiled.  See the
     description of the `GNAT LIBRARY' command in the OpenVMS version
     of the GNAT User's Guide for details on the use of this command.

`pragma Machine_Attribute ...'
     Syntax:

          pragma Machine_Attribute (
            [Attribute_Name =>] string_EXPRESSION,
            [Entity         =>] LOCAL_NAME);

     Machine dependent attributes can be specified for types and/or
     declarations.  Currently only subprogram entities are supported.
     This pragma is semantically equivalent to
     `__attribute__((STRING_EXPRESSION))' in GNU C, where
     `STRING_EXPRESSION' is recognized by the GNU C macros
     `VALID_MACHINE_TYPE_ATTRIBUTE' and `VALID_MACHINE_DECL_ATTRIBUTE'
     which are defined in the configuration header file `tm.h' for each
     machine.  See the GCC manual for further information.

`pragma Main_Storage'
     Syntax:

          pragma Main_Storage
            (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);
          
          MAIN_STORAGE_OPTION ::=
            [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
          | [TOP_GUARD       =>] static_SIMPLE_EXPRESSION

     This pragma is provided for compatibility with OpenVMS Vax
     Systems.  It has no effect in GNAT, other than being syntax
     checked.  Note that the pragma also has no effect in DEC Ada 83
     for OpenVMS Alpha Systems.

`pragma No_Return'
     Syntax:

          pragma No_Return (procedure_LOCAL_NAME);

     PROCEDURE_LOCAL_NAME must refer to one or more procedure
     declarations in the current declarative part.  A procedure to
     which this pragma is applied may not contain any explicit `return'
     statements, and also may not contain any implicit return
     statements from falling off the end of a statement sequence.  One
     use of this pragma is to identify procedures whose only purpose is
     to raise an exception.

     Another use of this pragma is to suppress incorrect warnings about
     missing returns in functions, where the last statement of a
     function statement sequence is a call to such a procedure.

`pragma Passive'
     Syntax:

          pragma Passive ([Semaphore | No]);

     Syntax checked, but otherwise ignored by GNAT.  This is recognized
     for compatibility with DEC Ada 83 implementations, where it is
     used within a task definition to request that a task be made
     passive.  If the argument `Semaphore' is present, or no argument
     is omitted, then DEC Ada 83 treats the pragma as an assertion that
     the containing task is passive and that optimization of context
     switch with this task is permitted and desired.  If the argument
     `No' is present, the task must not be optimized.  GNAT does not
     attempt to optimize any tasks in this manner (since protected
     objects are available in place of passive tasks).

`pragma Polling'
     Syntax:

          pragma Polling (ON | OFF);

     This pragma controls the generation of polling code.  This is
     normally off.  If `pragma Polling (ON)' is used then periodic
     calls are generated to the routine `Ada.Exceptions.Poll'.  This
     routine is a separate unit in the runtime library, and can be
     found in file `a-excpol.adb'.

     Pragma `Polling' can appear as a configuration pragma (for example
     it can be placed in the `gnat.adc' file) to enable polling
     globally, or it can be used in the statement or declaration
     sequence to control polling more locally.

     A call to the polling routine is generated at the start of every
     loop and at the start of every subprogram call.  This guarantees
     that the `Poll' routine is called frequently, and places an upper
     bound (determined by the complexity of the code) on the period
     between two `Poll' calls.

     The primary purpose of the polling interface is to enable
     asynchronous aborts on targets that cannot otherwise support it
     (for example Windows NT), but it may be used for any other purpose
     requiring periodic polling.  The standard version is null, and can
     be replaced by a user program.  This will require re-compilation
     of the `Ada.Exceptions' package that can be found in files
     `a-except.ads' and `a-except.adb'.

     A standard alternative unit (in file `4wexcpol.adb' in the
     standard GNAT distribution) is used to enable the asynchronous
     abort capability on targets that do not normally support the
     capability.  The version of `Poll' in this file makes a call to
     the appropriate runtime routine to test for an abort condition.

     Note that polling can also be enabled by use of the `-gnatP'
     switch.  See the `GNAT User's Guide' for details.

`pragma Propagate_Exceptions'
     Syntax:

          pragma Propagate_Exceptions (subprogram_LOCAL_NAME);

     This pragma indicates that the given entity, which is the name of
     an imported foreign-language subprogram may receive an Ada
     exception, and that the exception should be propagated.  It is
     relevant only if zero cost exception handling is in use, and is
     thus never needed if the alternative `longjmp' / `setjmp'
     implementation of exceptions is used (although it is harmless to
     use it in such cases).

     The implementation of fast exceptions always properly propagates
     exceptions through Ada code, as described in the Ada Reference
     Manual.  However, this manual is silent about the propagation of
     exceptions through foreign code.  For example, consider the
     situation where `P1' calls `P2', and `P2' calls `P3', where `P1'
     and `P3' are in Ada, but `P2' is in C.  `P3' raises an Ada
     exception.  The question is whether or not it will be propagated
     through `P2' and can be handled in `P1'.

     For the `longjmp' / `setjmp' implementation of exceptions, the
     answer is always yes.  For some targets on which zero cost
     exception handling is implemented, the answer is also always yes.
     However, there are some targets, notably in the current version
     all x86 architecture targets, in which the answer is that such
     propagation does not happen automatically.  If such propagation is
     required on these targets, it is mandatory to use
     `Propagate_Exceptions' to name all foreign language routines
     through which Ada exceptions may be propagated.

`pragma Psect_Object'
     Syntax:

          pragma Psect_Object
               [Internal =>] LOCAL_NAME,
            [, [External =>] EXTERNAL_SYMBOL]
            [, [Size     =>] EXTERNAL_SYMBOL]
          
          EXTERNAL_SYMBOL ::=
            IDENTIFIER
          | static_string_EXPRESSION

     This pragma is identical in effect to pragma `Common_Object'.

`pragma Pure_Function'
     Syntax:

          pragma Pure_Function ([Entity =>] function_LOCAL_NAME);

     This pragma appears in the same declarative part as a function
     declaration (or a set of function declarations if more than one
     overloaded declaration exists, in which case the pragma applies to
     all entities).  If specifies that the function `Entity' is to be
     considered pure for the purposes of code generation.  This means
     that the compiler can assume that there are no side effects, and
     in particular that two calls with identical arguments produce the
     same result.  It also means that the function can be used in an
     address clause.

     Note that, quite deliberately, there are no static checks to try
     to ensure that this promise is met, so `Pure_Function' can be used
     with functions that are conceptually pure, even if they do modify
     global variables.  For example, a square root function that is
     instrumented to count the number of times it is called is still
     conceptually pure, and can still be optimized, even though it
     modifies a global variable (the count).  Memo functions are another
     example (where a table of previous calls is kept and consulted to
     avoid re-computation).

     Note: Most functions in a `Pure' package are automatically pure,
     and there is no need to use pragma `Pure_Function' for such
     functions.  An exception is any function that has at least one
     formal of type `System.Address' or a type derived from it.  Such
     functions are not considered pure by default, since the compiler
     assumes that the `Address' parameter may be functioning as a
     pointer and that the referenced data may change even if the
     address value does not.  The use of pragma `Pure_Function' for
     such a function will override this default assumption, and cause
     the compiler to treat such a function as pure.

     Note: If pragma `Pure_Function' is applied to a renamed function,
     it applies to the underlying renamed function.  This can be used to
     disambiguate cases of overloading where some but not all functions
     in a set of overloaded functions are to be designated as pure.

`pragma Ravenscar'
     Syntax:

          pragma Ravenscar

     A configuration pragma that establishes the following set of
     restrictions:

    `No_Abort_Statements'
          [RM D.7] There are no abort_statements, and there are no
          calls to Task_Identification.Abort_Task.

    `No_Select_Statements'
          There are no select_statements.

    `No_Task_Hierarchy'
          [RM D.7] All (non-environment) tasks depend directly on the
          environment task of the partition.

    `No_Task_Allocators'
          [RM D.7] There are no allocators for task types or types
          containing task subcomponents.

    `No_Dynamic_Priorities'
          [RM D.7] There are no semantic dependencies on the package
          Dynamic_Priorities.

    `No_Terminate_Alternatives'
          [RM D.7] There are no selective_accepts with
          terminate_alternatives

    `No_Dynamic_Interrupts'
          There are no semantic dependencies on Ada.Interrupts.

    `No_Protected_Type_Allocators'
          There are no allocators for protected types or types
          containing protected subcomponents.

    `No_Local_Protected_Objects'
          Protected objects and access types that designate such
          objects shall be declared only at library level.

    `No_Requeue'
          Requeue statements are not allowed.

    `No_Calendar'
          There are no semantic dependencies on the package
          Ada.Calendar.

    `No_Relative_Delay'
          There are no delay_relative_statements.

    `No_Task_Attributes'
          There are no semantic dependencies on the Ada.Task_Attributes
          package and there are no references to the attributes
          Callable and Terminated [RM 9.9].

    `Static_Storage_Size'
          The expression for pragma Storage_Size is static.

    `Boolean_Entry_Barriers'
          Entry barrier condition expressions shall be boolean objects
          which are declared in the protected type which contains the
          entry.

    `Max_Asynchronous_Select_Nesting = 0'
          [RM D.7] Specifies the maximum dynamic nesting level of
          asynchronous_selects.  A value of zero prevents the use of
          any asynchronous_select.

    `Max_Task_Entries = 0'
          [RM D.7] Specifies the maximum number of entries per task.
          The bounds of every entry family of a task unit shall be
          static, or shall be defined by a discriminant of a subtype
          whose corresponding bound is static.  A value of zero
          indicates that no rendezvous are possible.  For the Ravenscar
          pragma, the value of Max_Task_Entries is always 0 (zero).

    `Max_Protected_Entries = 1'
          [RM D.7] Specifies the maximum number of entries per
          protected type.  The bounds of every entry family of a
          protected unit shall be static, or shall be defined by a
          discriminant of a subtype whose corresponding bound is
          static.  For the Ravenscar pragma the value of
          Max_Protected_Entries is always 1.

    `Max_Select_Alternatives = 0'
          [RM D.7] Specifies the maximum number of alternatives in a
          selective_accept.  For the Ravenscar pragma the value if
          always 0.

    `No_Task_Termination'
          Tasks which terminate are erroneous.

    `No_Entry_Queue'
          No task can be queued on a protected entry.  Note that this
          restrictions is checked at run time.  The violation of this
          restriction generates a Program_Error exception.

     This set of restrictions corresponds to the definition of the
     "Ravenscar Profile" for limited tasking, devised and published by
     the `International Real-Time Ada Workshop', 1997.

     The above set is a superset of the restrictions provided by pragma
     `Restricted_Run_Time', it includes six additional restrictions
     (`Boolean_Entry_Barriers', `No_Select_Statements', `No_Calendar',
     `Static_Storage_Size', `No_Relative_Delay' and
     `No_Task_Termination').  This means that pragma `Ravenscar', like
     the pragma `Restricted_Run_Time', automatically causes the use of
     a simplified, more efficient version of the tasking run-time
     system.

`pragma Restricted_Run_Time'
     Syntax:

          pragma Restricted_Run_Time

     A configuration pragma that establishes the following set of
     restrictions:

        * No_Abort_Statements

        * No_Asynchronous_Control

        * No_Entry_Queue

        * No_Task_Hierarchy

        * No_Task_Allocators

        * No_Dynamic_Priorities

        * No_Terminate_Alternatives

        * No_Dynamic_Interrupts

        * No_Protected_Type_Allocators

        * No_Local_Protected_Objects

        * No_Requeue

        * No_Task_Attributes

        * Max_Asynchronous_Select_Nesting =  0

        * Max_Task_Entries =  0

        * Max_Protected_Entries = 1

        * Max_Select_Alternatives = 0

     This set of restrictions causes the automatic selection of a
     simplified version of the run time that provides improved
     performance for the limited set of tasking functionality permitted
     by this set of restrictions.

`pragma Share_Generic'
     Syntax:

          pragma Share_Generic (NAME {, NAME});

     This pragma is recognized for compatibility with other Ada
     compilers but is ignored by GNAT.  GNAT does not provide the
     capability for sharing of generic code.  All generic
     instantiations result in making an inlined copy of the template
     with appropriate substitutions.

`pragma Source_File_Name'
     Syntax:

          pragma Source_File_Name (
            [Unit_Name   =>] unit_NAME,
            Spec_File_Name =>  STRING_LITERAL);
          
          pragma Source_File_Name (
            [Unit_Name   =>] unit_NAME,
            Body_File_Name =>  STRING_LITERAL);

     Use this to override the normal naming convention.  It is a
     configuration pragma, and so has the usual applicability of
     configuration pragmas (i.e. it applies to either an entire
     partition, or to all units in a compilation, or to a single unit,
     depending on how it is used.  UNIT_NAME is mapped to
     FILE_NAME_LITERAL.  The identifier for the second argument is
     required, and indicates whether this is the file name for the spec
     or for the body.

     Another form of the `Source_File_Name' pragma allows the
     specification of patterns defining alternative file naming schemes
     to apply to all files.

          pragma Source_File_Name
            (Spec_File_Name => STRING_LITERAL
             [,Casing => CASING_SPEC]
             [,Dot_Replacement => STRING_LITERAL]);
          
          pragma Source_File_Name
            (Body_File_Name => STRING_LITERAL
             [,Casing => CASING_SPEC]
             [,Dot_Replacement => STRING_LITERAL]);
          
          pragma Source_File_Name
            (Subunit_File_Name => STRING_LITERAL
             [,Casing => CASING_SPEC]
             [,Dot_Replacement => STRING_LITERAL]);
          
          CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

     The first argument is a pattern that contains a single asterisk
     indicating the point at which the unit name is to be inserted in
     the pattern string to form the file name.  The second argument is
     optional.  If present it specifies the casing of the unit name in
     the resulting file name string.  The default is lower case.
     Finally the third argument allows for systematic replacement of
     any dots in the unit name by the specified string literal.

     For more details on the use of the `Source_File_Name' pragma, see
     the sections "Using Other File Names" and "Alternative File Naming
     Schemes" in the `GNAT User's Guide'.

`pragma Source_Reference'
     Syntax:

          pragma Source_Reference (INTEGER_LITERAL,
                                   STRING_LITERAL);

     This pragma must appear as the first line of a source file.
     INTEGER_LITERAL is the logical line number of the line following
     the pragma line (for use in error messages and debugging
     information).  STRING_LITERAL is a static string constant that
     specifies the file name to be used in error messages and debugging
     information.  This is most notably used for the output of
     `gnatchop' with the `-r' switch, to make sure that the original
     unchopped source file is the one referred to.

     The second argument must be a string literal, it cannot be a static
     string expression other than a string literal.  This is because
     its value is needed for error messages issued by all phases of the
     compiler.

`pragma Stream_Convert'
     Syntax:

          pragma Stream_Convert (
            [Entity =>] type_LOCAL_NAME,
            [Read   =>] function_NAME,
            [Write  =>] function NAME);

     This pragma provides an efficient way of providing stream
     functions for types defined in packages.  Not only is it simpler
     to use than declaring the necessary functions with attribute
     representation clauses, but more significantly, it allows the
     declaration to made in such a way that the stream packages are not
     loaded unless they are needed.  The use of the Stream_Convert
     pragma adds no overhead at all, unless the stream attributes are
     actually used on the designated type.

     The first argument specifies the type for which stream functions
     are provided.  The second parameter provides a function used to
     read values of this type.  It must name a function whose argument
     type may be any subtype, and whose returned type must be the type
     given as the first argument to the pragma.

     The meaning of the READ parameter is that if a stream attribute
     directly or indirectly specifies reading of the type given as the
     first parameter, then a value of the type given as the argument to
     the Read function is read from the stream, and then the Read
     function is used to convert this to the required target type.

     Similarly the WRITE parameter specifies how to treat write
     attributes that directly or indirectly apply to the type given as
     the first parameter.  It must have an input parameter of the type
     specified by the first parameter, and the return type must be the
     same as the input type of the Read function.  The effect is to
     first call the Write function to convert to the given stream type,
     and then write the result type to the stream.

     The Read and Write functions must not be overloaded subprograms.
     If necessary renamings can be supplied to meet this requirement.
     The usage of this attribute is best illustrated by a simple
     example, taken from the GNAT implementation of package
     Ada.Strings.Unbounded:

          function To_Unbounded (S : String)
                     return Unbounded_String
            renames To_Unbounded_String;
          
          pragma Stream_Convert
            (Unbounded_String, To_Unbounded, To_String);

     The specifications of the referenced functions, as given in the
     Ada 95 Reference Manual are:

          function To_Unbounded_String (Source : String)
            return Unbounded_String;
          
          function To_String (Source : Unbounded_String)
            return String;

     The effect is that if the value of an unbounded string is written
     to a stream, then the representation of the item in the stream is
     in the same format used for `Standard.String', and this same
     representation is expected when a value of this type is read from
     the stream.

`pragma Style_Checks'
     Syntax:

          pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                               On | Off [, LOCAL_NAME]);

     This pragma is used in conjunction with compiler switches to
     control the built in style checking provided by GNAT.  The
     compiler switches, if set provide an initial setting for the
     switches, and this pragma may be used to modify these settings, or
     the settings may be provided entirely by the use of the pragma.
     This pragma can be used anywhere that a pragma is legal, including
     use as a configuration pragma (including use in the `gnat.adc'
     file).

     The form with a string literal specifies which style options are
     to be activated.  These are additive, so they apply in addition to
     any previously set style check options.  The codes for the options
     are the same as those used in the `-gnaty' switch to `gcc' or
     `gnatmake'.  For example the following two methods can be used to
     enable layout checking:

          pragma Style_Checks ("l");
          gcc -c -gnatyl ...

     The form ALL_CHECKS activates all standard checks (its use is
     equivalent to the use of the `gnaty' switch with no options.  See
     GNAT User's Guide for details.

     The forms with `Off' and `On' can be used to temporarily disable
     style checks as shown in the following example:

          pragma Style_Checks ("k"); -- requires keywords in lower case
          pragma Style_Checks (Off); -- turn off style checks
          NULL;                      -- this will not generate an error message
          pragma Style_Checks (On);  -- turn style checks back on
          NULL;                      -- this will generate an error message

     Finally the two argument form is allowed only if the first
     argument is `On' or `Off'.  The effect is to turn of semantic
     style checks for the specified entity, as shown in the following
     example:

          pragma Style_Checks ("r"); -- require consistency of identifier casing
          Arg : Integer;
          Rf1 : Integer := ARG;      -- incorrect, wrong case
          pragma Style_Checks (Off, Arg);
          Rf2 : Integer := ARG;      -- OK, no error

`pragma Subtitle'
     Syntax:

          pragma Subtitle ([Subtitle =>] STRING_LITERAL);

     This pragma is recognized for compatibility with other Ada
     compilers but is ignored by GNAT.

`pragma Suppress_All'
     Syntax:

          pragma Suppress_All;

     This pragma can only appear immediately following a compilation
     unit.  The effect is to apply `Suppress (All_Checks)' to the unit
     which it follows.  This pragma is implemented for compatibility
     with DEC Ada 83 usage.  The use of pragma `Suppress (All_Checks)'
     as a normal configuration pragma is the preferred usage in GNAT.

`pragma Suppress_Initialization'
     Syntax:

          pragma Suppress_Initialization ([Entity =>] type_Name);

     This pragma suppresses any implicit or explicit initialization
     associated with the given type name for all variables of this type.

`pragma Task_Info'
     Syntax

          pragma Task_Info (EXPRESSION);

     This pragma appears within a task definition (like pragma
     `Priority') and applies to the task in which it appears.  The
     argument must be of type `System.Task_Info.Task_Info_Type'.  The
     `Task_Info' pragma provides system dependent control over aspect
     of tasking implementation, for example, the ability to map tasks
     to specific processors.  For details on the facilities available
     for the version of GNAT that you are using, see the documentation
     in the specification of package System.Task_Info in the runtime
     library.

`pragma Task_Name'
     Syntax

          pragma Task_Name (string_EXPRESSION);

     This pragma appears within a task definition (like pragma
     `Priority') and applies to the task in which it appears.  The
     argument must be of type String, and provides a name to be used for
     the task instance when the task is created.  Note that this
     expression is not required to be static, and in particular, it can
     contain references to task discriminants.  This facility can be
     used to provide different names for different tasks as they are
     created, as illustrated in the example below.

     The task name is recorded internally in the run-time structures
     and is accessible to tools like the debugger.  In addition the
     routine `Ada.Task_Identification.Image' will return this string,
     with a unique task address appended.

          --  Example of the use of pragma Task_Name
          
          with Ada.Task_Identification;
          use Ada.Task_Identification;
          with Text_IO; use Text_IO;
          procedure t3 is
          
             type Astring is access String;
          
             task type Task_Typ (Name : access String) is
                pragma Task_Name (Name.all);
             end Task_Typ;
          
             task body Task_Typ is
                Nam : constant String := Image (Current_Task);
             begin
                Put_Line ("-->" & Nam (1 .. 14) & "<--");
             end Task_Typ;
          
             type Ptr_Task is access Task_Typ;
             Task_Var : Ptr_Task;
          
          begin
             Task_Var :=
               new Task_Typ (new String'("This is task 1"));
             Task_Var :=
               new Task_Typ (new String'("This is task 2"));
          end;

`pragma Task_Storage'
     Syntax:

          pragma Task_Storage
            [Task_Type =>] LOCAL_NAME,
            [Top_Guard =>] static_integer_EXPRESSION);

     This pragma specifies the length of the guard area for tasks.  The
     guard area is an additional storage area allocated to a task.  A
     value of zero means that either no guard area is created or a
     minimal guard area is created, depending on the target.  This
     pragma can appear anywhere a `Storage_Size' attribute definition
     clause is allowed for a task type.

`pragma Time_Slice'
     Syntax:

          pragma Time_Slice (static_duration_EXPRESSION);

     For implementations of GNAT on operating systems where it is
     possible to supply a time slice value, this pragma may be used for
     this purpose.  It is ignored if it is used in a system that does
     not allow this control, or if it appears in other than the main
     program unit.  Note that the effect of this pragma is identical to
     the effect of the DEC Ada 83 pragma of the same name when
     operating under OpenVMS systems.

`pragma Title'
     Syntax:

          pragma Title (TITLING_OPTION [, TITLING OPTION]);
          
          TITLING_OPTION ::=
            [Title    =>] STRING_LITERAL,
          | [Subtitle =>] STRING_LITERAL

     Syntax checked but otherwise ignored by GNAT.  This is a listing
     control pragma used in DEC Ada 83 implementations to provide a
     title and/or subtitle for the program listing.  The program
     listing generated by GNAT does not have titles or subtitles.

     Unlike other pragmas, the full flexibility of named notation is
     allowed for this pragma, i.e. the parameters may be given in any
     order if named notation is used, and named and positional notation
     can be mixed following the normal rules for procedure calls in Ada.

`pragma Unchecked_Union'
     Syntax:

          pragma Unchecked_Union (first_subtype_LOCAL_NAME)

     This pragma is used to declare that the specified type should be
     represented in a manner equivalent to a C union type, and is
     intended only for use in interfacing with C code that uses union
     types.  In Ada terms, the named type must obey the following rules:

        * It is a non-tagged non-limited record type.

        * It has a single discrete discriminant with a default value.

        * The component list consists of a single variant part.

        * Each variant has a component list with a single component.

        * No nested variants are allowed.

        * No component has an explicit default value.

        * No component has a non-static constraint.

     In addition, given a type that meets the above requirements, the
     following restrictions apply to its use throughout the program:

        * The discriminant name can be mentioned only in an aggregate.

        * No subtypes may be created of this type.

        * The type may not be constrained by giving a discriminant
          value.

        * The type cannot be passed as the actual for a generic formal
          with a discriminant.

     Equality and inequality operations on `unchecked_unions' are not
     available, since there is no discriminant to compare and the
     compiler does not even know how many bits to compare.  It is
     implementation dependent whether this is detected at compile time
     as an illegality or whether it is undetected and considered to be
     an erroneous construct.  In GNAT, a direct comparison is illegal,
     but GNAT does not attempt to catch the composite case (where two
     composites are compared that contain an unchecked union
     component), so such comparisons are simply considered erroneous.

     The layout of the resulting type corresponds exactly to a C union,
     where each branch of the union corresponds to a single variant in
     the Ada record.  The semantics of the Ada program is not changed
     in any way by the pragma, i.e. provided the above restrictions are
     followed, and no erroneous incorrect references to fields or
     erroneous comparisons occur, the semantics is exactly as described
     by the Ada reference manual.  Pragma `Suppress
     (Discriminant_Check)' applies implicitly to the type and the
     default convention is C

`pragma Unimplemented_Unit'
     Syntax:

          pragma Unimplemented_Unit;

     If this pragma occurs in a unit that is processed by the compiler,
     GNAT aborts with the message `XXX not implemented', where XXX is
     the name of the current compilation unit.  This pragma is intended
     to allow the compiler to handle unimplemented library units in a
     clean manner.

     The abort only happens if code is being generated.  Thus you can
     use specs of unimplemented packages in syntax or semantic checking
     mode.

`pragma Unreferenced'
     Syntax:

          pragma Unreferenced (local_Name {, local_Name});

     This pragma signals that the entities whose names are listed are
     deliberately not referenced. This suppresses warnings about the
     entities being unreferenced, and in addition a warning will be
     generated if one of these entities is in fact referenced.

     This is particularly useful for clearly signalling that a
     particular parameter is not referenced in some particular
     subprogram implementation and that this is deliberate. It can also
     be useful in the case of objects declared only for their
     initialization or finalization side effects.

     If `local_Name' identifies more than one matching homonym in the
     current scope, then the entity most recently declared is the one
     to which the pragma applies.

`pragma Unreserve_All_Interrupts'
     Syntax:

          pragma Unreserve_All_Interrupts;

     Normally certain interrupts are reserved to the implementation.
     Any attempt to attach an interrupt causes Program_Error to be
     raised, as described in RM C.3.2(22).  A typical example is the
     `SIGINT' interrupt used in many systems for an `Ctrl-C' interrupt.
     Normally this interrupt is reserved to the implementation, so
     that `Ctrl-C' can be used to interrupt execution.

     If the pragma `Unreserve_All_Interrupts' appears anywhere in any
     unit in a program, then all such interrupts are unreserved.  This
     allows the program to handle these interrupts, but disables their
     standard functions.  For example, if this pragma is used, then
     pressing `Ctrl-C' will not automatically interrupt execution.
     However, a program can then handle the `SIGINT' interrupt as it
     chooses.

     For a full list of the interrupts handled in a specific
     implementation, see the source code for the specification of
     `Ada.Interrupts.Names' in file `a-intnam.ads'.  This is a target
     dependent file that contains the list of interrupts recognized for
     a given target.  The documentation in this file also specifies
     what interrupts are affected by the use of the
     `Unreserve_All_Interrupts' pragma.

`pragma Unsuppress'
     Syntax:

          pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);

     This pragma undoes the effect of a previous pragma `Suppress'.  If
     there is no corresponding pragma `Suppress' in effect, it has no
     effect.  The range of the effect is the same as for pragma
     `Suppress'.  The meaning of the arguments is identical to that used
     in pragma `Suppress'.

     One important application is to ensure that checks are on in cases
     where code depends on the checks for its correct functioning, so
     that the code will compile correctly even if the compiler switches
     are set to suppress checks.

`pragma Use_VADS_Size'
     Syntax:

          pragma Use_VADS_Size;

     This is a configuration pragma.  In a unit to which it applies,
     any use of the 'Size attribute is automatically interpreted as a
     use of the 'VADS_Size attribute.  Note that this may result in
     incorrect semantic processing of valid Ada 95 programs.  This is
     intended to aid in the handling of legacy code which depends on
     the interpretation of Size as implemented in the VADS compiler.
     See description of the VADS_Size attribute for further details.

`pragma Validity_Checks'
     Syntax:

          pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);

     This pragma is used in conjunction with compiler switches to
     control the built in validity checking provided by GNAT.  The
     compiler switches, if set provide an initial setting for the
     switches, and this pragma may be used to modify these settings, or
     the settings may be provided entirely by the use of the pragma.
     This pragma can be used anywhere that a pragma is legal, including
     use as a configuration pragma (including use in the `gnat.adc'
     file).

     The form with a string literal specifies which validity options
     are to be activated.  The validity checks are first set to include
     only the default reference manual settings, and then a string of
     letters in the string specifies the exact set of options required.
     The form of this string is exactly as described for the `-gnatVx'
     compiler switch (see the GNAT users guide for details).  For
     example the following two methods can be used to enable validity
     checking for mode `in' and `in out' subprogram parameters:

          pragma Validity_Checks ("im");
          gcc -c -gnatVim ...

     The form ALL_CHECKS activates all standard checks (its use is
     equivalent to the use of the `gnatva' switch.

     The forms with `Off' and `On' can be used to temporarily disable
     validity checks as shown in the following example:

          pragma Validity_Checks ("c"); -- validity checks for copies
          pragma Validity_Checks (Off); -- turn off validity checks
          A := B;                       -- B will not be validity checked
          pragma Validity_Checks (On);  -- turn validity checks back on
          A := C;                       -- C will be validity checked

`pragma Volatile'
     Syntax:

          pragma Volatile (local_NAME)

     This pragma is defined by the Ada 95 Reference Manual, and the GNAT
     implementation is fully conformant with this definition.  The
     reason it is mentioned in this section is that a pragma of the
     same name was supplied in some Ada 83 compilers, including DEC Ada
     83.  The Ada 95 implementation of pragma Volatile is upwards
     compatible with the implementation in Dec Ada 83.

`pragma Warnings'
     Syntax:

          pragma Warnings (On | Off [, LOCAL_NAME]);

     Normally warnings are enabled, with the output being controlled by
     the command line switch.  Warnings (`Off') turns off generation of
     warnings until a Warnings (`On') is encountered or the end of the
     current unit.  If generation of warnings is turned off using this
     pragma, then no warning messages are output, regardless of the
     setting of the command line switches.

     The form with a single argument is a configuration pragma.

     If the LOCAL_NAME parameter is present, warnings are suppressed for
     the specified entity.  This suppression is effective from the
     point where it occurs till the end of the extended scope of the
     variable (similar to the scope of `Suppress').

`pragma Weak_External'
     Syntax:

          pragma Weak_External ([Entity =>] LOCAL_NAME);

     This pragma specifies that the given entity should be marked as a
     weak external (one that does not have to be resolved) for the
     linker.  For further details, consult the GCC manual.


File: gnat_rm.info,  Node: Implementation Defined Attributes,  Next: Implementation Advice,  Prev: Implementation Defined Pragmas,  Up: Top

Implementation Defined Attributes
*********************************

   Ada 95 defines (throughout the Ada 95 reference manual, summarized
in annex K), a set of attributes that provide useful additional
functionality in all areas of the language.  These language defined
attributes are implemented in GNAT and work as described in the Ada 95
Reference Manual.

   In addition, Ada 95 allows implementations to define additional
attributes whose meaning is defined by the implementation.  GNAT
provides a number of these implementation-dependent attributes which
can be used to extend and enhance the functionality of the compiler.
This section of the GNAT reference manual describes these additional
attributes.

   Note that any program using these attributes may not be portable to
other compilers (although GNAT implements this set of attributes on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these attributes.

`Abort_Signal'
     `Standard'Abort_Signal' (`Standard' is the only allowed prefix)
     provides the entity for the special exception used to signal task
     abort or asynchronous transfer of control.  Normally this attribute
     should only be used in the tasking runtime (it is highly peculiar,
     and completely outside the normal semantics of Ada, for a user
     program to intercept the abort exception).

`Address_Size'
     `Standard'Address_Size' (`Standard' is the only allowed prefix) is
     a static constant giving the number of bits in an `Address'.  It
     is used primarily for constructing the definition of `Memory_Size'
     in package `Standard', but may be freely used in user programs and
     has the advantage of being static, while a direct reference to
     System.Address'Size is non-static because Address is a private
     type.

`Asm_Input'
     The `Asm_Input' attribute denotes a function that takes two
     parameters.  The first is a string, the second is an expression of
     the type designated by the prefix.  The first (string) argument is
     required to be a static expression, and is the constraint for the
     parameter, (e.g. what kind of register is required).  The second
     argument is the value to be used as the input argument.  The
     possible values for the constant are the same as those used in the
     RTL, and are dependent on the configuration file used to built the
     GCC back end.  *Note Machine Code Insertions::

`Asm_Output'
     The `Asm_Output' attribute denotes a function that takes two
     parameters.  The first is a string, the second is the name of a
     variable of the type designated by the attribute prefix.  The
     first (string) argument is required to be a static expression and
     designates the constraint for the parameter (e.g. what kind of
     register is required).  The second argument is the variable to be
     updated with the result.  The possible values for constraint are
     the same as those used in the RTL, and are dependent on the
     configuration file used to build the GCC back end.  If there are
     no output operands, then this argument may either be omitted, or
     explicitly given as `No_Output_Operands'.  *Note Machine Code
     Insertions::

`AST_Entry'
     This attribute is implemented only in OpenVMS versions of GNAT.
     Applied to the name of an entry, it yields a value of the
     predefined type AST_Handler (declared in the predefined package
     System, as extended by the use of pragma `Extend_System
     (Aux_DEC)').  This value enables the given entry to be called when
     an AST occurs.  For further details, refer to the `DEC Ada
     Language Reference Manual', section 9.12a.

`Bit'
     `OBJ'Bit', where OBJ is any object, yields the bit offset within
     the storage unit (byte) that contains the first bit of storage
     allocated for the object.  The value of this attribute is of the
     type `Universal_Integer', and is always a non-negative number not
     exceeding the value of `System.Storage_Unit'.

     For an object that is a variable or a constant allocated in a
     register, the value is zero.  (The use of this attribute does not
     force the allocation of a variable to memory).

     For an object that is a formal parameter, this attribute applies
     to either the matching actual parameter or to a copy of the
     matching actual parameter.

     For an access object the value is zero.  Note that `OBJ.all'Bit'
     is subject to an `Access_Check' for the designated object.
     Similarly for a record component `X.C'Bit' is subject to a
     discriminant check and `X(I).Bit' and `X(I1..I2)'Bit' are subject
     to index checks.

     This attribute is designed to be compatible with the DEC Ada 83
     definition and implementation of the `Bit' attribute.

`Bit_Position'
     `R.C'Bit', where R is a record object and C is one of the fields
     of the record type, yields the bit offset within the record
     contains the first bit of storage allocated for the object.  The
     value of this attribute is of the type `Universal_Integer'.  The
     value depends only on the field C and is independent of the
     alignment of the containing record R.

`Code_Address'
     The `'Address' attribute may be applied to subprograms in Ada 95,
     but the intended effect from the Ada 95 reference manual seems to
     be to provide an address value which can be used to call the
     subprogram by means of an address clause as in the following
     example:

          procedure K is ...
          
          procedure L;
          for L'Address use K'Address;
          pragma Import (Ada, L);

     A call to `L' is then expected to result in a call to `K'.  In Ada
     83, where there were no access-to-subprogram values, this was a
     common work around for getting the effect of an indirect call.
     GNAT implements the above use of `Address' and the technique
     illustrated by the example code works correctly.

     However, for some purposes, it is useful to have the address of
     the start of the generated code for the subprogram.  On some
     architectures, this is not necessarily the same as the `Address'
     value described above.  For example, the `Address' value may
     reference a subprogram descriptor rather than the subprogram
     itself.

     The `'Code_Address' attribute, which can only be applied to
     subprogram entities, always returns the address of the start of the
     generated code of the specified subprogram, which may or may not be
     the same value as is returned by the corresponding `'Address'
     attribute.

`Default_Bit_Order'
     `Standard'Default_Bit_Order' (`Standard' is the only permissible
     prefix), provides the value `System.Default_Bit_Order' as a `Pos'
     value (0 for `High_Order_First', 1 for `Low_Order_First').  This
     is used to construct the definition of `Default_Bit_Order' in
     package `System'.

`Elaborated'
     The prefix of the `'Elaborated' attribute must be a unit name.  The
     value is a Boolean which indicates whether or not the given unit
     has been elaborated.  This attribute is primarily intended for
     internal use by the generated code for dynamic elaboration
     checking, but it can also be used in user programs.  The value
     will always be True once elaboration of all units has been
     completed.

`Elab_Body'
     This attribute can only be applied to a program unit name.  It
     returns the entity for the corresponding elaboration procedure for
     elaborating the body of the referenced unit.  This is used in the
     main generated elaboration procedure by the binder and is not
     normally used in any other context.  However, there may be
     specialized situations in which it is useful to be able to call
     this elaboration procedure from Ada code, e.g. if it is necessary
     to do selective re-elaboration to fix some error.

`Elab_Spec'
     This attribute can only be applied to a program unit name.  It
     returns the entity for the corresponding elaboration procedure for
     elaborating the specification of the referenced unit.  This is
     used in the main generated elaboration procedure by the binder and
     is not normally used in any other context.  However, there may be
     specialized situations in which it is useful to be able to call
     this elaboration procedure from Ada code, e.g. if it is necessary
     to do selective re-elaboration to fix some error.

`Emax'
     The `Emax' attribute is provided for compatibility with Ada 83.
     See the Ada 83 reference manual for an exact description of the
     semantics of this attribute.

`Enum_Rep'
     For every enumeration subtype S, `S'Enum_Rep' denotes a function
     with the following specification:

          function S'Enum_Rep (Arg : S'Base)
            return Universal_Integer;

     It is also allowable to apply `Enum_Rep' directly to an object of
     an enumeration type or to a non-overloaded enumeration literal.
     In this case `S'Enum_Rep' is equivalent to `TYP'Enum_Rep(S)' where
     TYP is the type of the enumeration literal or object.

     The function returns the representation value for the given
     enumeration value.  This will be equal to value of the `Pos'
     attribute in the absence of an enumeration representation clause.
     This is a static attribute (i.e. the result is static if the
     argument is static).

     `S'Enum_Rep' can also be used with integer types and objects, in
     which case it simply returns the integer value.  The reason for
     this is to allow it to be used for `(<>)' discrete formal
     arguments in a generic unit that can be instantiated with either
     enumeration types or integer types.  Note that if `Enum_Rep' is
     used on a modular type whose upper bound exceeds the upper bound
     of the largest signed integer type, and the argument is a
     variable, so that the universal integer calculation is done at
     run-time, then the call to `Enum_Rep' may raise `Constraint_Error'.

`Epsilon'
     The `Epsilon' attribute is provided for compatibility with Ada 83.
     See the Ada 83 reference manual for an exact description of the
     semantics of this attribute.

`Fixed_Value'
     For every fixed-point type S, `S'Fixed_Value' denotes a function
     with the following specification:

          function S'Fixed_Value (Arg : Universal_Integer)
            return S;

     The value returned is the fixed-point value V such that

          V = Arg * S'Small

     The effect is thus equivalent to first converting the argument to
     the integer type used to represent S, and then doing an unchecked
     conversion to the fixed-point type.  This attribute is primarily
     intended for use in implementation of the input-output functions
     for fixed-point values.

`Has_Discriminants'
     The prefix of the `Has_Discriminants' attribute is a type.  The
     result is a Boolean value which is True if the type has
     discriminants, and False otherwise.  The intended use of this
     attribute is in conjunction with generic definitions.  If the
     attribute is applied to a generic private type, it indicates
     whether or not the corresponding actual type has discriminants.

`Img'
     The `Img' attribute differs from `Image' in that it may be applied
     to objects as well as types, in which case it gives the `Image'
     for the subtype of the object.  This is convenient for debugging:

          Put_Line ("X = " & X'Img);

     has the same meaning as the more verbose:

          Put_Line ("X = " & TYPE'Image (X));

     where TYPE is the subtype of the object X.

`Integer_Value'
     For every integer type S, `S'Integer_Value' denotes a function
     with the following specification:

          function S'Integer_Value (Arg : Universal_Fixed)
            return S;

     The value returned is the integer value V, such that

          Arg = V * TYPE'Small

     The effect is thus equivalent to first doing an unchecked convert
     from the fixed-point type to its corresponding implementation
     type, and then converting the result to the target integer type.
     This attribute is primarily intended for use in implementation of
     the standard input-output functions for fixed-point values.

`Large'
     The `Large' attribute is provided for compatibility with Ada 83.
     See the Ada 83 reference manual for an exact description of the
     semantics of this attribute.

`Machine_Size'
     This attribute is identical to the `Object_Size' attribute.  It is
     provided for compatibility with the DEC Ada 83 attribute of this
     name.

`Mantissa'
     The `Mantissa' attribute is provided for compatibility with Ada
     83.  See the Ada 83 reference manual for an exact description of
     the semantics of this attribute.

`Max_Interrupt_Priority'
     `Standard'Max_Interrupt_Priority' (`Standard' is the only
     permissible prefix), provides the value
     `System.Max_Interrupt_Priority' and is intended primarily for
     constructing this definition in package `System'.

`Max_Priority'
     `Standard'Max_Priority' (`Standard' is the only permissible
     prefix) provides the value `System.Max_Priority' and is intended
     primarily for constructing this definition in package `System'.

`Maximum_Alignment'
     `Standard'Maximum_Alignment' (`Standard' is the only permissible
     prefix) provides the maximum useful alignment value for the
     target.  This is a static value that can be used to specify the
     alignment for an object, guaranteeing that it is properly aligned
     in all cases.  This is useful when an external object is imported
     and its alignment requirements are unknown.

`Mechanism_Code'
     `FUNCTION'Mechanism_Code' yields an integer code for the mechanism
     used for the result of function, and `SUBPROGRAM'Mechanism_Code
     (N)' yields the mechanism used for formal parameter number N (a
     static integer value with 1 meaning the first parameter) of
     SUBPROGRAM.  The code returned is:

    1
          by copy (value)

    2
          by reference

    3
          by descriptor (default descriptor class)

    4
          by descriptor (UBS: unaligned bit string)

    5
          by descriptor (UBSB: aligned bit string with arbitrary bounds)

    6
          by descriptor (UBA: unaligned bit array)

    7
          by descriptor (S: string, also scalar access type parameter)

    8
          by descriptor (SB: string with arbitrary bounds)

    9
          by descriptor (A: contiguous array)

    10
          by descriptor (NCA: non-contiguous array)

     Values from 3 through 10 are only relevant to Digital OpenVMS
     implementations.

`Null_Parameter'
     A reference `T'Null_Parameter' denotes an imaginary object of type
     or subtype T allocated at machine address zero.  The attribute is
     allowed only as the default expression of a formal parameter, or as
     an actual expression of a subprogram call.  In either case, the
     subprogram must be imported.

     The identity of the object is represented by the address zero in
     the argument list, independent of the passing mechanism (explicit
     or default).

     This capability is needed to specify that a zero address should be
     passed for a record or other composite object passed by reference.
     There is no way of indicating this without the `Null_Parameter'
     attribute.

`Object_Size'
     The size of an object is not necessarily the same as the size of
     the type of an object.  This is because by default object sizes
     are increased to be a multiple of the alignment of the object.
     For example, `Natural'Size' is 31, but by default objects of type
     `Natural' will have a size of 32 bits.  Similarly, a record
     containing an integer and a character:

          type Rec is record
             I : Integer;
             C : Character;
          end record;

     will have a size of 40 (that is `Rec'Size' will be 40.  The
     alignment will be 4, because of the integer field, and so the
     default size of record objects for this type will be 64 (8 bytes).

     The `TYPE'Object_Size' attribute has been added to GNAT to allow
     the default object size of a type to be easily determined.  For
     example, `Natural'Object_Size' is 32, and `Rec'Object_Size' (for
     the record type in the above example) will be 64.  Note also that,
     unlike the situation with the `Size' attribute as defined in the
     Ada RM, the `Object_Size' attribute can be specified individually
     for different subtypes.  For example:

          type R is new Integer;
          subtype R1 is R range 1 .. 10;
          subtype R2 is R range 1 .. 10;
          for R2'Object_Size use 8;

     In this example, `R'Object_Size' and `R1'Object_Size' are both 32
     since the default object size for a subtype is the same as the
     object size for the parent subtype.  This means that objects of
     type `R' or `R1' will by default be 32 bits (four bytes).  But
     objects of type `R2' will be only 8 bits (one byte), since
     `R2'Object_Size' has been set to 8.

`Passed_By_Reference'
     `TYPE'Passed_By_Reference' for any subtype TYPE returns a value of
     type `Boolean' value that is `True' if the type is normally passed
     by reference and `False' if the type is normally passed by copy in
     calls.  For scalar types, the result is always `False' and is
     static.  For non-scalar types, the result is non-static.

`Range_Length'
     `TYPE'Range_Length' for any discrete type TYPE yields the number
     of values represented by the subtype (zero for a null range).  The
     result is static for static subtypes.  `Range_Length' applied to
     the index subtype of a one dimensional array always gives the same
     result as `Range' applied to the array itself.

`Safe_Emax'
     The `Safe_Emax' attribute is provided for compatibility with Ada
     83.  See the Ada 83 reference manual for an exact description of
     the semantics of this attribute.

`Safe_Large'
     The `Safe_Large' attribute is provided for compatibility with Ada
     83.  See the Ada 83 reference manual for an exact description of
     the semantics of this attribute.

`Safe_Large'
     The `Safe_Large' attribute is provided for compatibility with Ada
     83.  See the Ada 83 reference manual for an exact description of
     the semantics of this attribute.

`Small'
     The `Small' attribute is defined in Ada 95 only for fixed-point
     types.  GNAT also allows this attribute to be applied to
     floating-point types for compatibility with Ada 83.  See the Ada
     83 reference manual for an exact description of the semantics of
     this attribute when applied to floating-point types.

`Storage_Unit'
     `Standard'Storage_Unit' (`Standard' is the only permissible
     prefix) provides the value `System.Storage_Unit' and is intended
     primarily for constructing this definition in package `System'.

`Tick'
     `Standard'Tick' (`Standard' is the only permissible prefix)
     provides the value of `System.Tick' and is intended primarily for
     constructing this definition in package `System'.

`To_Address'
     The `System'To_Address' (`System' is the only permissible prefix)
     denotes a function identical to
     `System.Storage_Elements.To_Address' except that it is a static
     attribute.  This means that if its argument is a static
     expression, then the result of the attribute is a static
     expression.  The result is that such an expression can be used in
     contexts (e.g. preelaborable packages) which require a static
     expression and where the function call could not be used (since
     the function call is always non-static, even if its argument is
     static).

`Type_Class'
     `TYPE'Type_Class' for any type or subtype TYPE yields the value of
     the type class for the full type of TYPE.  If TYPE is a generic
     formal type, the value is the value for the corresponding actual
     subtype.  The value of this attribute is of type
     `System.Aux_DEC.Type_Class', which has the following definition:

            type Type_Class is
              (Type_Class_Enumeration,
               Type_Class_Integer,
               Type_Class_Fixed_Point,
               Type_Class_Floating_Point,
               Type_Class_Array,
               Type_Class_Record,
               Type_Class_Access,
               Type_Class_Task,
               Type_Class_Address);

     Protected types yield the value `Type_Class_Task', which thus
     applies to all concurrent types.  This attribute is designed to be
     compatible with the DEC Ada 83 attribute of the same name.

`UET_Address'
     The `UET_Address' attribute can only be used for a prefix which
     denotes a library package.  It yields the address of the unit
     exception table when zero cost exception handling is used.  This
     attribute is intended only for use within the GNAT implementation.
     See the unit `Ada.Exceptions' in files `a-except.ads' and
     `a-except.adb' for details on how this attribute is used in the
     implementation.

`Universal_Literal_String'
     The prefix of `Universal_Literal_String' must be a named number.
     The static result is the string consisting of the characters of
     the number as defined in the original source.  This allows the user
     program to access the actual text of named numbers without
     intermediate conversions and without the need to enclose the
     strings in quotes (which would preclude their use as numbers).
     This is used internally for the construction of values of the
     floating-point attributes from the file `ttypef.ads', but may also
     be used by user programs.

`Unrestricted_Access'
     The `Unrestricted_Access' attribute is similar to `Access' except
     that all accessibility and aliased view checks are omitted.  This
     is a user-beware attribute.  It is similar to `Address', for which
     it is a desirable replacement where the value desired is an access
     type.  In other words, its effect is identical to first applying
     the `Address' attribute and then doing an unchecked conversion to
     a desired access type.  In GNAT, but not necessarily in other
     implementations, the use of static chains for inner level
     subprograms means that `Unrestricted_Access' applied to a
     subprogram yields a value that can be called as long as the
     subprogram is in scope (normal Ada 95 accessibility rules restrict
     this usage).

`VADS_Size'
     The `'VADS_Size' attribute is intended to make it easier to port
     legacy code which relies on the semantics of `'Size' as implemented
     by the VADS Ada 83 compiler.  GNAT makes a best effort at
     duplicating the same semantic interpretation.  In particular,
     `'VADS_Size' applied to a predefined or other primitive type with
     no Size clause yields the Object_Size (for example, `Natural'Size'
     is 32 rather than 31 on typical machines).  In addition
     `'VADS_Size' applied to an object gives the result that would be
     obtained by applying the attribute to the corresponding type.

`Value_Size'
     `TYPE'Value_Size' is the number of bits required to represent a
     value of the given subtype.  It is the same as `TYPE'Size', but,
     unlike `Size', may be set for non-first subtypes.

`Wchar_T_Size'
     `Standard'Wchar_T_Size' (`Standard' is the only permissible
     prefix) provides the size in bits of the C `wchar_t' type
     primarily for constructing the definition of this type in package
     `Interfaces.C'.

`Word_Size'
     `Standard'Word_Size' (`Standard' is the only permissible prefix)
     provides the value `System.Word_Size' and is intended primarily
     for constructing this definition in package `System'.


File: gnat_rm.info,  Node: Implementation Advice,  Next: Implementation Defined Characteristics,  Prev: Implementation Defined Attributes,  Up: Top

Implementation Advice
*********************

   The main text of the Ada 95 Reference Manual describes the required
behavior of all Ada 95 compilers, and the GNAT compiler conforms to
these requirements.

   In addition, there are sections throughout the Ada 95 reference
manual headed by the phrase "implementation advice".  These sections
are not normative, i.e. they do not specify requirements that all
compilers must follow.  Rather they provide advice on generally
desirable behavior.  You may wonder why they are not requirements.  The
most typical answer is that they describe behavior that seems generally
desirable, but cannot be provided on all systems, or which may be
undesirable on some systems.

   As far as practical, GNAT follows the implementation advice sections
in the Ada 95 Reference Manual.  This chapter contains a table giving
the reference manual section number, paragraph number and several
keywords for each advice.  Each entry consists of the text of the
advice followed by the GNAT interpretation of this advice.  Most often,
this simply says "followed", which means that GNAT follows the advice.
However, in a number of cases, GNAT deliberately deviates from this
advice, in which case the text describes what GNAT does and why.

*1.1.3(20): Error Detection*

     If an implementation detects the use of an unsupported Specialized
     Needs Annex feature at run time, it should raise `Program_Error' if
     feasible.
     Not relevant.  All specialized needs annex features are either
     supported, or diagnosed at compile time.

*1.1.3(31): Child Units*

     If an implementation wishes to provide implementation-defined
     extensions to the functionality of a language-defined library
     unit, it should normally do so by adding children to the library
     unit.
     Followed.

*1.1.5(12): Bounded Errors*

     If an implementation detects a bounded error or erroneous
     execution, it should raise `Program_Error'.
     Followed in all cases in which the implementation detects a bounded
     error or erroneous execution.  Not all such situations are
     detected at runtime.

*2.8(16): Pragmas*

     Normally, implementation-defined pragmas should have no semantic
     effect for error-free programs; that is, if the
     implementation-defined pragmas are removed from a working program,
     the program should still be legal, and should still have the same
     semantics.
     The following implementation defined pragmas are exceptions to this
     rule:

    `Abort_Defer'
          Affects semantics

    `Ada_83'
          Affects legality

    `Assert'
          Affects semantics

    `CPP_Class'
          Affects semantics

    `CPP_Constructor'
          Affects semantics

    `CPP_Virtual'
          Affects semantics

    `CPP_Vtable'
          Affects semantics

    `Debug'
          Affects semantics

    `Interface_Name'
          Affects semantics

    `Machine_Attribute'
          Affects semantics

    `Unimplemented_Unit'
          Affects legality

    `Unchecked_Union'
          Affects semantics

     In each of the above cases, it is essential to the purpose of the
     pragma that this advice not be followed.  For details see the
     separate section on implementation defined pragmas.

*2.8(17-19): Pragmas*

     Normally, an implementation should not define pragmas that can
     make an illegal program legal, except as follows:


     A pragma used to complete a declaration, such as a pragma `Import';


     A pragma used to configure the environment by adding, removing, or
     replacing `library_items'.
     See response to paragraph 16 of this same section.

*3.5.2(5): Alternative Character Sets*

     If an implementation supports a mode with alternative
     interpretations for `Character' and `Wide_Character', the set of
     graphic characters of `Character' should nevertheless remain a
     proper subset of the set of graphic characters of
     `Wide_Character'.  Any character set "localizations" should be
     reflected in the results of the subprograms defined in the
     language-defined package `Characters.Handling' (see A.3) available
     in such a mode.  In a mode with an alternative interpretation of
     `Character', the implementation should also support a
     corresponding change in what is a legal `identifier_letter'.
     Not all wide character modes follow this advice, in particular the
     JIS and IEC modes reflect standard usage in Japan, and in these
     encoding, the upper half of the Latin-1 set is not part of the
     wide-character subset, since the most significant bit is used for
     wide character encoding.  However, this only applies to the
     external forms.  Internally there is no such restriction.

*3.5.4(28): Integer Types*

     An implementation should support `Long_Integer' in addition to
     `Integer' if the target machine supports 32-bit (or longer)
     arithmetic.  No other named integer subtypes are recommended for
     package `Standard'.  Instead, appropriate named integer subtypes
     should be provided in the library package `Interfaces' (see B.2).
     `Long_Integer' is supported.  Other standard integer types are
     supported so this advice is not fully followed.  These types are
     supported for convenient interface to C, and so that all hardware
     types of the machine are easily available.

*3.5.4(29): Integer Types*

     An implementation for a two's complement machine should support
     modular types with a binary modulus up to `System.Max_Int*2+2'.  An
     implementation should support a non-binary modules up to
     `Integer'Last'.
     Followed.

*3.5.5(8): Enumeration Values*

     For the evaluation of a call on `S'Pos' for an enumeration
     subtype, if the value of the operand does not correspond to the
     internal code for any enumeration literal of its type (perhaps due
     to an un-initialized variable), then the implementation should
     raise `Program_Error'.  This is particularly important for
     enumeration types with noncontiguous internal codes specified by an
     enumeration_representation_clause.
     Followed.

*3.5.7(17): Float Types*

     An implementation should support `Long_Float' in addition to
     `Float' if the target machine supports 11 or more digits of
     precision.  No other named floating point subtypes are recommended
     for package `Standard'.  Instead, appropriate named floating point
     subtypes should be provided in the library package `Interfaces'
     (see B.2).
     `Short_Float' and `Long_Long_Float' are also provided.  The former
     provides improved compatibility with other implementations
     supporting this type.  The latter corresponds to the highest
     precision floating-point type supported by the hardware.  On most
     machines, this will be the same as `Long_Float', but on some
     machines, it will correspond to the IEEE extended form.  The
     notable case is all ia32 (x86) implementations, where
     `Long_Long_Float' corresponds to the 80-bit extended precision
     format supported in hardware on this processor.  Note that the
     128-bit format on SPARC is not supported, since this is a software
     rather than a hardware format.

*3.6.2(11): Multidimensional Arrays*

     An implementation should normally represent multidimensional
     arrays in row-major order, consistent with the notation used for
     multidimensional array aggregates (see 4.3.3).  However, if a
     pragma `Convention' (`Fortran', ...) applies to a multidimensional
     array type, then column-major order should be used instead (see
     B.5, "Interfacing with Fortran").
     Followed.

*9.6(30-31): Duration'Small*

     Whenever possible in an implementation, the value of
     `Duration'Small' should be no greater than 100 microseconds.
     Followed.  (`Duration'Small' = 10**(-9)).


     The time base for `delay_relative_statements' should be monotonic;
     it need not be the same time base as used for `Calendar.Clock'.
     Followed.

*10.2.1(12): Consistent Representation*

     In an implementation, a type declared in a pre-elaborated package
     should have the same representation in every elaboration of a
     given version of the package, whether the elaborations occur in
     distinct executions of the same program, or in executions of
     distinct programs or partitions that include the given version.
     Followed, except in the case of tagged types.  Tagged types involve
     implicit pointers to a local copy of a dispatch table, and these
     pointers have representations which thus depend on a particular
     elaboration of the package.  It is not easy to see how it would be
     possible to follow this advice without severely impacting
     efficiency of execution.

*11.4.1(19): Exception Information*

     `Exception_Message' by default and `Exception_Information' should
     produce information useful for debugging.  `Exception_Message'
     should be short, about one line.  `Exception_Information' can be
     long.  `Exception_Message' should not include the
     `Exception_Name'.  `Exception_Information' should include both the
     `Exception_Name' and the `Exception_Message'.
     Followed.  For each exception that doesn't have a specified
     `Exception_Message', the compiler generates one containing the
     location of the raise statement.  This location has the form
     "file:line", where file is the short file name (without path
     information) and line is the line number in the file.  Note that
     in the case of the Zero Cost Exception mechanism, these messages
     become redundant with the Exception_Information that contains a
     full backtrace of the calling sequence, so they are disabled.  To
     disable explicitly the generation of the source location message,
     use the Pragma `Discard_Names'.

*11.5(28): Suppression of Checks*

     The implementation should minimize the code executed for checks
     that have been suppressed.
     Followed.

*13.1 (21-24): Representation Clauses*

     The recommended level of support for all representation items is
     qualified as follows:


     An implementation need not support representation items containing
     non-static expressions, except that an implementation should
     support a representation item for a given entity if each
     non-static expression in the representation item is a name that
     statically denotes a constant declared before the entity.
     Followed.  GNAT does not support non-static expressions in
     representation clauses unless they are constants declared before
     the entity.  For example:

          X : typ;
          for X'Address use To_address (16#2000#);

     will be rejected, since the To_Address expression is non-static.
     Instead write:

          X_Address : constant Address : =
          To_Address    ((16#2000#);
          X : typ;
          for X'Address use X_Address;


     An implementation need not support a specification for the `Size'
     for a given composite subtype, nor the size or storage place for an
     object (including a component) of a given composite subtype,
     unless the constraints on the subtype and its composite
     subcomponents (if any) are all static constraints.
     Followed.  Size Clauses are not permitted on non-static
     components, as described above.


     An aliased component, or a component whose type is by-reference,
     should always be allocated at an addressable location.
     Followed.

*13.2(6-8): Packed Types*

     If a type is packed, then the implementation should try to minimize
     storage allocated to objects of the type, possibly at the expense
     of speed of accessing components, subject to reasonable complexity
     in addressing calculations.


     The recommended level of support pragma `Pack' is:

     For a packed record type, the components should be packed as
     tightly as possible subject to the Sizes of the component
     subtypes, and subject to any `record_representation_clause' that
     applies to the type; the implementation may, but need not, reorder
     components or cross aligned word boundaries to improve the
     packing.  A component whose `Size' is greater than the word size
     may be allocated an integral number of words.
     Followed.  Tight packing of arrays is supported for all component
     sizes up to 64-bits.


     An implementation should support Address clauses for imported
     subprograms.
     Followed.

*13.3(14-19): Address Clauses*

     For an array X, `X'Address' should point at the first component of
     the array, and not at the array bounds.
     Followed.


     The recommended level of support for the `Address' attribute is:

     `X'Address' should produce a useful result if X is an object that
     is aliased or of a by-reference type, or is an entity whose
     `Address' has been specified.
     Followed.  A valid address will be produced even if none of those
     conditions have been met.  If necessary, the object is forced into
     memory to ensure the address is valid.


     An implementation should support `Address' clauses for imported
     subprograms.
     Followed.


     Objects (including subcomponents) that are aliased or of a
     by-reference type should be allocated on storage element
     boundaries.
     Followed.


     If the `Address' of an object is specified, or it is imported or
     exported, then the implementation should not perform optimizations
     based on assumptions of no aliases.
     Followed.

*13.3(29-35): Alignment Clauses*

     The recommended level of support for the `Alignment' attribute for
     subtypes is:

     An implementation should support specified Alignments that are
     factors and multiples of the number of storage elements per word,
     subject to the following:
     Followed.


     An implementation need not support specified `Alignment's for
     combinations of `Size's and `Alignment's that cannot be easily
     loaded and stored by available machine instructions.
     Followed.


     An implementation need not support specified `Alignment's that are
     greater than the maximum `Alignment' the implementation ever
     returns by default.
     Followed.


     The recommended level of support for the `Alignment' attribute for
     objects is:

     Same as above, for subtypes, but in addition:
     Followed.


     For stand-alone library-level objects of statically constrained
     subtypes, the implementation should support all `Alignment's
     supported by the target linker.  For example, page alignment is
     likely to be supported for such objects, but not for subtypes.
     Followed.

*13.3(42-43): Size Clauses*

     The recommended level of support for the `Size' attribute of
     objects is:

     A `Size' clause should be supported for an object if the specified
     `Size' is at least as large as its subtype's `Size', and
     corresponds to a size in storage elements that is a multiple of the
     object's `Alignment' (if the `Alignment' is nonzero).
     Followed.

*13.3(50-56): Size Clauses*

     If the `Size' of a subtype is specified, and allows for efficient
     independent addressability (see 9.10) on the target architecture,
     then the `Size' of the following objects of the subtype should
     equal the `Size' of the subtype:

     Aliased objects (including components).
     Followed.


     `Size' clause on a composite subtype should not affect the
     internal layout of components.
     Followed.


     The recommended level of support for the `Size' attribute of
     subtypes is:


     The `Size' (if not specified) of a static discrete or fixed point
     subtype should be the number of bits needed to represent each value
     belonging to the subtype using an unbiased representation, leaving
     space for a sign bit only if the subtype contains negative values.
     If such a subtype is a first subtype, then an implementation
     should support a specified `Size' for it that reflects this
     representation.
     Followed.


     For a subtype implemented with levels of indirection, the `Size'
     should include the size of the pointers, but not the size of what
     they point at.
     Followed.

*13.3(71-73): Component Size Clauses*

     The recommended level of support for the `Component_Size'
     attribute is:


     An implementation need not support specified `Component_Sizes'
     that are less than the `Size' of the component subtype.
     Followed.


     An implementation should support specified `Component_Size's that
     are factors and multiples of the word size.  For such
     `Component_Size's, the array should contain no gaps between
     components.  For other `Component_Size's (if supported), the array
     should contain no gaps between components when packing is also
     specified; the implementation should forbid this combination in
     cases where it cannot support a no-gaps representation.
     Followed.

*13.4(9-10): Enumeration Representation Clauses*

     The recommended level of support for enumeration representation
     clauses is:

     An implementation need not support enumeration representation
     clauses for boolean types, but should at minimum support the
     internal codes in the range `System.Min_Int.System.Max_Int'.
     Followed.

*13.5.1(17-22): Record Representation Clauses*

     The recommended level of support for
     `record_representation_clauses' is:

     An implementation should support storage places that can be
     extracted with a load, mask, shift sequence of machine code, and
     set with a load, shift, mask, store sequence, given the available
     machine instructions and run-time model.
     Followed.


     A storage place should be supported if its size is equal to the
     `Size' of the component subtype, and it starts and ends on a
     boundary that obeys the `Alignment' of the component subtype.
     Followed.


     If the default bit ordering applies to the declaration of a given
     type, then for a component whose subtype's `Size' is less than the
     word size, any storage place that does not cross an aligned word
     boundary should be supported.
     Followed.


     An implementation may reserve a storage place for the tag field of
     a tagged type, and disallow other components from overlapping that
     place.
     Followed.  The storage place for the tag field is the beginning of
     the tagged record, and its size is Address'Size.  GNAT will reject
     an explicit component clause for the tag field.


     An implementation need not support a `component_clause' for a
     component of an extension part if the storage place is not after
     the storage places of all components of the parent type, whether
     or not those storage places had been specified.
     Followed.  The above advice on record representation clauses is
     followed, and all mentioned features are implemented.

*13.5.2(5): Storage Place Attributes*

     If a component is represented using some form of pointer (such as
     an offset) to the actual data of the component, and this data is
     contiguous with the rest of the object, then the storage place
     attributes should reflect the place of the actual data, not the
     pointer.  If a component is allocated discontinuously from the
     rest of the object, then a warning should be generated upon
     reference to one of its storage place attributes.
     Followed.  There are no such components in GNAT.

*13.5.3(7-8): Bit Ordering*

     The recommended level of support for the non-default bit ordering
     is:


     If `Word_Size' = `Storage_Unit', then the implementation should
     support the non-default bit ordering in addition to the default
     bit ordering.
     Followed.  Word size does not equal storage size in this
     implementation.  Thus non-default bit ordering is not supported.

*13.7(37): Address as Private*

     `Address' should be of a private type.
     Followed.

*13.7.1(16): Address Operations*

     Operations in `System' and its children should reflect the target
     environment semantics as closely as is reasonable.  For example,
     on most machines, it makes sense for address arithmetic to "wrap
     around".  Operations that do not make sense should raise
     `Program_Error'.
     Followed.  Address arithmetic is modular arithmetic that wraps
     around.  No operation raises `Program_Error', since all operations
     make sense.

*13.9(14-17): Unchecked Conversion*

     The `Size' of an array object should not include its bounds; hence,
     the bounds should not be part of the converted data.
     Followed.


     The implementation should not generate unnecessary run-time checks
     to ensure that the representation of S is a representation of the
     target type.  It should take advantage of the permission to return
     by reference when possible.  Restrictions on unchecked conversions
     should be avoided unless required by the target environment.
     Followed.  There are no restrictions on unchecked conversion.  A
     warning is generated if the source and target types do not have
     the same size since the semantics in this case may be target
     dependent.


     The recommended level of support for unchecked conversions is:


     Unchecked conversions should be supported and should be reversible
     in the cases where this clause defines the result.  To enable
     meaningful use of unchecked conversion, a contiguous
     representation should be used for elementary subtypes, for
     statically constrained array subtypes whose component subtype is
     one of the subtypes described in this paragraph, and for record
     subtypes without discriminants whose component subtypes are
     described in this paragraph.
     Followed.

*13.11(23-25): Implicit Heap Usage*

     An implementation should document any cases in which it dynamically
     allocates heap storage for a purpose other than the evaluation of
     an allocator.
     Followed, the only other points at which heap storage is
     dynamically allocated are as follows:

        * At initial elaboration time, to allocate dynamically sized
          global objects.

        * To allocate space for a task when a task is created.

        * To extend the secondary stack dynamically when needed.  The
          secondary stack is used for returning variable length results.


     A default (implementation-provided) storage pool for an
     access-to-constant type should not have overhead to support
     deallocation of individual objects.
     Followed.


     A storage pool for an anonymous access type should be created at
     the point of an allocator for the type, and be reclaimed when the
     designated object becomes inaccessible.
     Followed.

*13.11.2(17): Unchecked De-allocation*

     For a standard storage pool, `Free' should actually reclaim the
     storage.
     Followed.

*13.13.2(17): Stream Oriented Attributes*

     If a stream element is the same size as a storage element, then the
     normal in-memory representation should be used by `Read' and
     `Write' for scalar objects.  Otherwise, `Read' and `Write' should
     use the smallest number of stream elements needed to represent all
     values in the base range of the scalar type.
     Followed.  In particular, the interpretation chosen is that of
     AI-195, which specifies that the size to be used is that of the
     first subtype.

*A.1(52): Implementation Advice*

     If an implementation provides additional named predefined integer
     types, then the names should end with `Integer' as in
     `Long_Integer'.  If an implementation provides additional named
     predefined floating point types, then the names should end with
     `Float' as in `Long_Float'.
     Followed.

*A.3.2(49): `Ada.Characters.Handling'*

     If an implementation provides a localized definition of `Character'
     or `Wide_Character', then the effects of the subprograms in
     `Characters.Handling' should reflect the localizations.  See also
     3.5.2.
     Followed.  GNAT provides no such localized definitions.

*A.4.4(106): Bounded-Length String Handling*

     Bounded string objects should not be implemented by implicit
     pointers and dynamic allocation.
     Followed.  No implicit pointers or dynamic allocation are used.

*A.5.2(46-47): Random Number Generation*

     Any storage associated with an object of type `Generator' should be
     reclaimed on exit from the scope of the object.
     Followed.


     If the generator period is sufficiently long in relation to the
     number of distinct initiator values, then each possible value of
     `Initiator' passed to `Reset' should initiate a sequence of random
     numbers that does not, in a practical sense, overlap the sequence
     initiated by any other value.  If this is not possible, then the
     mapping between initiator values and generator states should be a
     rapidly varying function of the initiator value.
     Followed.  The generator period is sufficiently long for the first
     condition here to hold true.

*A.10.7(23): `Get_Immediate'*

     The `Get_Immediate' procedures should be implemented with
     unbuffered input.  For a device such as a keyboard, input should be
     "available" if a key has already been typed, whereas for a disk
     file, input should always be available except at end of file.  For
     a file associated with a keyboard-like device, any line-editing
     features of the underlying operating system should be disabled
     during the execution of `Get_Immediate'.
     Followed.

*B.1(39-41): Pragma `Export'*

     If an implementation supports pragma `Export' to a given language,
     then it should also allow the main subprogram to be written in that
     language.  It should support some mechanism for invoking the
     elaboration of the Ada library units included in the system, and
     for invoking the finalization of the environment task.  On typical
     systems, the recommended mechanism is to provide two subprograms
     whose link names are `adainit' and `adafinal'.  `adainit' should
     contain the elaboration code for library units.  `adafinal' should
     contain the finalization code.  These subprograms should have no
     effect the second and subsequent time they are called.
     Followed.


     Automatic elaboration of pre-elaborated packages should be
     provided when pragma `Export' is supported.
     Followed when the main program is in Ada.  If the main program is
     in a foreign language, then `adainit' must be called to elaborate
     pre-elaborated packages.


     For each supported convention L other than `Intrinsic', an
     implementation should support `Import' and `Export' pragmas for
     objects of L-compatible types and for subprograms, and pragma
     `Convention' for L-eligible types and for subprograms, presuming
     the other language has corresponding features.  Pragma
     `Convention' need not be supported for scalar types.
     Followed.

*B.2(12-13): Package `Interfaces'*

     For each implementation-defined convention identifier, there
     should be a child package of package Interfaces with the
     corresponding name.  This package should contain any declarations
     that would be useful for interfacing to the language
     (implementation) represented by the convention.  Any declarations
     useful for interfacing to any language on the given hardware
     architecture should be provided directly in `Interfaces'.
     Followed.  An additional package not defined in the Ada 95
     Reference Manual is `Interfaces.CPP', used for interfacing to C++.


     An implementation supporting an interface to C, COBOL, or Fortran
     should provide the corresponding package or packages described in
     the following clauses.
     Followed.  GNAT provides all the packages described in this
     section.

*B.3(63-71): Interfacing with C*

     An implementation should support the following interface
     correspondences between Ada and C.
     Followed.


     An Ada procedure corresponds to a void-returning C function.
     Followed.


     An Ada function corresponds to a non-void C function.
     Followed.


     An Ada `in' scalar parameter is passed as a scalar argument to a C
     function.
     Followed.


     An Ada `in' parameter of an access-to-object type with designated
     type T is passed as a `T*' argument to a C function, where T is
     the C type corresponding to the Ada type T.
     Followed.


     An Ada access T parameter, or an Ada `out' or `in out' parameter
     of an elementary type T, is passed as a `T*' argument to a C
     function, where T is the C type corresponding to the Ada type T.
     In the case of an elementary `out' or `in out' parameter, a
     pointer to a temporary copy is used to preserve by-copy semantics.
     Followed.


     An Ada parameter of a record type T, of any mode, is passed as a
     `T*' argument to a C function, where T is the C structure
     corresponding to the Ada type T.
     Followed.  This convention may be overridden by the use of the
     C_Pass_By_Copy pragma, or Convention, or by explicitly specifying
     the mechanism for a given call using an extended import or export
     pragma.


     An Ada parameter of an array type with component type T, of any
     mode, is passed as a `T*' argument to a C function, where T is the
     C type corresponding to the Ada type T.
     Followed.


     An Ada parameter of an access-to-subprogram type is passed as a
     pointer to a C function whose prototype corresponds to the
     designated subprogram's specification.
     Followed.

*B.4(95-98): Interfacing with COBOL*

     An Ada implementation should support the following interface
     correspondences between Ada and COBOL.
     Followed.


     An Ada access T parameter is passed as a `BY REFERENCE' data item
     of the COBOL type corresponding to T.
     Followed.


     An Ada in scalar parameter is passed as a `BY CONTENT' data item of
     the corresponding COBOL type.
     Followed.


     Any other Ada parameter is passed as a `BY REFERENCE' data item of
     the COBOL type corresponding to the Ada parameter type; for
     scalars, a local copy is used if necessary to ensure by-copy
     semantics.
     Followed.

*B.5(22-26): Interfacing with Fortran*

     An Ada implementation should support the following interface
     correspondences between Ada and Fortran:
     Followed.


     An Ada procedure corresponds to a Fortran subroutine.
     Followed.


     An Ada function corresponds to a Fortran function.
     Followed.


     An Ada parameter of an elementary, array, or record type T is
     passed as a T argument to a Fortran procedure, where T is the
     Fortran type corresponding to the Ada type T, and where the INTENT
     attribute of the corresponding dummy argument matches the Ada
     formal parameter mode; the Fortran implementation's parameter
     passing conventions are used.  For elementary types, a local copy
     is used if necessary to ensure by-copy semantics.
     Followed.


     An Ada parameter of an access-to-subprogram type is passed as a
     reference to a Fortran procedure whose interface corresponds to the
     designated subprogram's specification.
     Followed.

*C.1(3-5): Access to Machine Operations*

     The machine code or intrinsic support should allow access to all
     operations normally available to assembly language programmers for
     the target environment, including privileged instructions, if any.
     Followed.


     The interfacing pragmas (see Annex B) should support interface to
     assembler; the default assembler should be associated with the
     convention identifier `Assembler'.
     Followed.


     If an entity is exported to assembly language, then the
     implementation should allocate it at an addressable location, and
     should ensure that it is retained by the linking process, even if
     not otherwise referenced from the Ada code.  The implementation
     should assume that any call to a machine code or assembler
     subprogram is allowed to read or update every object that is
     specified as exported.
     Followed.

*C.1(10-16): Access to Machine Operations*

     The implementation should ensure that little or no overhead is
     associated with calling intrinsic and machine-code subprograms.
     Followed for both intrinsics and machine-code subprograms.


     It is recommended that intrinsic subprograms be provided for
     convenient access to any machine operations that provide special
     capabilities or efficiency and that are not otherwise available
     through the language constructs.
     Followed.  A full set of machine operation intrinsic subprograms
     is provided.


     Atomic read-modify-write operations--e.g., test and set, compare
     and swap, decrement and test, enqueue/dequeue.
     Followed on any target supporting such operations.


     Standard numeric functions--e.g., sin, log.
     Followed on any target supporting such operations.


     String manipulation operations--e.g., translate and test.
     Followed on any target supporting such operations.


     Vector operations--e.g., compare vector against thresholds.
     Followed on any target supporting such operations.


     Direct operations on I/O ports.
     Followed on any target supporting such operations.

*C.3(28): Interrupt Support*

     If the `Ceiling_Locking' policy is not in effect, the
     implementation should provide means for the application to specify
     which interrupts are to be blocked during protected actions, if
     the underlying system allows for a finer-grain control of
     interrupt blocking.
     Followed.  The underlying system does not allow for finer-grain
     control of interrupt blocking.

*C.3.1(20-21): Protected Procedure Handlers*

     Whenever possible, the implementation should allow interrupt
     handlers to be called directly by the hardware.
     Followed on any target where the underlying operating system
     permits such direct calls.


     Whenever practical, violations of any implementation-defined
     restrictions should be detected before run time.
     Followed.  Compile time warnings are given when possible.

*C.3.2(25): Package `Interrupts'*

     If implementation-defined forms of interrupt handler procedures are
     supported, such as protected procedures with parameters, then for
     each such form of a handler, a type analogous to
     `Parameterless_Handler' should be specified in a child package of
     `Interrupts', with the same operations as in the predefined
     package Interrupts.
     Followed.

*C.4(14): Pre-elaboration Requirements*

     It is recommended that pre-elaborated packages be implemented in
     such a way that there should be little or no code executed at run
     time for the elaboration of entities not already covered by the
     Implementation Requirements.
     Followed.  Executable code is generated in some cases, e.g. loops
     to initialize large arrays.

*C.5(8): Pragma `Discard_Names'*

     If the pragma applies to an entity, then the implementation should
     reduce the amount of storage used for storing names associated
     with that entity.
     Followed.

*C.7.2(30): The Package Task_Attributes*

     Some implementations are targeted to domains in which memory use
     at run time must be completely deterministic.  For such
     implementations, it is recommended that the storage for task
     attributes will be pre-allocated statically and not from the heap.
     This can be accomplished by either placing restrictions on the
     number and the size of the task's attributes, or by using the
     pre-allocated storage for the first N attribute objects, and the
     heap for the others.  In the latter case, N should be documented.
     Not followed.  This implementation is not targeted to such a
     domain.

*D.3(17): Locking Policies*

     The implementation should use names that end with `_Locking' for
     locking policies defined by the implementation.
     Followed.  A single implementation-defined locking policy is
     defined, whose name (`Inheritance_Locking') follows this
     suggestion.

*D.4(16): Entry Queuing Policies*

     Names that end with `_Queuing' should be used for all
     implementation-defined queuing policies.
     Followed.  No such implementation-defined queueing policies exist.

*D.6(9-10): Preemptive Abort*

     Even though the `abort_statement' is included in the list of
     potentially blocking operations (see 9.5.1), it is recommended
     that this statement be implemented in a way that never requires
     the task executing the `abort_statement' to block.
     Followed.


     On a multi-processor, the delay associated with aborting a task on
     another processor should be bounded; the implementation should use
     periodic polling, if necessary, to achieve this.
     Followed.

*D.7(21): Tasking Restrictions*

     When feasible, the implementation should take advantage of the
     specified restrictions to produce a more efficient implementation.
     GNAT currently takes advantage of these restrictions by providing
     an optimized run time when the Ravenscar profile and the GNAT
     restricted run time set of restrictions are specified.  See pragma
     `Ravenscar' and pragma `Restricted_Run_Time' for more details.

*D.8(47-49): Monotonic Time*

     When appropriate, implementations should provide configuration
     mechanisms to change the value of `Tick'.
     Such configuration mechanisms are not appropriate to this
     implementation and are thus not supported.


     It is recommended that `Calendar.Clock' and `Real_Time.Clock' be
     implemented as transformations of the same time base.
     Followed.


     It is recommended that the "best" time base which exists in the
     underlying system be available to the application through `Clock'.
     "Best" may mean highest accuracy or largest range.
     Followed.

*E.5(28-29): Partition Communication Subsystem*

     Whenever possible, the PCS on the called partition should allow for
     multiple tasks to call the RPC-receiver with different messages and
     should allow them to block until the corresponding subprogram body
     returns.
     Followed by GLADE, a separately supplied PCS that can be used with
     GNAT.


     The `Write' operation on a stream of type `Params_Stream_Type'
     should raise `Storage_Error' if it runs out of space trying to
     write the `Item' into the stream.
     Followed by GLADE, a separately supplied PCS that can be used with
     GNAT.

*F(7): COBOL Support*

     If COBOL (respectively, C) is widely supported in the target
     environment, implementations supporting the Information Systems
     Annex should provide the child package `Interfaces.COBOL'
     (respectively, `Interfaces.C') specified in Annex B and should
     support a `convention_identifier' of COBOL (respectively, C) in
     the interfacing pragmas (see Annex B), thus allowing Ada programs
     to interface with programs written in that language.
     Followed.

*F.1(2): Decimal Radix Support*

     Packed decimal should be used as the internal representation for
     objects of subtype S when S'Machine_Radix = 10.
     Not followed.  GNAT ignores S'Machine_Radix and always uses binary
     representations.

*G: Numerics*


     If Fortran (respectively, C) is widely supported in the target
     environment, implementations supporting the Numerics Annex should
     provide the child package `Interfaces.Fortran' (respectively,
     `Interfaces.C') specified in Annex B and should support a
     `convention_identifier' of Fortran (respectively, C) in the
     interfacing pragmas (see Annex B), thus allowing Ada programs to
     interface with programs written in that language.
     Followed.

*G.1.1(56-58): Complex Types*


     Because the usual mathematical meaning of multiplication of a
     complex operand and a real operand is that of the scaling of both
     components of the former by the latter, an implementation should
     not perform this operation by first promoting the real operand to
     complex type and then performing a full complex multiplication.
     In systems that, in the future, support an Ada binding to IEC
     559:1989, the latter technique will not generate the required
     result when one of the components of the complex operand is
     infinite.  (Explicit multiplication of the infinite component by
     the zero component obtained during promotion yields a NaN that
     propagates into the final result.) Analogous advice applies in the
     case of multiplication of a complex operand and a pure-imaginary
     operand, and in the case of division of a complex operand by a
     real or pure-imaginary operand.
     Not followed.


     Similarly, because the usual mathematical meaning of addition of a
     complex operand and a real operand is that the imaginary operand
     remains unchanged, an implementation should not perform this
     operation by first promoting the real operand to complex type and
     then performing a full complex addition.  In implementations in
     which the `Signed_Zeros' attribute of the component type is `True'
     (and which therefore conform to IEC 559:1989 in regard to the
     handling of the sign of zero in predefined arithmetic operations),
     the latter technique will not generate the required result when
     the imaginary component of the complex operand is a negatively
     signed zero.  (Explicit addition of the negative zero to the zero
     obtained during promotion yields a positive zero.)  Analogous
     advice applies in the case of addition of a complex operand and a
     pure-imaginary operand, and in the case of subtraction of a
     complex operand and a real or pure-imaginary operand.
     Not followed.


     Implementations in which `Real'Signed_Zeros' is `True' should
     attempt to provide a rational treatment of the signs of zero
     results and result components.  As one example, the result of the
     `Argument' function should have the sign of the imaginary
     component of the parameter `X' when the point represented by that
     parameter lies on the positive real axis; as another, the sign of
     the imaginary component of the `Compose_From_Polar' function
     should be the same as (respectively, the opposite of) that of the
     `Argument' parameter when that parameter has a value of zero and
     the `Modulus' parameter has a nonnegative (respectively, negative)
     value.
     Followed.

*G.1.2(49): Complex Elementary Functions*

     Implementations in which `Complex_Types.Real'Signed_Zeros' is
     `True' should attempt to provide a rational treatment of the signs
     of zero results and result components.  For example, many of the
     complex elementary functions have components that are odd
     functions of one of the parameter components; in these cases, the
     result component should have the sign of the parameter component
     at the origin.  Other complex elementary functions have zero
     components whose sign is opposite that of a parameter component at
     the origin, or is always positive or always negative.
     Followed.

*G.2.4(19): Accuracy Requirements*

     The versions of the forward trigonometric functions without a
     `Cycle' parameter should not be implemented by calling the
     corresponding version with a `Cycle' parameter of
     `2.0*Numerics.Pi', since this will not provide the required
     accuracy in some portions of the domain.  For the same reason, the
     version of `Log' without a `Base' parameter should not be
     implemented by calling the corresponding version with a `Base'
     parameter of `Numerics.e'.
     Followed.

*G.2.6(15): Complex Arithmetic Accuracy*

     The version of the `Compose_From_Polar' function without a `Cycle'
     parameter should not be implemented by calling the corresponding
     version with a `Cycle' parameter of `2.0*Numerics.Pi', since this
     will not provide the required accuracy in some portions of the
     domain.
     Followed.


File: gnat_rm.info,  Node: Implementation Defined Characteristics,  Next: Intrinsic Subprograms,  Prev: Implementation Advice,  Up: Top

Implementation Defined Characteristics
**************************************

   In addition to the implementation dependent pragmas and attributes,
and the implementation advice, there are a number of other features of
Ada 95 that are potentially implementation dependent.  These are
mentioned throughout the Ada 95 Reference Manual, and are summarized in
annex M.

   A requirement for conforming Ada compilers is that they provide
documentation describing how the implementation deals with each of these
issues.  In this chapter, you will find each point in annex M listed
followed by a description in italic font of how GNAT handles the
implementation dependence.

   You can use this chapter as a guide to minimizing implementation
dependent features in your programs if portability to other compilers
and other operating systems is an important consideration.  The numbers
in each section below correspond to the paragraph number in the Ada 95
Reference Manual.


*2*.  Whether or not each recommendation given in Implementation Advice
is followed.  See 1.1.2(37).

*Note Implementation Advice::.


*3*.  Capacity limitations of the implementation.  See 1.1.3(3).

The complexity of programs that can be processed is limited only by the
total amount of available virtual memory, and disk space for the
generated object files.


*4*.  Variations from the standard that are impractical to avoid given
the implementation's execution environment.  See 1.1.3(6).

There are no variations from the standard.


*5*.  Which `code_statement's cause external interactions.  See
1.1.3(10).

Any `code_statement' can potentially cause external interactions.


*6*.  The coded representation for the text of an Ada program.  See
2.1(4).

See separate section on source representation.


*7*.  The control functions allowed in comments.  See 2.1(14).

See separate section on source representation.


*8*.  The representation for an end of line.  See 2.2(2).

See separate section on source representation.


*9*.  Maximum supported line length and lexical element length.  See
2.2(15).

The maximum line length is 255 characters an the maximum length of a
lexical element is also 255 characters.


*10*.  Implementation defined pragmas.  See 2.8(14).

*Note Implementation Defined Pragmas::.


*11*.  Effect of pragma `Optimize'.  See 2.8(27).

Pragma `Optimize', if given with a `Time' or `Space' parameter, checks
that the optimization flag is set, and aborts if it is not.


*12*.  The sequence of characters of the value returned by `S'Image'
when some of the graphic characters of `S'Wide_Image' are not defined
in `Character'.  See 3.5(37).

The sequence of characters is as defined by the wide character encoding
method used for the source.  See section on source representation for
further details.


*13*.  The predefined integer types declared in `Standard'.  See
3.5.4(25).

`Short_Short_Integer'
     8 bit signed

`Short_Integer'
     (Short) 16 bit signed

`Integer'
     32 bit signed

`Long_Integer'
     64 bit signed (Alpha OpenVMS only) 32 bit signed (all other
     targets)

`Long_Long_Integer'
     64 bit signed


*14*.  Any nonstandard integer types and the operators defined for
them.  See 3.5.4(26).

There are no nonstandard integer types.


*15*.  Any nonstandard real types and the operators defined for them.
See 3.5.6(8).

There are no nonstandard real types.


*16*.  What combinations of requested decimal precision and range are
supported for floating point types.  See 3.5.7(7).

The precision and range is as defined by the IEEE standard.


*17*.  The predefined floating point types declared in `Standard'.  See
3.5.7(16).

`Short_Float'
     32 bit IEEE short

`Float'
     (Short) 32 bit IEEE short

`Long_Float'
     64 bit IEEE long

`Long_Long_Float'
     64 bit IEEE long (80 bit IEEE long on x86 processors)


*18*.  The small of an ordinary fixed point type.  See 3.5.9(8).

`Fine_Delta' is 2**(-63)


*19*.  What combinations of small, range, and digits are supported for
fixed point types.  See 3.5.9(10).

Any combinations are permitted that do not result in a small less than
`Fine_Delta' and do not result in a mantissa larger than 63 bits.  If
the mantissa is larger than 53 bits on machines where Long_Long_Float
is 64 bits (true of all architectures except ia32), then the output from
Text_IO is accurate to only 53 bits, rather than the full mantissa.
This is because floating-point conversions are used to convert fixed
point.


*20*.  The result of `Tags.Expanded_Name' for types declared within an
unnamed `block_statement'.  See 3.9(10).

Block numbers of the form `BNNN', where NNN is a decimal integer are
allocated.


*21*.  Implementation-defined attributes.  See 4.1.4(12).

*Note Implementation Defined Attributes::.


*22*.  Any implementation-defined time types.  See 9.6(6).

There are no implementation-defined time types.


*23*.  The time base associated with relative delays.

See 9.6(20).  The time base used is that provided by the C library
function `gettimeofday'.


*24*.  The time base of the type `Calendar.Time'.  See 9.6(23).

The time base used is that provided by the C library function
`gettimeofday'.


*25*.  The time zone used for package `Calendar' operations.  See
9.6(24).

The time zone used by package `Calendar' is the current system time zone
setting for local time, as accessed by the C library function
`localtime'.


*26*.  Any limit on `delay_until_statements' of `select_statements'.
See 9.6(29).

There are no such limits.


*27*.  Whether or not two non overlapping parts of a composite object
are independently addressable, in the case where packing, record
layout, or `Component_Size' is specified for the object.  See 9.10(1).

Separate components are independently addressable if they do not share
overlapping storage units.


*28*.  The representation for a compilation.  See 10.1(2).

A compilation is represented by a sequence of files presented to the
compiler in a single invocation of the `gcc' command.


*29*.  Any restrictions on compilations that contain multiple
compilation_units.  See 10.1(4).

No single file can contain more than one compilation unit, but any
sequence of files can be presented to the compiler as a single
compilation.


*30*.  The mechanisms for creating an environment and for adding and
replacing compilation units.  See 10.1.4(3).

See separate section on compilation model.


*31*.  The manner of explicitly assigning library units to a partition.
See 10.2(2).

If a unit contains an Ada main program, then the Ada units for the
partition are determined by recursive application of the rules in the
Ada Reference Manual section 10.2(2-6).  In other words, the Ada units
will be those that are needed by the main program, and then this
definition of need is applied recursively to those units, and the
partition contains the transitive closure determined by this
relationship.  In short, all the necessary units are included, with no
need to explicitly specify the list.  If additional units are required,
e.g. by foreign language units, then all units must be mentioned in the
context clause of one of the needed Ada units.

   If the partition contains no main program, or if the main program is
in a language other than Ada, then GNAT provides the binder options
`-z' and `-n' respectively, and in this case a list of units can be
explicitly supplied to the binder for inclusion in the partition (all
units needed by these units will also be included automatically).  For
full details on the use of these options, refer to the `GNAT User's
Guide' sections on Binding and Linking.


*32*.  The implementation-defined means, if any, of specifying which
compilation units are needed by a given compilation unit.  See 10.2(2).

The units needed by a given compilation unit are as defined in the Ada
Reference Manual section 10.2(2-6).  There are no
implementation-defined pragmas or other implementation-defined means
for specifying needed units.


*33*.  The manner of designating the main subprogram of a partition.
See 10.2(7).

The main program is designated by providing the name of the
corresponding `ALI' file as the input parameter to the binder.


*34*.  The order of elaboration of `library_items'.  See 10.2(18).

The first constraint on ordering is that it meets the requirements of
chapter 10 of the Ada 95 Reference Manual.  This still leaves some
implementation dependent choices, which are resolved by first
elaborating bodies as early as possible (i.e. in preference to specs
where there is a choice), and second by evaluating the immediate with
clauses of a unit to determine the probably best choice, and third by
elaborating in alphabetical order of unit names where a choice still
remains.


*35*.  Parameter passing and function return for the main subprogram.
See 10.2(21).

The main program has no parameters.  It may be a procedure, or a
function returning an integer type.  In the latter case, the returned
integer value is the return code of the program.


*36*.  The mechanisms for building and running partitions.  See
10.2(24).

GNAT itself supports programs with only a single partition.  The
GNATDIST tool provided with the GLADE package (which also includes an
implementation of the PCS) provides a completely flexible method for
building and running programs consisting of multiple partitions.  See
the separate GLADE manual for details.


*37*.  The details of program execution, including program termination.
See 10.2(25).

See separate section on compilation model.


*38*.  The semantics of any non-active partitions supported by the
implementation.  See 10.2(28).

Passive partitions are supported on targets where shared memory is
provided by the operating system.  See the GLADE reference manual for
further details.


*39*.  The information returned by `Exception_Message'.  See 11.4.1(10).

Exception message returns the null string unless a specific message has
been passed by the program.


*40*.  The result of `Exceptions.Exception_Name' for types declared
within an unnamed `block_statement'.  See 11.4.1(12).

Blocks have implementation defined names of the form `BNNN' where NNN
is an integer.


*41*.  The information returned by `Exception_Information'.  See
11.4.1(13).

`Exception_Information' returns a string in the following format:

     _Exception_Name:_ nnnnn
     _Message:_ mmmmm
     _PID:_ ppp
     _Call stack traceback locations:_
     0xhhhh 0xhhhh 0xhhhh ... 0xhhh

where

   * `nnnn' is the fully qualified name of the exception in all upper
     case letters. This line is always present.

   * `mmmm' is the message (this line present only if message is
     non-null)

   * `ppp' is the Process Id value as a decimal integer (this line is
     present only if the Process Id is non-zero). Currently we are not
     making use of this field.

   * The Call stack traceback locations line and the following values
     are present only if at least one traceback location was recorded.
     The values are given in C style format, with lower case letters
     for a-f, and only as many digits present as are necessary.

The line terminator sequence at the end of each line, including the
last line is a single `LF' character (`16#0A#').


*42*.  Implementation-defined check names.  See 11.5(27).

No implementation-defined check names are supported.


*43*.  The interpretation of each aspect of representation.  See
13.1(20).

See separate section on data representations.


*44*.  Any restrictions placed upon representation items.  See 13.1(20).

See separate section on data representations.


*45*.  The meaning of `Size' for indefinite subtypes.  See 13.3(48).

Size for an indefinite subtype is the maximum possible size, except that
for the case of a subprogram parameter, the size of the parameter object
is the actual size.


*46*.  The default external representation for a type tag.  See
13.3(75).

The default external representation for a type tag is the fully expanded
name of the type in upper case letters.


*47*.  What determines whether a compilation unit is the same in two
different partitions.  See 13.3(76).

A compilation unit is the same in two different partitions if and only
if it derives from the same source file.


*48*.  Implementation-defined components.  See 13.5.1(15).

The only implementation defined component is the tag for a tagged type,
which contains a pointer to the dispatching table.


*49*.  If `Word_Size' = `Storage_Unit', the default bit ordering.  See
13.5.3(5).

`Word_Size' (32) is not the same as `Storage_Unit' (8) for this
implementation, so no non-default bit ordering is supported.  The
default bit ordering corresponds to the natural endianness of the
target architecture.


*50*.  The contents of the visible part of package `System' and its
language-defined children.  See 13.7(2).

See the definition of these packages in files `system.ads' and
`s-stoele.ads'.


*51*.  The contents of the visible part of package
`System.Machine_Code', and the meaning of `code_statements'.  See
13.8(7).

See the definition and documentation in file `s-maccod.ads'.


*52*.  The effect of unchecked conversion.  See 13.9(11).

Unchecked conversion between types of the same size and results in an
uninterpreted transmission of the bits from one type to the other.  If
the types are of unequal sizes, then in the case of discrete types, a
shorter source is first zero or sign extended as necessary, and a
shorter target is simply truncated on the left.  For all non-discrete
types, the source is first copied if necessary to ensure that the
alignment requirements of the target are met, then a pointer is
constructed to the source value, and the result is obtained by
dereferencing this pointer after converting it to be a pointer to the
target type.


*53*.  The manner of choosing a storage pool for an access type when
`Storage_Pool' is not specified for the type.  See 13.11(17).

There are 3 different standard pools used by the compiler when
`Storage_Pool' is not specified depending whether the type is local to
a subprogram or defined at the library level and whether
`Storage_Size'is specified or not.  See documentation in the runtime
library units `System.Pool_Global', `System.Pool_Size' and
`System.Pool_Local' in files `s-poosiz.ads', `s-pooglo.ads' and
`s-pooloc.ads' for full details on the default pools used.


*54*.  Whether or not the implementation provides user-accessible names
for the standard pool type(s).  See 13.11(17).

See documentation in the sources of the run time mentioned in paragraph
*53* .  All these pools are accessible by means of `with''ing these
units.


*55*.  The meaning of `Storage_Size'.  See 13.11(18).

`Storage_Size' is measured in storage units, and refers to the total
space available for an access type collection, or to the primary stack
space for a task.


*56*.  Implementation-defined aspects of storage pools.  See 13.11(22).

See documentation in the sources of the run time mentioned in paragraph
*53* for details on GNAT-defined aspects of storage pools.


*57*.  The set of restrictions allowed in a pragma `Restrictions'.  See
13.12(7).

All RM defined Restriction identifiers are implemented.  The following
additional restriction identifiers are provided.  There are two separate
lists of implementation dependent restriction identifiers.  The first
set requires consistency throughout a partition (in other words, if the
restriction identifier is used for any compilation unit in the
partition, then all compilation units in the partition must obey the
restriction.

`Boolean_Entry_Barriers'
     This restriction ensures at compile time that barriers in entry
     declarations for protected types are restricted to references to
     simple boolean variables defined in the private part of the
     protected type.  No other form of entry barriers is permitted.
     This is one of the restrictions of the Ravenscar profile for
     limited tasking (see also pragma `Ravenscar').

`Max_Entry_Queue_Depth => Expr'
     This restriction is a declaration that any protected entry
     compiled in the scope of the restriction has at most the specified
     number of tasks waiting on the entry at any one time, and so no
     queue is required.  This restriction is not checked at compile
     time.  A program execution is erroneous if an attempt is made to
     queue more than the specified number of tasks on such an entry.

`No_Calendar'
     This restriction ensures at compile time that there is no implicit
     or explicit dependence on the package `Ada.Calendar'.

`No_Dynamic_Interrupts'
     This restriction ensures at compile time that there is no attempt
     to dynamically associate interrupts.  Only static association is
     allowed.

`No_Enumeration_Maps'
     This restriction ensures at compile time that no operations
     requiring enumeration maps are used (that is Image and Value
     attributes applied to enumeration types).

`No_Entry_Calls_In_Elaboration_Code'
     This restriction ensures at compile time that no task or protected
     entry calls are made during elaboration code.  As a result of the
     use of this restriction, the compiler can assume that no code past
     an accept statement in a task can be executed at elaboration time.

`No_Exception_Handlers'
     This restriction ensures at compile time that there are no explicit
     exception handlers.

`No_Implicit_Conditionals'
     This restriction ensures that the generated code does not contain
     any implicit conditionals, either by modifying the generated code
     where possible, or by rejecting any construct that would otherwise
     generate an implicit conditional.  The details and use of this
     restriction are described in more detail in the High Integrity
     product documentation.

`No_Implicit_Loops'
     This restriction ensures that the generated code does not contain
     any implicit `for' loops, either by modifying the generated code
     where possible, or by rejecting any construct that would otherwise
     generate an implicit `for' loop.  The details and use of this
     restriction are described in more detail in the High Integrity
     product documentation.

`No_Local_Protected_Objects'
     This restriction ensures at compile time that protected objects are
     only declared at the library level.

`No_Protected_Type_Allocators'
     This restriction ensures at compile time that there are no
     allocator expressions that attempt to allocate protected objects.

`No_Secondary_Stack'
     This restriction ensures at compile time that the generated code
     does not contain any reference to the secondary stack.  The
     secondary stack is used to implement functions returning
     unconstrained objects (arrays or records) on some targets.  The
     details and use of this restriction are described in more detail
     in the High Integrity product documentation.

`No_Select_Statements'
     This restriction ensures at compile time no select statements of
     any kind are permitted, that is the keyword `select' may not
     appear.  This is one of the restrictions of the Ravenscar profile
     for limited tasking (see also pragma `Ravenscar').

`No_Standard_Storage_Pools'
     This restriction ensures at compile time that no access types use
     the standard default storage pool.  Any access type declared must
     have an explicit Storage_Pool attribute defined specifying a
     user-defined storage pool.

`No_Streams'
     This restriction ensures at compile time that there are no
     implicit or explicit dependencies on the package `Ada.Streams'.

`No_Task_Attributes'
     This restriction ensures at compile time that there are no
     implicit or explicit dependencies on the package
     `Ada.Task_Attributes'.

`No_Task_Termination'
     This restriction ensures at compile time that no terminate
     alternatives appear in any task body.

`No_Tasking'
     This restriction prevents the declaration of tasks or task types
     throughout the partition.  It is similar in effect to the use of
     `Max_Tasks => 0' except that violations are caught at compile time
     and cause an error message to be output either by the compiler or
     binder.

`No_Wide_Characters'
     This restriction ensures at compile time that no uses of the types
     `Wide_Character' or `Wide_String' appear, and that no wide
     character literals appear in the program (that is literals
     representing characters not in type `Character'.

`Static_Priorities'
     This restriction ensures at compile time that all priority
     expressions are static, and that there are no dependencies on the
     package `Ada.Dynamic_Priorities'.

`Static_Storage_Size'
     This restriction ensures at compile time that any expression
     appearing in a Storage_Size pragma or attribute definition clause
     is static.

The second set of implementation dependent restriction identifiers does
not require partition-wide consistency.  The restriction may be
enforced for a single compilation unit without any effect on any of the
other compilation units in the partition.

`No_Elaboration_Code'
     This restriction ensures at compile time that no elaboration code
     is generated.  Note that this is not the same condition as is
     enforced by pragma `Preelaborate'.  There are cases in which
     pragma `Preelaborate' still permits code to be generated (e.g.
     code to initialize a large array to all zeroes), and there are
     cases of units which do not meet the requirements for pragma
     `Preelaborate', but for which no elaboration code is generated.
     Generally, it is the case that preelaborable units will meet the
     restrictions, with the exception of large aggregates initialized
     with an others_clause, and exception declarations (which generate
     calls to a run-time registry procedure).  Note that this
     restriction is enforced on a unit by unit basis, it need not be
     obeyed consistently throughout a partition.

`No_Entry_Queue'
     This restriction is a declaration that any protected entry
     compiled in the scope of the restriction has at most one task
     waiting on the entry at any one time, and so no queue is required.
     This restriction is not checked at compile time.  A program
     execution is erroneous if an attempt is made to queue a second
     task on such an entry.

`No_Implementation_Attributes'
     This restriction checks at compile time that no GNAT-defined
     attributes are present.  With this restriction, the only
     attributes that can be used are those defined in the Ada 95
     Reference Manual.

`No_Implementation_Pragmas'
     This restriction checks at compile time that no GNAT-defined
     pragmas are present.  With this restriction, the only pragmas that
     can be used are those defined in the Ada 95 Reference Manual.

`No_Implementation_Restrictions'
     This restriction checks at compile time that no GNAT-defined
     restriction identifiers (other than
     `No_Implementation_Restrictions' itself) are present.  With this
     restriction, the only other restriction identifiers that can be
     used are those defined in the Ada 95 Reference Manual.


*58*.  The consequences of violating limitations on `Restrictions'
pragmas.  See 13.12(9).

Restrictions that can be checked at compile time result in illegalities
if violated.  Currently there are no other consequences of violating
restrictions.


*59*.  The representation used by the `Read' and `Write' attributes of
elementary types in terms of stream elements.  See 13.13.2(9).

The representation is the in-memory representation of the base type of
the type, using the number of bits corresponding to the `TYPE'Size'
value, and the natural ordering of the machine.


*60*.  The names and characteristics of the numeric subtypes declared
in the visible part of package `Standard'.  See A.1(3).

See items describing the integer and floating-point types supported.


*61*.  The accuracy actually achieved by the elementary functions.  See
A.5.1(1).

The elementary functions correspond to the functions available in the C
library.  Only fast math mode is implemented.


*62*.  The sign of a zero result from some of the operators or
functions in `Numerics.Generic_Elementary_Functions', when
`Float_Type'Signed_Zeros' is `True'.  See A.5.1(46).

The sign of zeroes follows the requirements of the IEEE 754 standard on
floating-point.


*63*.  The value of `Numerics.Float_Random.Max_Image_Width'.  See
A.5.2(27).

Maximum image width is 649, see library file `a-numran.ads'.


*64*.  The value of `Numerics.Discrete_Random.Max_Image_Width'.  See
A.5.2(27).

Maximum image width is 80, see library file `a-nudira.ads'.


*65*.  The algorithms for random number generation.  See A.5.2(32).

The algorithm is documented in the source files `a-numran.ads' and
`a-numran.adb'.


*66*.  The string representation of a random number generator's state.
See A.5.2(38).

See the documentation contained in the file `a-numran.adb'.


*67*.  The minimum time interval between calls to the time-dependent
Reset procedure that are guaranteed to initiate different random number
sequences.  See A.5.2(45).

The minimum period between reset calls to guarantee distinct series of
random numbers is one microsecond.


*68*.  The values of the `Model_Mantissa', `Model_Emin',
`Model_Epsilon', `Model', `Safe_First', and `Safe_Last' attributes, if
the Numerics Annex is not supported.  See A.5.3(72).

See the source file `ttypef.ads' for the values of all numeric
attributes.


*69*.  Any implementation-defined characteristics of the input-output
packages.  See A.7(14).

There are no special implementation defined characteristics for these
packages.


*70*.  The value of `Buffer_Size' in `Storage_IO'.  See A.9(10).

All type representations are contiguous, and the `Buffer_Size' is the
value of `TYPE'Size' rounded up to the next storage unit boundary.


*71*.  External files for standard input, standard output, and standard
error See A.10(5).

These files are mapped onto the files provided by the C streams
libraries.  See source file `i-cstrea.ads' for further details.


*72*.  The accuracy of the value produced by `Put'.  See A.10.9(36).

If more digits are requested in the output than are represented by the
precision of the value, zeroes are output in the corresponding least
significant digit positions.


*73*.  The meaning of `Argument_Count', `Argument', and `Command_Name'.
See A.15(1).

These are mapped onto the `argv' and `argc' parameters of the main
program in the natural manner.


*74*.  Implementation-defined convention names.  See B.1(11).

The following convention names are supported

`Ada'
     Ada

`Assembler'
     Assembly language

`Asm'
     Synonym for Assembler

`Assembly'
     Synonym for Assembler

`C'
     C

`C_Pass_By_Copy'
     Allowed only for record types, like C, but also notes that record
     is to be passed by copy rather than reference.

`COBOL'
     COBOL

`CPP'
     C++

`Default'
     Treated the same as C

`External'
     Treated the same as C

`Fortran'
     Fortran

`Intrinsic'
     For support of pragma `Import' with convention Intrinsic, see
     separate section on Intrinsic Subprograms.

`Stdcall'
     Stdcall (used for Windows implementations only).  This convention
     correspond to the WINAPI (previously called Pascal convention)
     C/C++ convention under Windows.  A function with this convention
     cleans the stack before exit.

`DLL'
     Synonym for Stdcall

`Win32'
     Synonym for Stdcall

`Stubbed'
     Stubbed is a special convention used to indicate that the body of
     the subprogram will be entirely ignored.  Any call to the
     subprogram is converted into a raise of the `Program_Error'
     exception.  If a pragma `Import' specifies convention `stubbed'
     then no body need be present at all.  This convention is useful
     during development for the inclusion of subprograms whose body has
     not yet been written.

In addition, all otherwise unrecognized convention names are also
treated as being synonymous with convention C.  In all implementations
except for VMS, use of such other names results in a warning.  In VMS
implementations, these names are accepted silently.


*75*.  The meaning of link names.  See B.1(36).

Link names are the actual names used by the linker.


*76*.  The manner of choosing link names when neither the link name nor
the address of an imported or exported entity is specified.  See
B.1(36).

The default linker name is that which would be assigned by the relevant
external language, interpreting the Ada name as being in all lower case
letters.


*77*.  The effect of pragma `Linker_Options'.  See B.1(37).

The string passed to `Linker_Options' is presented uninterpreted as an
argument to the link command, unless it contains Ascii.NUL characters.
NUL characters if they appear act as argument separators, so for example

     pragma Linker_Options ("-labc" & ASCII.Nul & "-ldef");

causes two separate arguments `-labc' and `-ldef' to be passed to the
linker. The order of linker options is preserved for a given unit. The
final list of options passed to the linker is in reverse order of the
elaboration order. For example, linker options fo a body always appear
before the options from the corresponding package spec.


*78*.  The contents of the visible part of package `Interfaces' and its
language-defined descendants.  See B.2(1).

See files with prefix `i-' in the distributed library.


*79*.  Implementation-defined children of package `Interfaces'.  The
contents of the visible part of package `Interfaces'.  See B.2(11).

See files with prefix `i-' in the distributed library.


*80*.  The types `Floating', `Long_Floating', `Binary', `Long_Binary',
`Decimal_ Element', and `COBOL_Character'; and the initialization of
the variables `Ada_To_COBOL' and `COBOL_To_Ada', in `Interfaces.COBOL'.
See B.4(50).

`Floating'
     Float

`Long_Floating'
     (Floating) Long_Float

`Binary'
     Integer

`Long_Binary'
     Long_Long_Integer

`Decimal_Element'
     Character

`COBOL_Character'
     Character

For initialization, see the file `i-cobol.ads' in the distributed
library.


*81*.  Support for access to machine instructions.  See C.1(1).

See documentation in file `s-maccod.ads' in the distributed library.


*82*.  Implementation-defined aspects of access to machine operations.
See C.1(9).

See documentation in file `s-maccod.ads' in the distributed library.


*83*.  Implementation-defined aspects of interrupts.  See C.3(2).

Interrupts are mapped to signals or conditions as appropriate.  See
definition of unit `Ada.Interrupt_Names' in source file `a-intnam.ads'
for details on the interrupts supported on a particular target.


*84*.  Implementation-defined aspects of pre-elaboration.  See C.4(13).

GNAT does not permit a partition to be restarted without reloading,
except under control of the debugger.


*85*.  The semantics of pragma `Discard_Names'.  See C.5(7).

Pragma `Discard_Names' causes names of enumeration literals to be
suppressed.  In the presence of this pragma, the Image attribute
provides the image of the Pos of the literal, and Value accepts Pos
values.


*86*.  The result of the `Task_Identification.Image' attribute.  See
C.7.1(7).

The result of this attribute is an 8-digit hexadecimal string
representing the virtual address of the task control block.


*87*.  The value of `Current_Task' when in a protected entry or
interrupt handler.  See C.7.1(17).

Protected entries or interrupt handlers can be executed by any
convenient thread, so the value of `Current_Task' is undefined.


*88*.  The effect of calling `Current_Task' from an entry body or
interrupt handler.  See C.7.1(19).

The effect of calling `Current_Task' from an entry body or interrupt
handler is to return the identification of the task currently executing
the code.


*89*.  Implementation-defined aspects of `Task_Attributes'.  See
C.7.2(19).

There are no implementation-defined aspects of `Task_Attributes'.


*90*.  Values of all `Metrics'.  See D(2).

The metrics information for GNAT depends on the performance of the
underlying operating system.  The sources of the run-time for tasking
implementation, together with the output from `-gnatG' can be used to
determine the exact sequence of operating systems calls made to
implement various tasking constructs.  Together with appropriate
information on the performance of the underlying operating system, on
the exact target in use, this information can be used to determine the
required metrics.


*91*.  The declarations of `Any_Priority' and `Priority'.  See D.1(11).

See declarations in file `system.ads'.


*92*.  Implementation-defined execution resources.  See D.1(15).

There are no implementation-defined execution resources.


*93*.  Whether, on a multiprocessor, a task that is waiting for access
to a protected object keeps its processor busy.  See D.2.1(3).

On a multi-processor, a task that is waiting for access to a protected
object does not keep its processor busy.


*94*.  The affect of implementation defined execution resources on task
dispatching.  See D.2.1(9).

Tasks map to threads in the threads package used by GNAT.  Where
possible and appropriate, these threads correspond to native threads of
the underlying operating system.


*95*.  Implementation-defined `policy_identifiers' allowed in a pragma
`Task_Dispatching_Policy'.  See D.2.2(3).

There are no implementation-defined policy-identifiers allowed in this
pragma.


*96*.  Implementation-defined aspects of priority inversion.  See
D.2.2(16).

Execution of a task cannot be preempted by the implementation processing
of delay expirations for lower priority tasks.


*97*.  Implementation defined task dispatching.  See D.2.2(18).

The policy is the same as that of the underlying threads implementation.


*98*.  Implementation-defined `policy_identifiers' allowed in a pragma
`Locking_Policy'.  See D.3(4).

The only implementation defined policy permitted in GNAT is
`Inheritance_Locking'.  On targets that support this policy, locking is
implemented by inheritance, i.e. the task owning the lock operates at a
priority equal to the highest priority of any task currently requesting
the lock.


*99*.  Default ceiling priorities.  See D.3(10).

The ceiling priority of protected objects of the type
`System.Interrupt_Priority'Last' as described in the Ada 95 Reference
Manual D.3(10),


*100*.  The ceiling of any protected object used internally by the
implementation.  See D.3(16).

The ceiling priority of internal protected objects is
`System.Priority'Last'.


*101*.  Implementation-defined queuing policies.  See D.4(1).

There are no implementation-defined queueing policies.


*102*.  On a multiprocessor, any conditions that cause the completion
of an aborted construct to be delayed later than what is specified for
a single processor.  See D.6(3).

The semantics for abort on a multi-processor is the same as on a single
processor, there are no further delays.


*103*.  Any operations that implicitly require heap storage allocation.
See D.7(8).

The only operation that implicitly requires heap storage allocation is
task creation.


*104*.  Implementation-defined aspects of pragma `Restrictions'.  See
D.7(20).

There are no such implementation-defined aspects.


*105*.  Implementation-defined aspects of package `Real_Time'.  See
D.8(17).

There are no implementation defined aspects of package `Real_Time'.


*106*.  Implementation-defined aspects of `delay_statements'.  See
D.9(8).

Any difference greater than one microsecond will cause the task to be
delayed (see D.9(7)).


*107*.  The upper bound on the duration of interrupt blocking caused by
the implementation.  See D.12(5).

The upper bound is determined by the underlying operating system.  In
no cases is it more than 10 milliseconds.


*108*.  The means for creating and executing distributed programs.  See
E(5).

The GLADE package provides a utility GNATDIST for creating and executing
distributed programs.  See the GLADE reference manual for further
details.


*109*.  Any events that can result in a partition becoming
inaccessible.  See E.1(7).

See the GLADE reference manual for full details on such events.


*110*.  The scheduling policies, treatment of priorities, and
management of shared resources between partitions in certain cases.  See
E.1(11).

See the GLADE reference manual for full details on these aspects of
multi-partition execution.


*111*.  Events that cause the version of a compilation unit to change.
See E.3(5).

Editing the source file of a compilation unit, or the source files of
any units on which it is dependent in a significant way cause the
version to change.  No other actions cause the version number to
change.  All changes are significant except those which affect only
layout, capitalization or comments.


*112*.  Whether the execution of the remote subprogram is immediately
aborted as a result of cancellation.  See E.4(13).

See the GLADE reference manual for details on the effect of abort in a
distributed application.


*113*.  Implementation-defined aspects of the PCS.  See E.5(25).

See the GLADE reference manual for a full description of all
implementation defined aspects of the PCS.


*114*.  Implementation-defined interfaces in the PCS.  See E.5(26).

See the GLADE reference manual for a full description of all
implementation defined interfaces.


*115*.  The values of named numbers in the package `Decimal'.  See
F.2(7).

`Max_Scale'
     +18

`Min_Scale'
     -18

`Min_Delta'
     1.0E-18

`Max_Delta'
     1.0E+18

`Max_Decimal_Digits'
     18


*116*.  The value of `Max_Picture_Length' in the package
`Text_IO.Editing'.  See F.3.3(16).

64


*117*.  The value of `Max_Picture_Length' in the package
`Wide_Text_IO.Editing'.  See F.3.4(5).

64


*118*.  The accuracy actually achieved by the complex elementary
functions and by other complex arithmetic operations.  See G.1(1).

Standard library functions are used for the complex arithmetic
operations.  Only fast math mode is currently supported.


*119*.  The sign of a zero result (or a component thereof) from any
operator or function in `Numerics.Generic_Complex_Types', when
`Real'Signed_Zeros' is True.  See G.1.1(53).

The signs of zero values are as recommended by the relevant
implementation advice.


*120*.  The sign of a zero result (or a component thereof) from any
operator or function in
`Numerics.Generic_Complex_Elementary_Functions', when
`Real'Signed_Zeros' is `True'.  See G.1.2(45).

The signs of zero values are as recommended by the relevant
implementation advice.


*121*.  Whether the strict mode or the relaxed mode is the default.
See G.2(2).

The strict mode is the default.  There is no separate relaxed mode.
GNAT provides a highly efficient implementation of strict mode.


*122*.  The result interval in certain cases of fixed-to-float
conversion.  See G.2.1(10).

For cases where the result interval is implementation dependent, the
accuracy is that provided by performing all operations in 64-bit IEEE
floating-point format.


*123*.  The result of a floating point arithmetic operation in overflow
situations, when the `Machine_Overflows' attribute of the result type
is `False'.  See G.2.1(13).

Infinite and Nan values are produced as dictated by the IEEE
floating-point standard.


*124*.  The result interval for division (or exponentiation by a
negative exponent), when the floating point hardware implements division
as multiplication by a reciprocal.  See G.2.1(16).

Not relevant, division is IEEE exact.


*125*.  The definition of close result set, which determines the
accuracy of certain fixed point multiplications and divisions.  See
G.2.3(5).

Operations in the close result set are performed using IEEE long format
floating-point arithmetic.  The input operands are converted to
floating-point, the operation is done in floating-point, and the result
is converted to the target type.


*126*.  Conditions on a `universal_real' operand of a fixed point
multiplication or division for which the result shall be in the perfect
result set.  See G.2.3(22).

The result is only defined to be in the perfect result set if the result
can be computed by a single scaling operation involving a scale factor
representable in 64-bits.


*127*.  The result of a fixed point arithmetic operation in overflow
situations, when the `Machine_Overflows' attribute of the result type
is `False'.  See G.2.3(27).

Not relevant, `Machine_Overflows' is `True' for fixed-point types.


*128*.  The result of an elementary function reference in overflow
situations, when the `Machine_Overflows' attribute of the result type
is `False'.  See G.2.4(4).

IEEE infinite and Nan values are produced as appropriate.


*129*.  The value of the angle threshold, within which certain
elementary functions, complex arithmetic operations, and complex
elementary functions yield results conforming to a maximum relative
error bound.  See G.2.4(10).

Information on this subject is not yet available.


*130*.  The accuracy of certain elementary functions for parameters
beyond the angle threshold.  See G.2.4(10).

Information on this subject is not yet available.


*131*.  The result of a complex arithmetic operation or complex
elementary function reference in overflow situations, when the
`Machine_Overflows' attribute of the corresponding real type is
`False'.  See G.2.6(5).

IEEE infinite and Nan values are produced as appropriate.


*132*.  The accuracy of certain complex arithmetic operations and
certain complex elementary functions for parameters (or components
thereof) beyond the angle threshold.  See G.2.6(8).

Information on those subjects is not yet available.


*133*.  Information regarding bounded errors and erroneous execution.
See H.2(1).

Information on this subject is not yet available.


*134*.  Implementation-defined aspects of pragma `Inspection_Point'.
See H.3.2(8).

Pragma `Inspection_Point' ensures that the variable is live and can be
examined by the debugger at the inspection point.


*135*.  Implementation-defined aspects of pragma `Restrictions'.  See
H.4(25).

There are no implementation-defined aspects of pragma `Restrictions'.
The use of pragma `Restrictions [No_Exceptions]' has no effect on the
generated code.  Checks must suppressed by use of pragma `Suppress'.


*136*.  Any restrictions on pragma `Restrictions'.  See H.4(27).

There are no restrictions on pragma `Restrictions'.


File: gnat_rm.info,  Node: Intrinsic Subprograms,  Next: Representation Clauses and Pragmas,  Prev: Implementation Defined Characteristics,  Up: Top

Intrinsic Subprograms
*********************

* Menu:

* Intrinsic Operators::
* Enclosing_Entity::
* Exception_Information::
* Exception_Message::
* Exception_Name::
* File::
* Line::
* Rotate_Left::
* Rotate_Right::
* Shift_Left::
* Shift_Right::
* Shift_Right_Arithmetic::
* Source_Location::

   GNAT allows a user application program to write the declaration:

        pragma Import (Intrinsic, name);

providing that the name corresponds to one of the implemented intrinsic
subprograms in GNAT, and that the parameter profile of the referenced
subprogram meets the requirements.  This chapter describes the set of
implemented intrinsic subprograms, and the requirements on parameter
profiles.  Note that no body is supplied; as with other uses of pragma
Import, the body is supplied elsewhere (in this case by the compiler
itself).  Note that any use of this feature is potentially
non-portable, since the Ada standard does not require Ada compilers to
implement this feature.


File: gnat_rm.info,  Node: Intrinsic Operators,  Next: Enclosing_Entity,  Up: Intrinsic Subprograms

Intrinsic Operators
===================

All the predefined numeric operators in package Standard in `pragma
Import (Intrinsic,..)' declarations.  In the binary operator case, the
operands must have the same size.  The operand or operands must also be
appropriate for the operator.  For example, for addition, the operands
must both be floating-point or both be fixed-point, and the right
operand for `"**"' must have a root type of `Standard.Integer'Base'.
You can use an intrinsic operator declaration as in the following
example:

        type Int1 is new Integer;
        type Int2 is new Integer;
     
        function "+" (X1 : Int1; X2 : Int2) return Int1;
        function "+" (X1 : Int1; X2 : Int2) return Int2;
        pragma Import (Intrinsic, "+");

This declaration would permit "mixed mode" arithmetic on items of the
differing types `Int1' and `Int2'.  It is also possible to specify such
operators for private types, if the full views are appropriate
arithmetic types.


File: gnat_rm.info,  Node: Enclosing_Entity,  Next: Exception_Information,  Prev: Intrinsic Operators,  Up: Intrinsic Subprograms

Enclosing_Entity
================

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Source_Info'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function `GNAT.Source_Info.Enclosing_Entity' to
obtain the name of the current subprogram, package, task, entry, or
protected subprogram.


File: gnat_rm.info,  Node: Exception_Information,  Next: Exception_Message,  Prev: Enclosing_Entity,  Up: Intrinsic Subprograms

Exception_Information
=====================

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Current_Exception'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function
`GNAT.Current_Exception.Exception_Information' to obtain the exception
information associated with the current exception.


File: gnat_rm.info,  Node: Exception_Message,  Next: Exception_Name,  Prev: Exception_Information,  Up: Intrinsic Subprograms

Exception_Message
=================

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Current_Exception'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function
`GNAT.Current_Exception.Exception_Message' to obtain the message
associated with the current exception.


File: gnat_rm.info,  Node: Exception_Name,  Next: File,  Prev: Exception_Message,  Up: Intrinsic Subprograms

Exception_Name
==============

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Current_Exception'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function `GNAT.Current_Exception.Exception_Name'
to obtain the name of the current exception.


File: gnat_rm.info,  Node: File,  Next: Line,  Prev: Exception_Name,  Up: Intrinsic Subprograms

File
====

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Source_Info'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function `GNAT.Source_Info.File' to obtain the
name of the current file.


File: gnat_rm.info,  Node: Line,  Next: Rotate_Left,  Prev: File,  Up: Intrinsic Subprograms

Line
====

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Source_Info'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function `GNAT.Source_Info.Line' to obtain the
number of the current source line.


File: gnat_rm.info,  Node: Rotate_Left,  Next: Rotate_Right,  Prev: Line,  Up: Intrinsic Subprograms

Rotate_Left
===========

In standard Ada 95, the `Rotate_Left' function is available only for
the predefined modular types in package `Interfaces'.  However, in GNAT
it is possible to define a Rotate_Left function for a user defined
modular type or any signed integer type as in this example:

        function Shift_Left
          (Value  : My_Modular_Type;
           Amount : Natural)
           return   My_Modular_Type;

The requirements are that the profile be exactly as in the example
above.  The only modifications allowed are in the formal parameter
names, and in the type of `Value' and the return type, which must be
the same, and must be either a signed integer type, or a modular
integer type with a binary modulus, and the size must be 8.  16, 32 or
64 bits.


File: gnat_rm.info,  Node: Rotate_Right,  Next: Shift_Left,  Prev: Rotate_Left,  Up: Intrinsic Subprograms

Rotate_Right
============

A `Rotate_Right' function can be defined for any user defined binary
modular integer type, or signed integer type, as described above for
`Rotate_Left'.


File: gnat_rm.info,  Node: Shift_Left,  Next: Shift_Right,  Prev: Rotate_Right,  Up: Intrinsic Subprograms

Shift_Left
==========

A `Shift_Left' function can be defined for any user defined binary
modular integer type, or signed integer type, as described above for
`Rotate_Left'.


File: gnat_rm.info,  Node: Shift_Right,  Next: Shift_Right_Arithmetic,  Prev: Shift_Left,  Up: Intrinsic Subprograms

Shift_Right
===========

A `Shift_Right' function can be defined for any user defined binary
modular integer type, or signed integer type, as described above for
`Rotate_Left'.


File: gnat_rm.info,  Node: Shift_Right_Arithmetic,  Next: Source_Location,  Prev: Shift_Right,  Up: Intrinsic Subprograms

Shift_Right_Arithmetic
======================

A `Shift_Right_Arithmetic' function can be defined for any user defined
binary modular integer type, or signed integer type, as described above
for `Rotate_Left'.


File: gnat_rm.info,  Node: Source_Location,  Prev: Shift_Right_Arithmetic,  Up: Intrinsic Subprograms

Source_Location
===============

This intrinsic subprogram is used in the implementation of the library
routine `GNAT.Source_Info'.  The only useful use of the intrinsic
import in this case is the one in this unit, so an application program
should simply call the function `GNAT.Source_Info.Source_Location' to
obtain the current source file location.


File: gnat_rm.info,  Node: Representation Clauses and Pragmas,  Next: Standard Library Routines,  Prev: Intrinsic Subprograms,  Up: Top

Representation Clauses and Pragmas
**********************************

* Menu:

* Alignment Clauses::
* Size Clauses::
* Storage_Size Clauses::
* Size of Variant Record Objects::
* Biased Representation ::
* Value_Size and Object_Size Clauses::
* Component_Size Clauses::
* Bit_Order Clauses::
* Effect of Bit_Order on Byte Ordering::
* Pragma Pack for Arrays::
* Pragma Pack for Records::
* Record Representation Clauses::
* Enumeration Clauses::
* Address Clauses::
* Effect of Convention on Representation::
* Determining the Representations chosen by GNAT::

This section describes the representation clauses accepted by GNAT, and
their effect on the representation of corresponding data objects.

   GNAT fully implements Annex C (Systems Programming).  This means
that all the implementation advice sections in chapter 13 are fully
implemented.  However, these sections only require a minimal level of
support for representation clauses.  GNAT provides much more extensive
capabilities, and this section describes the additional capabilities
provided.


File: gnat_rm.info,  Node: Alignment Clauses,  Next: Size Clauses,  Up: Representation Clauses and Pragmas

Alignment Clauses
=================

GNAT requires that all alignment clauses specify a power of 2, and all
default alignments are always a power of 2.  The default alignment
values are as follows:

   * Primitive Types For primitive types, the alignment is the maximum
     of the actual size of objects of the type, and the maximum
     alignment supported by the target.  For example, for type
     Long_Float, the object size is 8 bytes, and the default alignment
     will be 8 on any target that supports alignments this large, but
     on some targets, the maximum alignment may be smaller than 8, in
     which case objects of type Long_Float will be maximally aligned.

   * Arrays For arrays, the alignment is equal to the alignment of the
     component type for the normal case where no packing or component
     size is given.  If the array is packed, and the packing is
     effective (see separate section on packed arrays), then the
     alignment will be one for long packed arrays, or arrays whose
     length is not known at compile time.  For short packed arrays,
     which are handled internally as modular types, the alignment will
     be as described for primitive types, e.g. a packed array of length
     31 bits will have an object size of four bytes, and an alignment
     of 4.

   * Records For the normal non-packed case, the alignment of a record
     is equal to the maximum alignment of any of its components.  For
     tagged records, this includes the implicit access type used for
     the tag.  If a pragma `Pack' is used and all fields are packable
     (see separate section on pragma `Pack'), then the resulting
     alignment is 1.

     A special case is when the size of the record is given explicitly,
     or a full record representation clause is given, and the size of
     the record is 2, 4, or 8 bytes. In this case, an alignment is
     chosen to match the size of the record. For example, if we have:

             type Small is record
                A, B : Character;
             end record;

     then the default alignment of the record type `Small' is 2, not 1.
     This leads to more efficient code when the record is treated as a
     unit, and also allows the type to specified as `Atomic' on
     architectures requiring strict alignment.


An alignment clause may always specify a larger alignment than the
default value, up to some maximum value dependent on the target
(obtainable by using the attribute reference System'Maximum_Alignment).
The only case in which it is permissible to specify a smaller
alignment than the default value is in the case of a record for which a
record representation clause is given.  In this case, packable fields
for which a component clause is given still result in a default
alignment corresponding to the original type, but this may be
overridden, since these components in fact only require an alignment of
one byte.  For example, given

       type v is record
          a : integer;
       end record;
     
       for v use record
          a at 0  range 0 .. 31;
       end record;
     
       for v'alignment use 1;

The default alignment for the type `v' is 4, as a result of the integer
field in the record, but since this field is placed with a component
clause, it is permissible, as shown, to override the default alignment
of the record to a smaller value.


File: gnat_rm.info,  Node: Size Clauses,  Next: Storage_Size Clauses,  Prev: Alignment Clauses,  Up: Representation Clauses and Pragmas

Size Clauses
============

The default size of types is as specified in the reference manual.  For
objects, GNAT will generally increase the type size so that the object
size is a multiple of storage units, and also a multiple of the
alignment.  For example

        type Smallint is range 1 .. 6;
     
        type Rec is record
           y1 : integer;
           y2 : boolean;
        end record;

In this example, `Smallint' has a size of 3, as specified by the RM
rules, but objects of this type will have a size of 8, since objects by
default occupy an integral number of storage units.  On some targets,
notably older versions of the Digital Alpha, the size of stand alone
objects of this type may be 32, reflecting the inability of the
hardware to do byte load/stores.

   Similarly, the size of type `Rec' is 40 bits, but the alignment is
4, so objects of this type will have their size increased to 64 bits so
that it is a multiple of the alignment.  The reason for this decision,
which is in accordance with the specific note in RM 13.3(43):

     A Size clause should be supported for an object if the specified
     Size is at least as large as its subtype's Size, and corresponds
     to a size in storage elements that is a multiple of the object's
     Alignment (if the Alignment is nonzero).

An explicit size clause may be used to override the default size by
increasing it.  For example, if we have:

        type My_Boolean is new Boolean;
        for My_Boolean'Size use 32;

then objects of this type will always be 32 bits long.  In the case of
discrete types, the size can be increased up to 64 bits, with the effect
that the entire specified field is used to hold the value, sign- or
zero-extended as appropriate.  If more than 64 bits is specified, then
padding space is allocated after the value, and a warning is issued that
there are unused bits.

   Similarly the size of records and arrays may be increased, and the
effect is to add padding bits after the value.  This also causes a
warning message to be generated.

   The largest Size value permitted in GNAT is 2**32-1.  Since this is a
Size in bits, this corresponds to an object of size 256 megabytes (minus
one).  This limitation is true on all targets.  The reason for this
limitation is that it improves the quality of the code in many cases if
it is known that a Size value can be accommodated in an object of type
Integer.


File: gnat_rm.info,  Node: Storage_Size Clauses,  Next: Size of Variant Record Objects,  Prev: Size Clauses,  Up: Representation Clauses and Pragmas

Storage_Size Clauses
====================

For tasks, the `Storage_Size' clause specifies the amount of space to
be allocated for the task stack.  This cannot be extended, and if the
stack is exhausted, then `Storage_Error' will be raised if stack
checking is enabled.  If the default size of 20K bytes is insufficient,
then you need to use a `Storage_Size' attribute definition clause, or a
`Storage_Size' pragma in the task definition to set the appropriate
required size.  A useful technique is to include in every task
definition a pragma of the form:

        pragma Storage_Size (Default_Stack_Size);

Then Default_Stack_Size can be defined in a global package, and modified
as required.  Any tasks requiring different task stack sizes from the
default can have an appropriate alternative reference in the pragma.

   For access types, the `Storage_Size' clause specifies the maximum
space available for allocation of objects of the type.  If this space is
exceeded then `Storage_Error' will be raised by an allocation attempt.
In the case where the access type is declared local to a subprogram, the
use of a `Storage_Size' clause triggers automatic use of a special
predefined storage pool (`System.Pool_Size') that ensures that all
space for the pool is automatically reclaimed on exit from the scope in
which the type is declared.

   A special case recognized by the compiler is the specification of a
`Storage_Size' of zero for an access type.  This means that no items
can be allocated from the pool, and this is recognized at compile time,
and all the overhead normally associated with maintaining a fixed size
storage pool is eliminated.  Consider the following example:

        procedure p is
           type R is array (Natural) of Character;
           type P is access all R;
           for P'Storage_Size use 0;
           --  Above access type intended only for interfacing purposes
     
           y : P;
     
           procedure g (m : P);
           pragma Import (C, g);
     
           --  ...
     
        begin
           --  ...
           y := new R;
        end;

As indicated in this example, these dummy storage pools are often
useful in connection with interfacing where no object will ever be
allocated.  If you compile the above example, you get the warning:

        p.adb:16:09: warning: allocation from empty storage pool
        p.adb:16:09: warning: Storage_Error will be raised at run time

Of course in practice, there will not be any explicit allocators in the
case of such an access declaration.


File: gnat_rm.info,  Node: Size of Variant Record Objects,  Next: Biased Representation,  Prev: Storage_Size Clauses,  Up: Representation Clauses and Pragmas

Size of Variant Record Objects
==============================

An issue arises in the case of variant record objects of whether Size
gives information about a particular variant, or the maximum size
required for any variant.  Consider the following program

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;
     
        V1 : R1 (False);
        V2 : R1;
     
     begin
        Put_Line (Integer'Image (V1'Size));
        Put_Line (Integer'Image (V2'Size));
     end q;

Here we are dealing with a variant record, where the True variant
requires 16 bits, and the False variant requires 8 bits.  In the above
example, both V1 and V2 contain the False variant, which is only 8 bits
long.  However, the result of running the program is:

     8
     16

The reason for the difference here is that the discriminant value of V1
is fixed, and will always be False.  It is not possible to assign a
True variant value to V1, therefore 8 bits is sufficient.  On the other
hand, in the case of V2, the initial discriminant value is False (from
the default), but it is possible to assign a True variant value to V2,
therefore 16 bits must be allocated for V2 in the general case, even
fewer bits may be needed at any particular point during the program
execution.

   As can be seen from the output of this program, the `'Size'
attribute applied to such an object in GNAT gives the actual allocated
size of the variable, which is the largest size of any of the variants.
The Ada Reference Manual is not completely clear on what choice should
be made here, but the GNAT behavior seems most consistent with the
language in the RM.

   In some cases, it may be desirable to obtain the size of the current
variant, rather than the size of the largest variant.  This can be
achieved in GNAT by making use of the fact that in the case of a
subprogram parameter, GNAT does indeed return the size of the current
variant (because a subprogram has no way of knowing how much space is
actually allocated for the actual).

   Consider the following modified version of the above program:

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;
     
        V2 : R1;
     
        function Size (V : R1) return Integer is
        begin
           return V'Size;
        end Size;
     
     begin
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
        V2 := (True, 'x');
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
     end q;

The output from this program is

     16
     8
     16
     16

Here we see that while the `'Size' attribute always returns the maximum
size, regardless of the current variant value, the `Size' function does
indeed return the size of the current variant value.


File: gnat_rm.info,  Node: Biased Representation,  Next: Value_Size and Object_Size Clauses,  Prev: Size of Variant Record Objects,  Up: Representation Clauses and Pragmas

Biased Representation
=====================

In the case of scalars with a range starting at other than zero, it is
possible in some cases to specify a size smaller than the default
minimum value, and in such cases, GNAT uses an unsigned biased
representation, in which zero is used to represent the lower bound, and
successive values represent successive values of the type.

   For example, suppose we have the declaration:

        type Small is range -7 .. -4;
        for Small'Size use 2;

Although the default size of type `Small' is 4, the `Size' clause is
accepted by GNAT and results in the following representation scheme:

       -7 is represented as 2#00#
       -6 is represented as 2#01#
       -5 is represented as 2#10#
       -4 is represented as 2#11#

Biased representation is only used if the specified `Size' clause
cannot be accepted in any other manner.  These reduced sizes that force
biased representation can be used for all discrete types except for
enumeration types for which a representation clause is given.


File: gnat_rm.info,  Node: Value_Size and Object_Size Clauses,  Next: Component_Size Clauses,  Prev: Biased Representation,  Up: Representation Clauses and Pragmas

Value_Size and Object_Size Clauses
==================================

In Ada 95, the `Size' of a discrete type is the minimum number of bits
required to hold values of the type.  Although this interpretation was
allowed in Ada 83, it was not required, and this requirement in practice
can cause some significant difficulties.  For example, in most Ada 83
compilers, `Natural'Size' was 32.  However, in Ada-95, `Natural'Size' is
typically 31.  This means that code may change in behavior when moving
from Ada 83 to Ada 95.  For example, consider:

        type Rec is record;
           A : Natural;
           B : Natural;
        end record;
     
        for Rec use record
           for A use at 0  range 0 .. Natural'Size - 1;
           for B use at 0  range Natural'Size .. 2 * Natural'Size - 1;
        end record;

In the above code, since the typical size of `Natural' objects is 32
bits and `Natural'Size' is 31, the above code can cause unexpected
inefficient packing in Ada 95, and in general there are surprising
cases where the fact that the object size can exceed the size of the
type causes surprises.

   To help get around this problem GNAT provides two implementation
dependent attributes `Value_Size' and `Object_Size'.  When applied to a
type, these attributes yield the size of the type (corresponding to the
RM defined size attribute), and the size of objects of the type
respectively.

   The `Object_Size' is used for determining the default size of
objects and components.  This size value can be referred to using the
`Object_Size' attribute.  The phrase "is used" here means that it is
the basis of the determination of the size.  The backend is free to pad
this up if necessary for efficiency, e.g. an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.

   The default rules for the value of `Object_Size' for fixed-point and
discrete types are as follows:

   * The `Object_Size' for base subtypes reflect the natural hardware
     size in bits (run the utility `gnatpsta' to find those values for
     numeric types).  Enumeration types and fixed-point base subtypes
     have 8, 16, 32 or 64 bits for this size, depending on the range of
     values to be stored.

   * The `Object_Size' of a subtype is the same as the `Object_Size' of
     the type from which it is obtained.

   * The `Object_Size' of a derived base type is copied from the parent
     base type, and the `Object_Size' of a derived first subtype is
     copied from the parent first subtype.

The `Value_Size' attribute is the number of bits required to store a
value of the type.  This size can be referred to using the `Value_Size'
attribute.  This value is used to determine how tightly to pack records
or arrays with components of this type, and also affects the semantics
of unchecked conversion (unchecked conversions where the `Value_Size'
values differ generate a warning, and are potentially target dependent).

   The default rules for the value of `Value_Size' are as follows:

   * The `Value_Size' for a base subtype is the minimum number of bits
     required to store all values of the type (including the sign bit
     only if negative values are possible).

   * If a subtype statically matches the first subtype of a given type,
     then it has by default the same `Value_Size' as the first subtype.
     This is a consequence of RM 13.1(14) ("if two subtypes statically
     match, then their subtype-specific aspects are the same".)

   * All other subtypes have a `Value_Size' corresponding to the minimum
     number of bits required to store all values of the subtype.  For
     dynamic bounds, it is assumed that the value can range down or up
     to the corresponding bound of the ancestor

The RM defined attribute `Size' corresponds to the `Value_Size'
attribute.

   The `Size' attribute may be defined for a first-named subtype.  This
sets the `Value_Size' of the first-named subtype to the given value,
and the `Object_Size' of this first-named subtype to the given value
padded up to an appropriate boundary.  It is a consequence of the
default rules above that this `Object_Size' will apply to all further
subtypes.  On the other hand, `Value_Size' is affected only for the
first subtype, any dynamic subtypes obtained from it directly, and any
statically matching subtypes.  The `Value_Size' of any other static
subtypes is not affected.

   `Value_Size' and `Object_Size' may be explicitly set for any subtype
using an attribute definition clause.  Note that the use of these
attributes can cause the RM 13.1(14) rule to be violated.  If two
access types reference aliased objects whose subtypes have differing
`Object_Size' values as a result of explicit attribute definition
clauses, then it is erroneous to convert from one access subtype to the
other.

   At the implementation level, Esize stores the Object_SIze and the
RM_Size field stores the `Value_Size' (and hence the value of the
`Size' attribute, which, as noted above, is equivalent to `Value_Size').

   To get a feel for the difference, consider the following examples
(note that in each case the base is short_short_integer with a size of
8):

                                            Object_Size     Value_Size
     
     type x1 is range 0 .. 5;                    8               3
     
     type x2 is range 0 .. 5;
     for x2'size use 12;                        12              12
     
     subtype x3 is x2 range 0 .. 3;             12               2
     
     subtype x4 is x2'base range 0 .. 10;        8               4
     
     subtype x5 is x2 range 0 .. dynamic;       12              (7)
     
     subtype x6 is x2'base range 0 .. dynamic;   8              (7)

Note: the entries marked (7) are not actually specified by the Ada 95
RM, but it seems in the spirit of the RM rules to allocate the minimum
number of bits known to be large enough to hold the given range of
values.

   So far, so good, but GNAT has to obey the RM rules, so the question
is under what conditions must the RM `Size' be used.  The following is
a list of the occasions on which the RM `Size' must be used:

   * Component size for packed arrays or records

   * Value of the attribute `Size' for a type

   * Warning about sizes not matching for unchecked conversion

For types other than discrete and fixed-point types, the `Object_Size'
and Value_Size are the same (and equivalent to the RM attribute `Size').
Only `Size' may be specified for such types.


File: gnat_rm.info,  Node: Component_Size Clauses,  Next: Bit_Order Clauses,  Prev: Value_Size and Object_Size Clauses,  Up: Representation Clauses and Pragmas

Component_Size Clauses
======================

Normally, the value specified in a component clause must be consistent
with the subtype of the array component with regard to size and
alignment.  In other words, the value specified must be at least equal
to the size of this subtype, and must be a multiple of the alignment
value.

   In addition, component size clauses are allowed which cause the array
to be packed, by specifying a smaller value.  The cases in which this
is allowed are for component size values in the range 1 through 63.
The value specified must not be smaller than the Size of the subtype.
GNAT will accurately honor all packing requests in this range.  For
example, if we have:

     type r is array (1 .. 8) of Natural;
     for r'Size use 31;

then the resulting array has a length of 31 bytes (248 bits = 8 * 31).
Of course access to the components of such an array is considerably
less efficient than if the natural component size of 32 is used.


File: gnat_rm.info,  Node: Bit_Order Clauses,  Next: Effect of Bit_Order on Byte Ordering,  Prev: Component_Size Clauses,  Up: Representation Clauses and Pragmas

Bit_Order Clauses
=================

For record subtypes, GNAT permits the specification of the `Bit_Order'
attribute.  The specification may either correspond to the default bit
order for the target, in which case the specification has no effect and
places no additional restrictions, or it may be for the non-standard
setting (that is the opposite of the default).

   In the case where the non-standard value is specified, the effect is
to renumber bits within each byte, but the ordering of bytes is not
affected.  There are certain restrictions placed on component clauses
as follows:

   * Components fitting within a single storage unit.

     These are unrestricted, and the effect is merely to renumber bits.
     For example if we are on a little-endian machine with
     `Low_Order_First' being the default, then the following two
     declarations have exactly the same effect:

             type R1 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;
          
             for R1 use record
                A at 0 range 0 .. 0;
                B at 0 range 1 .. 7;
             end record;
          
             type R2 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;
          
             for R2'Bit_Order use High_Order_First;
          
             for R2 use record
                A at 0 range 7 .. 7;
                B at 0 range 0 .. 6;
             end record;

     The useful application here is to write the second declaration
     with the `Bit_Order' attribute definition clause, and know that it
     will be treated the same, regardless of whether the target is
     little-endian or big-endian.

   * Components occupying an integral number of bytes.

     These are components that exactly fit in two or more bytes.  Such
     component declarations are allowed, but have no effect, since it
     is important to realize that the `Bit_Order' specification does
     not affect the ordering of bytes.  In particular, the following
     attempt at getting an endian-independent integer does not work:

             type R2 is record
                A : Integer;
             end record;
          
             for R2'Bit_Order use High_Order_First;
          
             for R2 use record
                A at 0 range 0 .. 31;
             end record;

     This declaration will result in a little-endian integer on a
     little-endian machine, and a big-endian integer on a big-endian
     machine.  If byte flipping is required for interoperability
     between big- and little-endian machines, this must be explicitly
     programmed.  This capability is not provided by `Bit_Order'.

   * Components that are positioned across byte boundaries

     but do not occupy an integral number of bytes.  Given that bytes
     are not reordered, such fields would occupy a non-contiguous
     sequence of bits in memory, requiring non-trivial code to
     reassemble.  They are for this reason not permitted, and any
     component clause specifying such a layout will be flagged as
     illegal by GNAT.


Since the misconception that Bit_Order automatically deals with all
endian-related incompatibilities is a common one, the specification of
a component field that is an integral number of bytes will always
generate a warning.  This warning may be suppressed using `pragma
Suppress' if desired.  The following section contains additional
details regarding the issue of byte ordering.


File: gnat_rm.info,  Node: Effect of Bit_Order on Byte Ordering,  Next: Pragma Pack for Arrays,  Prev: Bit_Order Clauses,  Up: Representation Clauses and Pragmas

Effect of Bit_Order on Byte Ordering
====================================

In this section we will review the effect of the `Bit_Order' attribute
definition clause on byte ordering.  Briefly, it has no effect at all,
but a detailed example will be helpful.  Before giving this example,
let us review the precise definition of the effect of defining
`Bit_Order'.  The effect of a non-standard bit order is described in
section 15.5.3 of the Ada Reference Manual:

     2   A bit ordering is a method of interpreting the meaning of
     the storage place attributes.

To understand the precise definition of storage place attributes in
this context, we visit section 13.5.1 of the manual:

     13   A record_representation_clause (without the mod_clause)
     specifies the layout.  The storage place attributes (see 13.5.2)
     are taken from the values of the position, first_bit, and last_bit
     expressions after normalizing those values so that first_bit is
     less than Storage_Unit.

The critical point here is that storage places are taken from the
values after normalization, not before.  So the `Bit_Order'
interpretation applies to normalized values.  The interpretation is
described in the later part of the 15.5.3 paragraph:

     2   A bit ordering is a method of interpreting the meaning of
     the storage place attributes.  High_Order_First (known in the
     vernacular as ``big endian'') means that the first bit of a
     storage element (bit 0) is the most significant bit (interpreting
     the sequence of bits that represent a component as an unsigned
     integer value).  Low_Order_First (known in the vernacular as
     ``little endian'') means the opposite: the first bit is the
     least significant.

Note that the numbering is with respect to the bits of a storage unit.
In other words, the specification affects only the numbering of bits
within a single storage unit.

   We can make the effect clearer by giving an example.

   Suppose that we have an external device which presents two bytes,
the first byte presented, which is the first (low addressed byte) of
the two byte record is called Master, and the second byte is called
Slave.

   The left most (most significant bit is called Control for each byte,
and the remaining 7 bits are called V1, V2, ... V7, where V7 is the
rightmost (least significant) bit.

   On a big-endian machine, we can write the following representation
clause

        type Data is record
           Master_Control : Bit;
           Master_V1      : Bit;
           Master_V2      : Bit;
           Master_V3      : Bit;
           Master_V4      : Bit;
           Master_V5      : Bit;
           Master_V6      : Bit;
           Master_V7      : Bit;
           Slave_Control  : Bit;
           Slave_V1       : Bit;
           Slave_V2       : Bit;
           Slave_V3       : Bit;
           Slave_V4       : Bit;
           Slave_V5       : Bit;
           Slave_V6       : Bit;
           Slave_V7       : Bit;
        end record;
     
        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Now if we move this to a little endian machine, then the bit ordering
within the byte is backwards, so we have to rewrite the record rep
clause as:

        for Data use record
           Master_Control at 0 range 7 .. 7;
           Master_V1      at 0 range 6 .. 6;
           Master_V2      at 0 range 5 .. 5;
           Master_V3      at 0 range 4 .. 4;
           Master_V4      at 0 range 3 .. 3;
           Master_V5      at 0 range 2 .. 2;
           Master_V6      at 0 range 1 .. 1;
           Master_V7      at 0 range 0 .. 0;
           Slave_Control  at 1 range 7 .. 7;
           Slave_V1       at 1 range 6 .. 6;
           Slave_V2       at 1 range 5 .. 5;
           Slave_V3       at 1 range 4 .. 4;
           Slave_V4       at 1 range 3 .. 3;
           Slave_V5       at 1 range 2 .. 2;
           Slave_V6       at 1 range 1 .. 1;
           Slave_V7       at 1 range 0 .. 0;
        end record;

   It is a nuisance to have to rewrite the clause, especially if the
code has to be maintained on both machines.  However, this is a case
that we can handle with the `Bit_Order' attribute if it is implemented.
Note that the implementation is not required on byte addressed
machines, but it is indeed implemented in GNAT.  This means that we can
simply use the first record clause, together with the declaration

        for Data'Bit_Order use High_Order_First;

and the effect is what is desired, namely the layout is exactly the
same, independent of whether the code is compiled on a big-endian or
little-endian machine.

   The important point to understand is that byte ordering is not
affected.  A `Bit_Order' attribute definition never affects which byte
a field ends up in, only where it ends up in that byte.  To make this
clear, let us rewrite the record rep clause of the previous example as:

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range  0 .. 0;
           Master_V1      at 0 range  1 .. 1;
           Master_V2      at 0 range  2 .. 2;
           Master_V3      at 0 range  3 .. 3;
           Master_V4      at 0 range  4 .. 4;
           Master_V5      at 0 range  5 .. 5;
           Master_V6      at 0 range  6 .. 6;
           Master_V7      at 0 range  7 .. 7;
           Slave_Control  at 0 range  8 .. 8;
           Slave_V1       at 0 range  9 .. 9;
           Slave_V2       at 0 range 10 .. 10;
           Slave_V3       at 0 range 11 .. 11;
           Slave_V4       at 0 range 12 .. 12;
           Slave_V5       at 0 range 13 .. 13;
           Slave_V6       at 0 range 14 .. 14;
           Slave_V7       at 0 range 15 .. 15;
        end record;

This is exactly equivalent to saying (a repeat of the first example):

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Why are they equivalent? Well take a specific field, the `Slave_V2'
field.  The storage place attributes are obtained by normalizing the
values given so that the `First_Bit' value is less than 8.  After
nromalizing the values (0,10,10) we get (1,2,2) which is exactly what
we specified in the other case.

   Now one might expect that the `Bit_Order' attribute might affect bit
numbering within the entire record component (two bytes in this case,
thus affecting which byte fields end up in), but that is not the way
this feature is defined, it only affects numbering of bits, not which
byte they end up in.

   Consequently it never makes sense to specify a starting bit number
greater than 7 (for a byte addressable field) if an attribute
definition for `Bit_Order' has been given, and indeed it may be
actively confusing to specify such a value, so the compiler generates a
warning for such usage.

   If you do need to control byte ordering then appropriate conditional
values must be used.  If in our example, the slave byte came first on
some machines we might write:

        Master_Byte_First constant Boolean := ...;
     
        Master_Byte : constant Natural :=
                        1 - Boolean'Pos (Master_Byte_First);
        Slave_Byte  : constant Natural :=
                        Boolean'Pos (Master_Byte_First);
     
        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at Master_Byte range 0 .. 0;
           Master_V1      at Master_Byte range 1 .. 1;
           Master_V2      at Master_Byte range 2 .. 2;
           Master_V3      at Master_Byte range 3 .. 3;
           Master_V4      at Master_Byte range 4 .. 4;
           Master_V5      at Master_Byte range 5 .. 5;
           Master_V6      at Master_Byte range 6 .. 6;
           Master_V7      at Master_Byte range 7 .. 7;
           Slave_Control  at Slave_Byte  range 0 .. 0;
           Slave_V1       at Slave_Byte  range 1 .. 1;
           Slave_V2       at Slave_Byte  range 2 .. 2;
           Slave_V3       at Slave_Byte  range 3 .. 3;
           Slave_V4       at Slave_Byte  range 4 .. 4;
           Slave_V5       at Slave_Byte  range 5 .. 5;
           Slave_V6       at Slave_Byte  range 6 .. 6;
           Slave_V7       at Slave_Byte  range 7 .. 7;
        end record;

Now to switch between machines, all that is necessary is to set the
boolean constant `Master_Byte_First' in an appropriate manner.


File: gnat_rm.info,  Node: Pragma Pack for Arrays,  Next: Pragma Pack for Records,  Prev: Effect of Bit_Order on Byte Ordering,  Up: Representation Clauses and Pragmas

Pragma Pack for Arrays
======================

Pragma `Pack' applied to an array has no effect unless the component
type is packable.  For a component type to be packable, it must be one
of the following cases:

   * Any scalar type

   * Any fixed-point type

   * Any type whose size is specified with a size clause

   * Any packed array type with a static size

For all these cases, if the component subtype size is in the range 1
through 63, then the effect of the pragma `Pack' is exactly as though a
component size were specified giving the component subtype size.  For
example if we have:

        type r is range 0 .. 17;
     
        type ar is array (1 .. 8) of r;
        pragma Pack (ar);

Then the component size of `ar' will be set to 5 (i.e. to `r'size', and
the size of the array `ar' will be exactly 40 bits.

   Note that in some cases this rather fierce approach to packing can
produce unexpected effects.  For example, in Ada 95, type Natural
typically has a size of 31, meaning that if you pack an array of
Natural, you get 31-bit close packing, which saves a few bits, but
results in far less efficient access.  Since many other Ada compilers
will ignore such a packing request, GNAT will generate a warning on
some uses of pragma `Pack' that it guesses might not be what is
intended.  You can easily remove this warning by using an explicit
`Component_Size' setting instead, which never generates a warning,
since the intention of the programmer is clear in this case.

   GNAT treats packed arrays in one of two ways.  If the size of the
array is known at compile time and is less than 64 bits, then
internally the array is represented as a single modular type, of
exactly the appropriate number of bits.  If the length is greater than
63 bits, or is not known at compile time, then the packed array is
represented as an array of bytes, and the length is always a multiple
of 8 bits.


File: gnat_rm.info,  Node: Pragma Pack for Records,  Next: Record Representation Clauses,  Prev: Pragma Pack for Arrays,  Up: Representation Clauses and Pragmas

Pragma Pack for Records
=======================

Pragma `Pack' applied to a record will pack the components to reduce
wasted space from alignment gaps and by reducing the amount of space
taken by components.  We distinguish between package components and
non-packable components.  Components of the following types are
considered packable:

   * All scalar types are packable.

   * All fixed-point types are represented internally as integers, and
     are packable.

   * Small packed arrays, whose size does not exceed 64 bits, and where
     the size is statically known at compile time, are represented
     internally as modular integers, and so they are also packable.


All packable components occupy the exact number of bits corresponding to
their `Size' value, and are packed with no padding bits, i.e. they can
start on an arbitrary bit boundary.

   All other types are non-packable, they occupy an integral number of
storage units, and are placed at a boundary corresponding to their
alignment requirements.

   For example, consider the record

        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);
     
        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);
     
        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);

The representation for the record x2 is as follows:

     for x2'Size use 224;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

Studying this example, we see that the packable fields `l1' and `l2' are
of length equal to their sizes, and placed at specific bit boundaries
(and not byte boundaries) to eliminate padding.  But `l3' is of a
non-packable float type, so it is on the next appropriate alignment
boundary.

   The next two fields are fully packable, so `l4' and `l5' are
minimally packed with no gaps.  However, type `Rb2' is a packed array
that is longer than 64 bits, so it is itself non-packable.  Thus the
`l6' field is aligned to the next byte boundary, and takes an integral
number of bytes, i.e. 72 bits.


File: gnat_rm.info,  Node: Record Representation Clauses,  Next: Enumeration Clauses,  Prev: Pragma Pack for Records,  Up: Representation Clauses and Pragmas

Record Representation Clauses
=============================

Record representation clauses may be given for all record types,
including types obtained by record extension.  Component clauses are
allowed for any static component.  The restrictions on component
clauses depend on the type of the component.

   For all components of an elementary type, the only restriction on
component clauses is that the size must be at least the 'Size value of
the type (actually the Value_Size).  There are no restrictions due to
alignment, and such components may freely cross storage boundaries.

   Packed arrays with a size up to and including 64 bits are represented
internally using a modular type with the appropriate number of bits, and
thus the same lack of restriction applies.  For example, if you declare:

        type R is array (1 .. 49) of Boolean;
        pragma Pack (R);
        for R'Size use 49;

then a component clause for a component of type R may start on any
specified bit boundary, and may specify a value of 49 bits or greater.

   For non-primitive types, including packed arrays with a size greater
than 64 bits, component clauses must respect the alignment requirement
of the type, in particular, always starting on a byte boundary, and the
length must be a multiple of the storage unit.

   The tag field of a tagged type always occupies an address sized
field at the start of the record.  No component clause may attempt to
overlay this tag.

   In the case of a record extension T1, of a type T, no component
clause applied to the type T1 can specify a storage location that would
overlap the first T'Size bytes of the record.


File: gnat_rm.info,  Node: Enumeration Clauses,  Next: Address Clauses,  Prev: Record Representation Clauses,  Up: Representation Clauses and Pragmas

Enumeration Clauses
===================

   The only restriction on enumeration clauses is that the range of
values must be representable.  For the signed case, if one or more of
the representation values are negative, all values must be in the range:

        System.Min_Int .. System.Max_Int

For the unsigned case, where all values are non negative, the values
must be in the range:

        0 .. System.Max_Binary_Modulus;

A _confirming_ representation clause is one in which the values range
from 0 in sequence, i.e. a clause that confirms the default
representation for an enumeration type.  Such a confirming
representation is permitted by these rules, and is specially recognized
by the compiler so that no extra overhead results from the use of such
a clause.

   If an array has an index type which is an enumeration type to which
an enumeration clause has been applied, then the array is stored in a
compact manner.  Consider the declarations:

        type r is (A, B, C);
        for r use (A => 1, B => 5, C => 10);
        type t is array (r) of Character;

The array type t corresponds to a vector with exactly three elements and
has a default size equal to `3*Character'Size'.  This ensures efficient
use of space, but means that accesses to elements of the array will
incur the overhead of converting representation values to the
corresponding positional values, (i.e. the value delivered by the `Pos'
attribute).


File: gnat_rm.info,  Node: Address Clauses,  Next: Effect of Convention on Representation,  Prev: Enumeration Clauses,  Up: Representation Clauses and Pragmas

Address Clauses
===============

   The reference manual allows a general restriction on representation
clauses, as found in RM 13.1(22):

        An implementation need not support representation
        items containing nonstatic expressions, except that
        an implementation should support a representation item
        for a given entity if each nonstatic expression in the
        representation item is a name that statically denotes
        a constant declared before the entity.

In practice this is applicable only to address clauses, since this is
the only case in which a non-static expression is permitted by the
syntax.  As the AARM notes in sections 13.1 (22.a-22.h):

       22.a   Reason: This is to avoid the following sort
              of thing:
     
       22.b        X : Integer := F(...);
                   Y : Address := G(...);
                   for X'Address use Y;
     
       22.c   In the above, we have to evaluate the
              initialization expression for X before we
              know where to put the result.  This seems
              like an unreasonable implementation burden.
     
       22.d   The above code should instead be written
              like this:
     
       22.e        Y : constant Address := G(...);
                   X : Integer := F(...);
                   for X'Address use Y;
     
       22.f   This allows the expression ``Y'' to be safely
              evaluated before X is created.
     
       22.g   The constant could be a formal parameter of mode in.
     
       22.h   An implementation can support other nonstatic
              expressions if it wants to.  Expressions of type
              Address are hardly ever static, but their value
              might be known at compile time anyway in many
              cases.

GNAT does indeed permit many additional cases of non-static
expressions.  In particular, if the type involved is elementary there
are no restrictions (since in this case, holding a temporary copy of
the initialization value, if one is present, is inexpensive).  In
addition, if there is no implicit or explicit initialization, then
there are no restrictions.  GNAT will reject only the case where all
three of these conditions hold:

   * The type of the item is non-elementary (e.g. a record or array).

   * There is explicit or implicit initialization required for the
     object.

   * The address value is non-static.  Here GNAT is more permissive
     than the RM, and allows the address value to be the address of a
     previously declared stand-alone variable, as long as it does not
     itself have an address clause.

                     Anchor : Some_Initialized_Type;
                     Overlay : Some_Initialized_Type;
                     for Overlay'Address use Anchor'Address;

     However, the prefix of the address clause cannot be an array
     component, or a component of a discriminated record.


As noted above in section 22.h, address values are typically
non-static.  In particular the To_Address function, even if applied to
a literal value, is a non-static function call.  To avoid this minor
annoyance, GNAT provides the implementation defined attribute
'To_Address.  The following two expressions have identical values:

   Another issue with address clauses is the interaction with alignment
requirements.  When an address clause is given for an object, the
address value must be consistent with the alignment of the object
(which is usually the same as the alignment of the type of the object).
If an address clause is given that specifies an inappropriately
aligned address value, then the program execution is erroneous.

   Since this source of erroneous behavior can have unfortunate
effects, GNAT checks (at compile time if possible, generating a
warning, or at execution time with a run-time check) that the alignment
is appropriate.  If the run-time check fails, then `Program_Error' is
raised.  This run-time check is suppressed if range checks are
suppressed, or if `pragma Restrictions (No_Elaboration_Code)' is in
effect.

        To_Address (16#1234_0000#)
        System'To_Address (16#1234_0000#);

except that the second form is considered to be a static expression, and
thus when used as an address clause value is always permitted.

Additionally, GNAT treats as static an address clause that is an
unchecked_conversion of a static integer value.  This simplifies the
porting of legacy code, and provides a portable equivalent to the GNAT
attribute To_Address.

   An address clause cannot be given for an exported object.  More
understandably the real restriction is that objects with an address
clause cannot be exported.  This is because such variables are not
defined by the Ada program, so there is no external object so export.

   It is permissible to give an address clause and a pragma Import for
the same object.  In this case, the variable is not really defined by
the Ada program, so there is no external symbol to be linked.  The link
name and the external name are ignored in this case.  The reason that
we allow this combination is that it provides a useful idiom to avoid
unwanted initializations on objects with address clauses.

   When an address clause is given for an object that has implicit or
explicit initialization, then by default initialization takes place.
This means that the effect of the object declaration is to overwrite the
memory at the specified address.  This is almost always not what the
programmer wants, so GNAT will output a warning:

       with System;
       package G is
          type R is record
             M : Integer := 0;
          end record;
     
          Ext : R;
          for Ext'Address use System'To_Address (16#1234_1234#);
              |
       >>> warning: implicit initialization of "Ext" may
           modify overlaid storage
       >>> warning: use pragma Import for "Ext" to suppress
           initialization (RM B(24))
     
       end G;

As indicated by the warning message, the solution is to use a (dummy)
pragma Import to suppress this initialization.  The pragma tell the
compiler that the object is declared and initialized elsewhere.  The
following package compiles without warnings (and the initialization is
suppressed):

        with System;
        package G is
           type R is record
              M : Integer := 0;
           end record;
     
           Ext : R;
           for Ext'Address use System'To_Address (16#1234_1234#);
           pragma Import (Ada, Ext);
        end G;


File: gnat_rm.info,  Node: Effect of Convention on Representation,  Next: Determining the Representations chosen by GNAT,  Prev: Address Clauses,  Up: Representation Clauses and Pragmas

Effect of Convention on Representation
======================================

Normally the specification of a foreign language convention for a type
or an object has no effect on the chosen representation.  In
particular, the representation chosen for data in GNAT generally meets
the standard system conventions, and for example records are laid out
in a manner that is consistent with C.  This means that specifying
convention C (for example) has no effect.

   There are three exceptions to this general rule:

   * Convention Fortran and array subtypes If pragma Convention Fortran
     is specified for an array subtype, then in accordance with the
     implementation advice in section 3.6.2(11) of the Ada Reference
     Manual, the array will be stored in a Fortran-compatible
     column-major manner, instead of the normal default row-major order.

   * Convention C and enumeration types GNAT normally stores
     enumeration types in 8, 16, or 32 bits as required to accommodate
     all values of the type.  For example, for the enumeration type
     declared by:

             type Color is (Red, Green, Blue);

     8 bits is sufficient to store all values of the type, so by
     default, objects of type `Color' will be represented using 8 bits.
     However, normal C convention is to use 32 bits for all enum
     values in C, since enum values are essentially of type int.  If
     pragma `Convention C' is specified for an Ada enumeration type,
     then the size is modified as necessary (usually to 32 bits) to be
     consistent with the C convention for enum values.

   * Convention C/Fortran and Boolean types In C, the usual convention
     for boolean values, that is values used for conditions, is that
     zero represents false, and nonzero values represent true.  In Ada,
     the normal convention is that two specific values, typically 0/1,
     are used to represent false/true respectively.

     Fortran has a similar convention for `LOGICAL' values (any nonzero
     value represents true).

     To accommodate the Fortran and C conventions, if a pragma
     Convention specifies C or Fortran convention for a derived
     Boolean, as in the following example:

             type C_Switch is new Boolean;
             pragma Convention (C, C_Switch);

     then the GNAT generated code will treat any nonzero value as true.
     For truth values generated by GNAT, the conventional value 1 will
     be used for True, but when one of these values is read, any
     nonzero value is treated as True.



File: gnat_rm.info,  Node: Determining the Representations chosen by GNAT,  Prev: Effect of Convention on Representation,  Up: Representation Clauses and Pragmas

Determining the Representations chosen by GNAT
==============================================

Although the descriptions in this section are intended to be complete,
it is often easier to simply experiment to see what GNAT accepts and
what the effect is on the layout of types and objects.

   As required by the Ada RM, if a representation clause is not
accepted, then it must be rejected as illegal by the compiler.
However, when a representation clause or pragma is accepted, there can
still be questions of what the compiler actually does.  For example, if
a partial record representation clause specifies the location of some
components and not others, then where are the non-specified components
placed? Or if pragma `Pack' is used on a record, then exactly where are
the resulting fields placed? The section on pragma `Pack' in this
chapter can be used to answer the second question, but it is often
easier to just see what the compiler does.

   For this purpose, GNAT provides the option `-gnatR'.  If you compile
with this option, then the compiler will output information on the
actual representations chosen, in a format similar to source
representation clauses.  For example, if we compile the package:

     package q is
        type r (x : boolean) is tagged record
           case x is
              when True => S : String (1 .. 100);
              when False => null;
           end case;
        end record;
     
        type r2 is new r (false) with record
           y2 : integer;
        end record;
     
        for r2 use record
           y2 at 16 range 0 .. 31;
        end record;
     
        type x is record
           y : character;
        end record;
     
        type x1 is array (1 .. 10) of x;
        for x1'component_size use 11;
     
        type ia is access integer;
     
        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);
     
        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);
     
        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);
     end q;

using the switch `-gnatR' we obtain the following output:

     Representation information for unit q
     -------------------------------------
     
     for r'Size use ??;
     for r'Alignment use 4;
     for r use record
        x    at 4 range  0 .. 7;
        _tag at 0 range  0 .. 31;
        s    at 5 range  0 .. 799;
     end record;
     
     for r2'Size use 160;
     for r2'Alignment use 4;
     for r2 use record
        x       at  4 range  0 .. 7;
        _tag    at  0 range  0 .. 31;
        _parent at  0 range  0 .. 63;
        y2      at 16 range  0 .. 31;
     end record;
     
     for x'Size use 8;
     for x'Alignment use 1;
     for x use record
        y at 0 range  0 .. 7;
     end record;
     
     for x1'Size use 112;
     for x1'Alignment use 1;
     for x1'Component_Size use 11;
     
     for rb1'Size use 13;
     for rb1'Alignment use 2;
     for rb1'Component_Size use 1;
     
     for rb2'Size use 72;
     for rb2'Alignment use 1;
     for rb2'Component_Size use 1;
     
     for x2'Size use 224;
     for x2'Alignment use 4;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

The Size values are actually the Object_Size, i.e. the default size that
will be allocated for objects of the type.  The ?? size for type r
indicates that we have a variant record, and the actual size of objects
will depend on the discriminant value.

   The Alignment values show the actual alignment chosen by the compiler
for each record or array type.

   The record representation clause for type r shows where all fields
are placed, including the compiler generated tag field (whose location
cannot be controlled by the programmer).

   The record representation clause for the type extension r2 shows all
the fields present, including the parent field, which is a copy of the
fields of the parent type of r2, i.e. r1.

   The component size and size clauses for types rb1 and rb2 show the
exact effect of pragma `Pack' on these arrays, and the record
representation clause for type x2 shows how pragma `Pack' affects this
record type.

   In some cases, it may be useful to cut and paste the representation
clauses generated by the compiler into the original source to fix and
guarantee the actual representation to be used.


File: gnat_rm.info,  Node: Standard Library Routines,  Next: The Implementation of Standard I/O,  Prev: Representation Clauses and Pragmas,  Up: Top

Standard Library Routines
*************************

The Ada 95 Reference Manual contains in Annex A a full description of an
extensive set of standard library routines that can be used in any Ada
program, and which must be provided by all Ada compilers.  They are
analogous to the standard C library used by C programs.

   GNAT implements all of the facilities described in annex A, and for
most purposes the description in the Ada 95 reference manual, or
appropriate Ada text book, will be sufficient for making use of these
facilities.

   In the case of the input-output facilities, *Note The Implementation
of Standard I/O::, gives details on exactly how GNAT interfaces to the
file system.  For the remaining packages, the Ada 95 reference manual
should be sufficient.  The following is a list of the packages included,
together with a brief description of the functionality that is provided.

   For completeness, references are included to other predefined library
routines defined in other sections of the Ada 95 reference manual
(these are cross-indexed from annex A).

`Ada (A.2)'
     This is a parent package for all the standard library packages.
     It is usually included implicitly in your program, and itself
     contains no useful data or routines.

`Ada.Calendar (9.6)'
     `Calendar' provides time of day access, and routines for
     manipulating times and durations.

`Ada.Characters (A.3.1)'
     This is a dummy parent package that contains no useful entities

`Ada.Characters.Handling (A.3.2)'
     This package provides some basic character handling capabilities,
     including classification functions for classes of characters (e.g.
     test for letters, or digits).

`Ada.Characters.Latin_1 (A.3.3)'
     This package includes a complete set of definitions of the
     characters that appear in type CHARACTER.  It is useful for
     writing programs that will run in international environments.  For
     example, if you want an upper case E with an acute accent in a
     string, it is often better to use the definition of `UC_E_Acute'
     in this package.  Then your program will print in an
     understandable manner even if your environment does not support
     these extended characters.

`Ada.Command_Line (A.15)'
     This package provides access to the command line parameters and
     the name of the current program (analogous to the use of `argc'
     and `argv' in C), and also allows the exit status for the program
     to be set in a system-independent manner.

`Ada.Decimal (F.2)'
     This package provides constants describing the range of decimal
     numbers implemented, and also a decimal divide routine (analogous
     to the COBOL verb DIVIDE .. GIVING .. REMAINDER ..)

`Ada.Direct_IO (A.8.4)'
     This package provides input-output using a model of a set of
     records of fixed-length, containing an arbitrary definite Ada
     type, indexed by an integer record number.

`Ada.Dynamic_Priorities (D.5)'
     This package allows the priorities of a task to be adjusted
     dynamically as the task is running.

`Ada.Exceptions (11.4.1)'
     This package provides additional information on exceptions, and
     also contains facilities for treating exceptions as data objects,
     and raising exceptions with associated messages.

`Ada.Finalization (7.6)'
     This package contains the declarations and subprograms to support
     the use of controlled types, providing for automatic
     initialization and finalization (analogous to the constructors and
     destructors of C++)

`Ada.Interrupts (C.3.2)'
     This package provides facilities for interfacing to interrupts,
     which includes the set of signals or conditions that can be raised
     and recognized as interrupts.

`Ada.Interrupts.Names (C.3.2)'
     This package provides the set of interrupt names (actually signal
     or condition names) that can be handled by GNAT.

`Ada.IO_Exceptions (A.13)'
     This package defines the set of exceptions that can be raised by
     use of the standard IO packages.

`Ada.Numerics'
     This package contains some standard constants and exceptions used
     throughout the numerics packages.  Note that the constants pi and
     e are defined here, and it is better to use these definitions than
     rolling your own.

`Ada.Numerics.Complex_Elementary_Functions'
     Provides the implementation of standard elementary functions (such
     as log and trigonometric functions) operating on complex numbers
     using the standard `Float' and the `Complex' and `Imaginary' types
     created by the package `Numerics.Complex_Types'.

`Ada.Numerics.Complex_Types'
     This is a predefined instantiation of
     `Numerics.Generic_Complex_Types' using `Standard.Float' to build
     the type `Complex' and `Imaginary'.

`Ada.Numerics.Discrete_Random'
     This package provides a random number generator suitable for
     generating random integer values from a specified range.

`Ada.Numerics.Float_Random'
     This package provides a random number generator suitable for
     generating uniformly distributed floating point values.

`Ada.Numerics.Generic_Complex_Elementary_Functions'
     This is a generic version of the package that provides the
     implementation of standard elementary functions (such as log and
     trigonometric functions) for an arbitrary complex type.

     The following predefined instantiations of this package are
     provided:

    `Short_Float'
          `Ada.Numerics.Short_Complex_Elementary_Functions'

    `Float'
          `Ada.Numerics.Complex_Elementary_Functions'

    `Long_Float'
          `Ada.Numerics.   Long_Complex_Elementary_Functions'

`Ada.Numerics.Generic_Complex_Types'
     This is a generic package that allows the creation of complex
     types, with associated complex arithmetic operations.

     The following predefined instantiations of this package exist
    `Short_Float'
          `Ada.Numerics.Short_Complex_Complex_Types'

    `Float'
          `Ada.Numerics.Complex_Complex_Types'

    `Long_Float'
          `Ada.Numerics.Long_Complex_Complex_Types'

`Ada.Numerics.Generic_Elementary_Functions'
     This is a generic package that provides the implementation of
     standard elementary functions (such as log an trigonometric
     functions) for an arbitrary float type.

     The following predefined instantiations of this package exist

    `Short_Float'
          `Ada.Numerics.Short_Elementary_Functions'

    `Float'
          `Ada.Numerics.Elementary_Functions'

    `Long_Float'
          `Ada.Numerics.Long_Elementary_Functions'

`Ada.Real_Time (D.8)'
     This package provides facilities similar to those of `Calendar',
     but operating with a finer clock suitable for real time control.
     Note that annex D requires that there be no backward clock jumps,
     and GNAT generally guarantees this behavior, but of course if the
     external clock on which the GNAT runtime depends is deliberately
     reset by some external event, then such a backward jump may occur.

`Ada.Sequential_IO (A.8.1)'
     This package provides input-output facilities for sequential files,
     which can contain a sequence of values of a single type, which can
     be any Ada type, including indefinite (unconstrained) types.

`Ada.Storage_IO (A.9)'
     This package provides a facility for mapping arbitrary Ada types
     to and from a storage buffer.  It is primarily intended for the
     creation of new IO packages.

`Ada.Streams (13.13.1)'
     This is a generic package that provides the basic support for the
     concept of streams as used by the stream attributes (`Input',
     `Output', `Read' and `Write').

`Ada.Streams.Stream_IO (A.12.1)'
     This package is a specialization of the type `Streams' defined in
     package `Streams' together with a set of operations providing
     Stream_IO capability.  The Stream_IO model permits both random and
     sequential access to a file which can contain an arbitrary set of
     values of one or more Ada types.

`Ada.Strings (A.4.1)'
     This package provides some basic constants used by the string
     handling packages.

`Ada.Strings.Bounded (A.4.4)'
     This package provides facilities for handling variable length
     strings.  The bounded model requires a maximum length.  It is thus
     somewhat more limited than the unbounded model, but avoids the use
     of dynamic allocation or finalization.

`Ada.Strings.Fixed (A.4.3)'
     This package provides facilities for handling fixed length strings.

`Ada.Strings.Maps (A.4.2)'
     This package provides facilities for handling character mappings
     and arbitrarily defined subsets of characters.  For instance it is
     useful in defining specialized translation tables.

`Ada.Strings.Maps.Constants (A.4.6)'
     This package provides a standard set of predefined mappings and
     predefined character sets.  For example, the standard upper to
     lower case conversion table is found in this package.  Note that
     upper to lower case conversion is non-trivial if you want to take
     the entire set of characters, including extended characters like E
     with an acute accent, into account.  You should use the mappings
     in this package (rather than adding 32 yourself) to do case
     mappings.

`Ada.Strings.Unbounded (A.4.5)'
     This package provides facilities for handling variable length
     strings.  The unbounded model allows arbitrary length strings, but
     requires the use of dynamic allocation and finalization.

`Ada.Strings.Wide_Bounded (A.4.7)'
`Ada.Strings.Wide_Fixed (A.4.7)'
`Ada.Strings.Wide_Maps (A.4.7)'
`Ada.Strings.Wide_Maps.Constants (A.4.7)'
`Ada.Strings.Wide_Unbounded (A.4.7)'
     These package provide analogous capabilities to the corresponding
     packages without `Wide_' in the name, but operate with the types
     `Wide_String' and `Wide_Character' instead of `String' and
     `Character'.

`Ada.Synchronous_Task_Control (D.10)'
     This package provides some standard facilities for controlling task
     communication in a synchronous manner.

`Ada.Tags'
     This package contains definitions for manipulation of the tags of
     tagged values.

`Ada.Task_Attributes'
     This package provides the capability of associating arbitrary
     task-specific data with separate tasks.

`Ada.Text_IO'
     This package provides basic text input-output capabilities for
     character, string and numeric data.  The subpackages of this
     package are listed next.

`Ada.Text_IO.Decimal_IO'
     Provides input-output facilities for decimal fixed-point types

`Ada.Text_IO.Enumeration_IO'
     Provides input-output facilities for enumeration types.

`Ada.Text_IO.Fixed_IO'
     Provides input-output facilities for ordinary fixed-point types.

`Ada.Text_IO.Float_IO'
     Provides input-output facilities for float types.  The following
     predefined instantiations of this generic package are available:

    `Short_Float'
          `Short_Float_Text_IO'

    `Float'
          `Float_Text_IO'

    `Long_Float'
          `Long_Float_Text_IO'

`Ada.Text_IO.Integer_IO'
     Provides input-output facilities for integer types.  The following
     predefined instantiations of this generic package are available:

    `Short_Short_Integer'
          `Ada.Short_Short_Integer_Text_IO'

    `Short_Integer'
          `Ada.Short_Integer_Text_IO'

    `Integer'
          `Ada.Integer_Text_IO'

    `Long_Integer'
          `Ada.Long_Integer_Text_IO'

    `Long_Long_Integer'
          `Ada.Long_Long_Integer_Text_IO'

`Ada.Text_IO.Modular_IO'
     Provides input-output facilities for modular (unsigned) types

`Ada.Text_IO.Complex_IO (G.1.3)'
     This package provides basic text input-output capabilities for
     complex data.

`Ada.Text_IO.Editing (F.3.3)'
     This package contains routines for edited output, analogous to the
     use of pictures in COBOL.  The picture formats used by this
     package are a close copy of the facility in COBOL.

`Ada.Text_IO.Text_Streams (A.12.2)'
     This package provides a facility that allows Text_IO files to be
     treated as streams, so that the stream attributes can be used for
     writing arbitrary data, including binary data, to Text_IO files.

`Ada.Unchecked_Conversion (13.9)'
     This generic package allows arbitrary conversion from one type to
     another of the same size, providing for breaking the type safety in
     special circumstances.

     If the types have the same Size (more accurately the same
     Value_Size), then the effect is simply to transfer the bits from
     the source to the target type without any modification.  This
     usage is well defined, and for simple types whose representation
     is typically the same across all implementations, gives a portable
     method of performing such conversions.

     If the types do not have the same size, then the result is
     implementation defined, and thus may be non-portable.  The
     following describes how GNAT handles such unchecked conversion
     cases.

     If the types are of different sizes, and are both discrete types,
     then the effect is of a normal type conversion without any
     constraint checking.  In particular if the result type has a
     larger size, the result will be zero or sign extended.  If the
     result type has a smaller size, the result will be truncated by
     ignoring high order bits.

     If the types are of different sizes, and are not both discrete
     types, then the conversion works as though pointers were created
     to the source and target, and the pointer value is converted.  The
     effect is that bits are copied from successive low order storage
     units and bits of the source up to the length of the target type.

     A warning is issued if the lengths differ, since the effect in this
     case is implementation dependent, and the above behavior may not
     match that of some other compiler.

     A pointer to one type may be converted to a pointer to another
     type using unchecked conversion.  The only case in which the
     effect is undefined is when one or both pointers are pointers to
     unconstrained array types.  In this case, the bounds information
     may get incorrectly transferred, and in particular, GNAT uses
     double size pointers for such types, and it is meaningless to
     convert between such pointer types.  GNAT will issue a warning if
     the alignment of the target designated type is more strict than
     the alignment of the source designated type (since the result may
     be unaligned in this case).

     A pointer other than a pointer to an unconstrained array type may
     be converted to and from System.Address.  Such usage is common in
     Ada 83 programs, but note that Ada.Address_To_Access_Conversions
     is the preferred method of performing such conversions in Ada 95.
     Neither unchecked conversion nor Ada.Address_To_Access_Conversions
     should be used in conjunction with pointers to unconstrained
     objects, since the bounds information cannot be handled correctly
     in this case.

`Ada.Unchecked_Deallocation (13.11.2)'
     This generic package allows explicit freeing of storage previously
     allocated by use of an allocator.

`Ada.Wide_Text_IO (A.11)'
     This package is similar to `Ada.Text_IO', except that the external
     file supports wide character representations, and the internal
     types are `Wide_Character' and `Wide_String' instead of `Character'
     and `String'.  It contains generic subpackages listed next.

`Ada.Wide_Text_IO.Decimal_IO'
     Provides input-output facilities for decimal fixed-point types

`Ada.Wide_Text_IO.Enumeration_IO'
     Provides input-output facilities for enumeration types.

`Ada.Wide_Text_IO.Fixed_IO'
     Provides input-output facilities for ordinary fixed-point types.

`Ada.Wide_Text_IO.Float_IO'
     Provides input-output facilities for float types.  The following
     predefined instantiations of this generic package are available:

    `Short_Float'
          `Short_Float_Wide_Text_IO'

    `Float'
          `Float_Wide_Text_IO'

    `Long_Float'
          `Long_Float_Wide_Text_IO'

`Ada.Wide_Text_IO.Integer_IO'
     Provides input-output facilities for integer types.  The following
     predefined instantiations of this generic package are available:

    `Short_Short_Integer'
          `Ada.Short_Short_Integer_Wide_Text_IO'

    `Short_Integer'
          `Ada.Short_Integer_Wide_Text_IO'

    `Integer'
          `Ada.Integer_Wide_Text_IO'

    `Long_Integer'
          `Ada.Long_Integer_Wide_Text_IO'

    `Long_Long_Integer'
          `Ada.Long_Long_Integer_Wide_Text_IO'

`Ada.Wide_Text_IO.Modular_IO'
     Provides input-output facilities for modular (unsigned) types

`Ada.Wide_Text_IO.Complex_IO (G.1.3)'
     This package is similar to `Ada.Text_IO.Complex_IO', except that
     the external file supports wide character representations.

`Ada.Wide_Text_IO.Editing (F.3.4)'
     This package is similar to `Ada.Text_IO.Editing', except that the
     types are `Wide_Character' and `Wide_String' instead of
     `Character' and `String'.

`Ada.Wide_Text_IO.Streams (A.12.3)'
     This package is similar to `Ada.Text_IO.Streams', except that the
     types are `Wide_Character' and `Wide_String' instead of
     `Character' and `String'.


File: gnat_rm.info,  Node: The Implementation of Standard I/O,  Next: The GNAT Library,  Prev: Standard Library Routines,  Up: Top

The Implementation of Standard I/O
**********************************

GNAT implements all the required input-output facilities described in
A.6 through A.14.  These sections of the Ada 95 reference manual
describe the required behavior of these packages from the Ada point of
view, and if you are writing a portable Ada program that does not need
to know the exact manner in which Ada maps to the outside world when it
comes to reading or writing external files, then you do not need to
read this chapter.  As long as your files are all regular files (not
pipes or devices), and as long as you write and read the files only
from Ada, the description in the Ada 95 reference manual is sufficient.

   However, if you want to do input-output to pipes or other devices,
such as the keyboard or screen, or if the files you are dealing with are
either generated by some other language, or to be read by some other
language, then you need to know more about the details of how the GNAT
implementation of these input-output facilities behaves.

   In this chapter we give a detailed description of exactly how GNAT
interfaces to the file system.  As always, the sources of the system are
available to you for answering questions at an even more detailed level,
but for most purposes the information in this chapter will suffice.

   Another reason that you may need to know more about how input-output
is implemented arises when you have a program written in mixed languages
where, for example, files are shared between the C and Ada sections of
the same program.  GNAT provides some additional facilities, in the form
of additional child library packages, that facilitate this sharing, and
these additional facilities are also described in this chapter.

* Menu:

* Standard I/O Packages::
* FORM Strings::
* Direct_IO::
* Sequential_IO::
* Text_IO::
* Wide_Text_IO::
* Stream_IO::
* Shared Files::
* Open Modes::
* Operations on C Streams::
* Interfacing to C Streams::


File: gnat_rm.info,  Node: Standard I/O Packages,  Next: FORM Strings,  Up: The Implementation of Standard I/O

Standard I/O Packages
=====================

The Standard I/O packages described in Annex A for

   * Ada.Text_IO

   * Ada.Text_IO.Complex_IO

   * Ada.Text_IO.Text_Streams,

   * Ada.Wide_Text_IO

   * Ada.Wide_Text_IO.Complex_IO,

   * Ada.Wide_Text_IO.Text_Streams

   * Ada.Stream_IO

   * Ada.Sequential_IO

   * Ada.Direct_IO

are implemented using the C library streams facility; where

   * All files are opened using `fopen'.

   * All input/output operations use `fread'/`fwrite'.

   There is no internal buffering of any kind at the Ada library level.
The only buffering is that provided at the system level in the
implementation of the C library routines that support streams.  This
facilitates shared use of these streams by mixed language programs.


File: gnat_rm.info,  Node: FORM Strings,  Next: Direct_IO,  Prev: Standard I/O Packages,  Up: The Implementation of Standard I/O

FORM Strings
============

The format of a FORM string in GNAT is:

     "keyword=value,keyword=value,...,keyword=value"

where letters may be in upper or lower case, and there are no spaces
between values.  The order of the entries is not important.  Currently
there are two keywords defined.

     SHARED=[YES|NO]
     WCEM=[n|h|u|s\e]

   The use of these parameters is described later in this section.


File: gnat_rm.info,  Node: Direct_IO,  Next: Sequential_IO,  Prev: FORM Strings,  Up: The Implementation of Standard I/O

Direct_IO
=========

Direct_IO can only be instantiated for definite types.  This is a
restriction of the Ada language, which means that the records are fixed
length (the length being determined by `TYPE'Size', rounded up to the
next storage unit boundary if necessary).

   The records of a Direct_IO file are simply written to the file in
index sequence, with the first record starting at offset zero, and
subsequent records following.  There is no control information of any
kind.  For example, if 32-bit integers are being written, each record
takes 4-bytes, so the record at index K starts at offset (K-1)*4.

   There is no limit on the size of Direct_IO files, they are expanded
as necessary to accommodate whatever records are written to the file.


File: gnat_rm.info,  Node: Sequential_IO,  Next: Text_IO,  Prev: Direct_IO,  Up: The Implementation of Standard I/O

Sequential_IO
=============

Sequential_IO may be instantiated with either a definite (constrained)
or indefinite (unconstrained) type.

   For the definite type case, the elements written to the file are
simply the memory images of the data values with no control information
of any kind.  The resulting file should be read using the same type, no
validity checking is performed on input.

   For the indefinite type case, the elements written consist of two
parts.  First is the size of the data item, written as the memory image
of a `Interfaces.C.size_t' value, followed by the memory image of the
data value.  The resulting file can only be read using the same
(unconstrained) type.  Normal assignment checks are performed on these
read operations, and if these checks fail, `Data_Error' is raised.  In
particular, in the array case, the lengths must match, and in the
variant record case, if the variable for a particular read operation is
constrained, the discriminants must match.

   Note that it is not possible to use Sequential_IO to write variable
length array items, and then read the data back into different length
arrays.  For example, the following will raise `Data_Error':

      package IO is new Sequential_IO (String);
      F : IO.File_Type;
      S : String (1..4);
      ...
      IO.Create (F)
      IO.Write (F, "hello!")
      IO.Reset (F, Mode=>In_File);
      IO.Read (F, S);
      Put_Line (S);

   On some Ada implementations, this will print `hell', but the program
is clearly incorrect, since there is only one element in the file, and
that element is the string `hello!'.

   In Ada 95, this kind of behavior can be legitimately achieved using
Stream_IO, and this is the preferred mechanism.  In particular, the
above program fragment rewritten to use Stream_IO will work correctly.


File: gnat_rm.info,  Node: Text_IO,  Next: Wide_Text_IO,  Prev: Sequential_IO,  Up: The Implementation of Standard I/O

Text_IO
=======

Text_IO files consist of a stream of characters containing the following
special control characters:

     LF (line feed, 16#0A#) Line Mark
     FF (form feed, 16#0C#) Page Mark

   A canonical Text_IO file is defined as one in which the following
conditions are met:

   * The character `LF' is used only as a line mark, i.e. to mark the
     end of the line.

   * The character `FF' is used only as a page mark, i.e. to mark the
     end of a page and consequently can appear only immediately
     following a `LF' (line mark) character.

   * The file ends with either `LF' (line mark) or `LF'-`FF' (line
     mark, page mark).  In the former case, the page mark is implicitly
     assumed to be present.

   A file written using Text_IO will be in canonical form provided that
no explicit `LF' or `FF' characters are written using `Put' or
`Put_Line'.  There will be no `FF' character at the end of the file
unless an explicit `New_Page' operation was performed before closing
the file.

   A canonical Text_IO file that is a regular file, i.e. not a device
or a pipe, can be read using any of the routines in Text_IO.  The
semantics in this case will be exactly as defined in the Ada 95
reference manual and all the routines in Text_IO are fully implemented.

   A text file that does not meet the requirements for a canonical
Text_IO file has one of the following:

   * The file contains `FF' characters not immediately following a `LF'
     character.

   * The file contains `LF' or `FF' characters written by `Put' or
     `Put_Line', which are not logically considered to be line marks or
     page marks.

   * The file ends in a character other than `LF' or `FF', i.e. there
     is no explicit line mark or page mark at the end of the file.

   Text_IO can be used to read such non-standard text files but
subprograms to do with line or page numbers do not have defined
meanings.  In particular, a `FF' character that does not follow a `LF'
character may or may not be treated as a page mark from the point of
view of page and line numbering.  Every `LF' character is considered to
end a line, and there is an implied `LF' character at the end of the
file.

* Menu:

* Text_IO Stream Pointer Positioning::
* Text_IO Reading and Writing Non-Regular Files::
* Get_Immediate::
* Treating Text_IO Files as Streams::
* Text_IO Extensions::
* Text_IO Facilities for Unbounded Strings::


File: gnat_rm.info,  Node: Text_IO Stream Pointer Positioning,  Next: Text_IO Reading and Writing Non-Regular Files,  Up: Text_IO

Stream Pointer Positioning
--------------------------

`Ada.Text_IO' has a definition of current position for a file that is
being read.  No internal buffering occurs in Text_IO, and usually the
physical position in the stream used to implement the file corresponds
to this logical position defined by Text_IO.  There are two exceptions:

   * After a call to `End_Of_Page' that returns `True', the stream is
     positioned past the `LF' (line mark) that precedes the page mark.
     Text_IO maintains an internal flag so that subsequent read
     operations properly handle the logical position which is unchanged
     by the `End_Of_Page' call.

   * After a call to `End_Of_File' that returns `True', if the Text_IO
     file was positioned before the line mark at the end of file before
     the call, then the logical position is unchanged, but the stream
     is physically positioned right at the end of file (past the line
     mark, and past a possible page mark following the line mark.
     Again Text_IO maintains internal flags so that subsequent read
     operations properly handle the logical position.

   These discrepancies have no effect on the observable behavior of
Text_IO, but if a single Ada stream is shared between a C program and
Ada program, or shared (using `shared=yes' in the form string) between
two Ada files, then the difference may be observable in some situations.


File: gnat_rm.info,  Node: Text_IO Reading and Writing Non-Regular Files,  Next: Get_Immediate,  Prev: Text_IO Stream Pointer Positioning,  Up: Text_IO

Reading and Writing Non-Regular Files
-------------------------------------

A non-regular file is a device (such as a keyboard), or a pipe.  Text_IO
can be used for reading and writing.  Writing is not affected and the
sequence of characters output is identical to the normal file case, but
for reading, the behavior of Text_IO is modified to avoid undesirable
look-ahead as follows:

   An input file that is not a regular file is considered to have no
page marks.  Any `Ascii.FF' characters (the character normally used for
a page mark) appearing in the file are considered to be data
characters.  In particular:

   * `Get_Line' and `Skip_Line' do not test for a page mark following a
     line mark.  If a page mark appears, it will be treated as a data
     character.

   * This avoids the need to wait for an extra character to be typed or
     entered from the pipe to complete one of these operations.

   * `End_Of_Page' always returns `False'

   * `End_Of_File' will return `False' if there is a page mark at the
     end of the file.

   Output to non-regular files is the same as for regular files.  Page
marks may be written to non-regular files using `New_Page', but as noted
above they will not be treated as page marks on input if the output is
piped to another Ada program.

   Another important discrepancy when reading non-regular files is that
the end of file indication is not "sticky".  If an end of file is
entered, e.g. by pressing the <EOT> key, then end of file is signalled
once (i.e. the test `End_Of_File' will yield `True', or a read will
raise `End_Error'), but then reading can resume to read data past that
end of file indication, until another end of file indication is entered.


File: gnat_rm.info,  Node: Get_Immediate,  Next: Treating Text_IO Files as Streams,  Prev: Text_IO Reading and Writing Non-Regular Files,  Up: Text_IO

Get_Immediate
-------------

Get_Immediate returns the next character (including control characters)
from the input file.  In particular, Get_Immediate will return LF or FF
characters used as line marks or page marks.  Such operations leave the
file positioned past the control character, and it is thus not treated
as having its normal function.  This means that page, line and column
counts after this kind of Get_Immediate call are set as though the mark
did not occur.  In the case where a Get_Immediate leaves the file
positioned between the line mark and page mark (which is not normally
possible), it is undefined whether the FF character will be treated as a
page mark.


File: gnat_rm.info,  Node: Treating Text_IO Files as Streams,  Next: Text_IO Extensions,  Prev: Get_Immediate,  Up: Text_IO

Treating Text_IO Files as Streams
---------------------------------

The package `Text_IO.Streams' allows a Text_IO file to be treated as a
stream.  Data written to a Text_IO file in this stream mode is binary
data.  If this binary data contains bytes 16#0A# (`LF') or 16#0C#
(`FF'), the resulting file may have non-standard format.  Similarly if
read operations are used to read from a Text_IO file treated as a
stream, then `LF' and `FF' characters may be skipped and the effect is
similar to that described above for `Get_Immediate'.


File: gnat_rm.info,  Node: Text_IO Extensions,  Next: Text_IO Facilities for Unbounded Strings,  Prev: Treating Text_IO Files as Streams,  Up: Text_IO

Text_IO Extensions
------------------

A package GNAT.IO_Aux in the GNAT library provides some useful
extensions to the standard `Text_IO' package:

   * function File_Exists (Name : String) return Boolean; Determines if
     a file of the given name exists and can be successfully opened
     (without actually performing the open operation).

   * function Get_Line return String; Reads a string from the standard
     input file.  The value returned is exactly the length of the line
     that was read.

   * function Get_Line (File : Ada.Text_IO.File_Type) return String;
     Similar, except that the parameter File specifies the file from
     which the string is to be read.



File: gnat_rm.info,  Node: Text_IO Facilities for Unbounded Strings,  Prev: Text_IO Extensions,  Up: Text_IO

Text_IO Facilities for Unbounded Strings
----------------------------------------

The package `Ada.Strings.Unbounded.Text_IO' in library files
`a-suteio.ads/adb' contains some GNAT-specific subprograms useful for
Text_IO operations on unbounded strings:

   * function Get_Line (File : File_Type) return Unbounded_String;
     Reads a line from the specified file and returns the result as an
     unbounded string.

   * procedure Put (File : File_Type; U : Unbounded_String); Writes the
     value of the given unbounded string to the specified file Similar
     to the effect of `Put (To_String (U))' except that an extra copy
     is avoided.

   * procedure Put_Line (File : File_Type; U : Unbounded_String);
     Writes the value of the given unbounded string to the specified
     file, followed by a `New_Line'.  Similar to the effect of
     `Put_Line (To_String (U))' except that an extra copy is avoided.

In the above procedures, `File' is of type `Ada.Text_IO.File_Type' and
is optional.  If the parameter is omitted, then the standard input or
output file is referenced as appropriate.

   The package `Ada.Strings.Wide_Unbounded.Wide_Text_IO' in library
files `a-swuwti.ads' and `a-swuwti.adb' provides similar extended
`Wide_Text_IO' functionality for unbounded wide strings.


File: gnat_rm.info,  Node: Wide_Text_IO,  Next: Stream_IO,  Prev: Text_IO,  Up: The Implementation of Standard I/O

Wide_Text_IO
============

`Wide_Text_IO' is similar in most respects to Text_IO, except that both
input and output files may contain special sequences that represent
wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:

     WCEM=X

as part of the FORM string (WCEM = wide character encoding method),
where X is one of the following characters

`h'
     Hex ESC encoding

`u'
     Upper half encoding

`s'
     Shift-JIS encoding

`e'
     EUC Encoding

`8'
     UTF-8 encoding

`b'
     Brackets encoding

   The encoding methods match those that can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

   The default encoding method for the standard files, and for opened
files for which no WCEM parameter is given in the FORM string matches
the wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).

Hex Coding
     In this encoding, a wide character is represented by a five
     character sequence:

          ESC a b c d

     where A, B, C, D are the four hexadecimal characters (using upper
     case letters) of the wide character code.  For example, ESC A345
     is used to represent the wide character with code 16#A345#.  This
     scheme is compatible with use of the full `Wide_Character' set.

Upper Half Coding
     The wide character with encoding 16#abcd#, where the upper bit is
     on (i.e. a is in the range 8-F) is represented as two bytes 16#ab#
     and 16#cd#.  The second byte may never be a format control
     character, but is not required to be in the upper half.  This
     method can be also used for shift-JIS or EUC where the internal
     coding matches the external coding.

Shift JIS Coding
     A wide character is represented by a two character sequence 16#ab#
     and 16#cd#, with the restrictions described for upper half
     encoding as described above.  The internal character code is the
     corresponding JIS character according to the standard algorithm
     for Shift-JIS conversion.  Only characters defined in the JIS code
     set table can be used with this encoding method.

EUC Coding
     A wide character is represented by a two character sequence 16#ab#
     and 16#cd#, with both characters being in the upper half.  The
     internal character code is the corresponding JIS character
     according to the EUC encoding algorithm.  Only characters defined
     in the JIS code set table can be used with this encoding method.

UTF-8 Coding
     A wide character is represented using UCS Transformation Format 8
     (UTF-8) as defined in Annex R of ISO 10646-1/Am.2.  Depending on
     the character value, the representation is a one, two, or three
     byte sequence:

          16#0000#-16#007f#: 2#0xxxxxxx#
          16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
          16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

     where the xxx bits correspond to the left-padded bits of the
     16-bit character value.  Note that all lower half ASCII characters
     are represented as ASCII bytes and all upper half characters and
     other wide characters are represented as sequences of upper-half
     (The full UTF-8 scheme allows for encoding 31-bit characters as
     6-byte sequences, but in this implementation, all UTF-8 sequences
     of four or more bytes length will raise a Constraint_Error, as
     will all invalid UTF-8 sequences.)

Brackets Coding
     In this encoding, a wide character is represented by the following
     eight character sequence:

          [ " a b c d " ]

     Where `a', `b', `c', `d' are the four hexadecimal characters
     (using uppercase letters) of the wide character code.  For
     example, `["A345"]' is used to represent the wide character with
     code `16#A345#'.  This scheme is compatible with use of the full
     Wide_Character set.  On input, brackets coding can also be used
     for upper half characters, e.g. `["C1"]' for lower case a.
     However, on output, brackets notation is only used for wide
     characters with a code greater than `16#FF#'.

   For the coding schemes other than Hex and Brackets encoding, not all
wide character values can be represented.  An attempt to output a
character that cannot be represented using the encoding scheme for the
file causes Constraint_Error to be raised.  An invalid wide character
sequence on input also causes Constraint_Error to be raised.

* Menu:

* Wide_Text_IO Stream Pointer Positioning::
* Wide_Text_IO Reading and Writing Non-Regular Files::


File: gnat_rm.info,  Node: Wide_Text_IO Stream Pointer Positioning,  Next: Wide_Text_IO Reading and Writing Non-Regular Files,  Up: Wide_Text_IO

Stream Pointer Positioning
--------------------------

`Ada.Wide_Text_IO' is similar to `Ada.Text_IO' in its handling of
stream pointer positioning (*note Text_IO::).  There is one additional
case:

   If `Ada.Wide_Text_IO.Look_Ahead' reads a character outside the
normal lower ASCII set (i.e. a character in the range:

     Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)

then although the logical position of the file pointer is unchanged by
the `Look_Ahead' call, the stream is physically positioned past the
wide character sequence.  Again this is to avoid the need for buffering
or backup, and all `Wide_Text_IO' routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using `Wide_Text_IO'.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.


File: gnat_rm.info,  Node: Wide_Text_IO Reading and Writing Non-Regular Files,  Prev: Wide_Text_IO Stream Pointer Positioning,  Up: Wide_Text_IO

Reading and Writing Non-Regular Files
-------------------------------------

As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and `End_Of_Page' always returns `False'.
Similarly, the end of file indication is not sticky, so it is possible
to read beyond an end of file.


File: gnat_rm.info,  Node: Stream_IO,  Next: Shared Files,  Prev: Wide_Text_IO,  Up: The Implementation of Standard I/O

Stream_IO
=========

A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada 95 reference manual.  The
type `Stream_Element' is simply a byte.  There are two ways to read or
write a stream file.

   * The operations `Read' and `Write' directly read or write a
     sequence of stream elements with no control information.

   * The stream attributes applied to a stream file transfer data in the
     manner described for stream attributes.


File: gnat_rm.info,  Node: Shared Files,  Next: Open Modes,  Prev: Stream_IO,  Up: The Implementation of Standard I/O

Shared Files
============

Section A.14 of the Ada 95 Reference Manual allows implementations to
provide a wide variety of behavior if an attempt is made to access the
same external file with two or more internal files.

   To provide a full range of functionality, while at the same time
minimizing the problems of portability caused by this implementation
dependence, GNAT handles file sharing as follows:

   * In the absence of a `shared=XXX' form parameter, an attempt to
     open two or more files with the same full name is considered an
     error and is not supported.  The exception `Use_Error' will be
     raised.  Note that a file that is not explicitly closed by the
     program remains open until the program terminates.

   * If the form parameter `shared=no' appears in the form string, the
     file can be opened or created with its own separate stream
     identifier, regardless of whether other files sharing the same
     external file are opened.  The exact effect depends on how the C
     stream routines handle multiple accesses to the same external
     files using separate streams.

   * If the form parameter `shared=yes' appears in the form string for
     each of two or more files opened using the same full name, the same
     stream is shared between these files, and the semantics are as
     described in Ada 95 Reference Manual, Section A.14.

   When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that `Use_Error'
is raised.

   The documentation of the original compiler and the documentation of
the program should then be examined to determine if file sharing was
expected, and `shared=XXX' parameters added to `Open' and `Create'
calls as required.

   When a program is ported from GNAT to some other Ada compiler, no
special attention is required unless the `shared=XXX' form parameter is
used in the program.  In this case, you must examine the documentation
of the new compiler to see if it supports the required file sharing
semantics, and form strings modified appropriately.  Of course it may
be the case that the program cannot be ported if the target compiler
does not support the required functionality.  The best approach in
writing portable code is to avoid file sharing (and hence the use of
the `shared=XXX' parameter in the form string) completely.

   One common use of file sharing in Ada 83 is the use of
instantiations of Sequential_IO on the same file with different types,
to achieve heterogeneous input-output.  Although this approach will
work in GNAT if `shared=yes' is specified, it is preferable in Ada 95
to use Stream_IO for this purpose (using the stream attributes)


File: gnat_rm.info,  Node: Open Modes,  Next: Operations on C Streams,  Prev: Shared Files,  Up: The Implementation of Standard I/O

Open Modes
==========

`Open' and `Create' calls result in a call to `fopen' using the mode
shown in Table 6.1



               Table 6-1 `Open' and `Create' Call Modes
                                    OPEN            CREATE
     Append_File                    "r+"             "w+"
     In_File                        "r"              "w+"
     Out_File (Direct_IO)           "r+"             "w"
     Out_File (all other cases)     "w"              "w"
     Inout_File                     "r+"             "w+"

   If text file translation is required, then either `b' or `t' is
added to the mode, depending on the setting of Text.  Text file
translation refers to the mapping of CR/LF sequences in an external file
to LF characters internally.  This mapping only occurs in DOS and
DOS-like systems, and is not relevant to other systems.

   A special case occurs with Stream_IO.  As shown in the above table,
the file is initially opened in `r' or `w' mode for the `In_File' and
`Out_File' cases.  If a `Set_Mode' operation subsequently requires
switching from reading to writing or vice-versa, then the file is
reopened in `r+' mode to permit the required operation.


File: gnat_rm.info,  Node: Operations on C Streams,  Next: Interfacing to C Streams,  Prev: Open Modes,  Up: The Implementation of Standard I/O

Operations on C Streams
=======================

   The package `Interfaces.C_Streams' provides an Ada program with
direct access to the C library functions for operations on C streams:

     package Interfaces.C_Streams is
       -- Note: the reason we do not use the types that are in
       -- Interfaces.C is that we want to avoid dragging in the
       -- code in this unit if possible.
       subtype chars is System.Address;
       -- Pointer to null-terminated array of characters
       subtype FILEs is System.Address;
       -- Corresponds to the C type FILE*
       subtype voids is System.Address;
       -- Corresponds to the C type void*
       subtype int is Integer;
       subtype long is Long_Integer;
       -- Note: the above types are subtypes deliberately, and it
       -- is part of this spec that the above correspondences are
       -- guaranteed.  This means that it is legitimate to, for
       -- example, use Integer instead of int.  We provide these
       -- synonyms for clarity, but in some cases it may be
       -- convenient to use the underlying types (for example to
       -- avoid an unnecessary dependency of a spec on the spec
       -- of this unit).
       type size_t is mod 2 ** Standard'Address_Size;
       NULL_Stream : constant FILEs;
       -- Value returned (NULL in C) to indicate an
       -- fdopen/fopen/tmpfile error
       ----------------------------------
       -- Constants Defined in stdio.h --
       ----------------------------------
       EOF : constant int;
       -- Used by a number of routines to indicate error or
       -- end of file
       IOFBF : constant int;
       IOLBF : constant int;
       IONBF : constant int;
       -- Used to indicate buffering mode for setvbuf call
       SEEK_CUR : constant int;
       SEEK_END : constant int;
       SEEK_SET : constant int;
       -- Used to indicate origin for fseek call
       function stdin return FILEs;
       function stdout return FILEs;
       function stderr return FILEs;
       -- Streams associated with standard files
       --------------------------
       -- Standard C functions --
       --------------------------
       -- The functions selected below are ones that are
       -- available in DOS, OS/2, UNIX and Xenix (but not
       -- necessarily in ANSI C).  These are very thin interfaces
       -- which copy exactly the C headers.  For more
       -- documentation on these functions, see the Microsoft C
       -- "Run-Time Library Reference" (Microsoft Press, 1990,
       -- ISBN 1-55615-225-6), which includes useful information
       -- on system compatibility.
       procedure clearerr (stream : FILEs);
       function fclose (stream : FILEs) return int;
       function fdopen (handle : int; mode : chars) return FILEs;
       function feof (stream : FILEs) return int;
       function ferror (stream : FILEs) return int;
       function fflush (stream : FILEs) return int;
       function fgetc (stream : FILEs) return int;
       function fgets (strng : chars; n : int; stream : FILEs)
           return chars;
       function fileno (stream : FILEs) return int;
       function fopen (filename : chars; Mode : chars)
           return FILEs;
       -- Note: to maintain target independence, use
       -- text_translation_required, a boolean variable defined in
       -- a-sysdep.c to deal with the target dependent text
       -- translation requirement.  If this variable is set,
       -- then  b/t should be appended to the standard mode
       -- argument to set the text translation mode off or on
       -- as required.
       function fputc (C : int; stream : FILEs) return int;
       function fputs (Strng : chars; Stream : FILEs) return int;
       function fread
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function freopen
          (filename : chars;
           mode : chars;
           stream : FILEs)
           return FILEs;
       function fseek
          (stream : FILEs;
           offset : long;
           origin : int)
           return int;
       function ftell (stream : FILEs) return long;
       function fwrite
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function isatty (handle : int) return int;
       procedure mktemp (template : chars);
       -- The return value (which is just a pointer to template)
       -- is discarded
       procedure rewind (stream : FILEs);
       function rmtmp return int;
       function setvbuf
          (stream : FILEs;
           buffer : chars;
           mode : int;
           size : size_t)
           return int;
     
       function tmpfile return FILEs;
       function ungetc (c : int; stream : FILEs) return int;
       function unlink (filename : chars) return int;
       ---------------------
       -- Extra functions --
       ---------------------
       -- These functions supply slightly thicker bindings than
       -- those above.  They are derived from functions in the
       -- C Run-Time Library, but may do a bit more work than
       -- just directly calling one of the Library functions.
       function is_regular_file (handle : int) return int;
       -- Tests if given handle is for a regular file (result 1)
       -- or for a non-regular file (pipe or device, result 0).
       ---------------------------------
       -- Control of Text/Binary Mode --
       ---------------------------------
       -- If text_translation_required is true, then the following
       -- functions may be used to dynamically switch a file from
       -- binary to text mode or vice versa.  These functions have
       -- no effect if text_translation_required is false (i.e.  in
       -- normal UNIX mode).  Use fileno to get a stream handle.
       procedure set_binary_mode (handle : int);
       procedure set_text_mode (handle : int);
       ----------------------------
       -- Full Path Name support --
       ----------------------------
       procedure full_name (nam : chars; buffer : chars);
       -- Given a NUL terminated string representing a file
       -- name, returns in buffer a NUL terminated string
       -- representing the full path name for the file name.
       -- On systems where it is relevant the   drive is also
       -- part of the full path name.  It is the responsibility
       -- of the caller to pass an actual parameter for buffer
       -- that is big enough for any full path name.  Use
       -- max_path_len given below as the size of buffer.
       max_path_len : integer;
       -- Maximum length of an allowable full path name on the
       -- system, including a terminating NUL character.
     end Interfaces.C_Streams;


File: gnat_rm.info,  Node: Interfacing to C Streams,  Prev: Operations on C Streams,  Up: The Implementation of Standard I/O

Interfacing to C Streams
========================

The packages in this section permit interfacing Ada files to C Stream
operations.

      with Interfaces.C_Streams;
      package Ada.Sequential_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Sequential_IO.C_Streams;
     
       with Interfaces.C_Streams;
       package Ada.Direct_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Direct_IO.C_Streams;
     
       with Interfaces.C_Streams;
       package Ada.Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Text_IO.C_Streams;
     
       with Interfaces.C_Streams;
       package Ada.Wide_Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
      end Ada.Wide_Text_IO.C_Streams;
     
      with Interfaces.C_Streams;
      package Ada.Stream_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Stream_IO.C_Streams;

   In each of these five packages, the `C_Stream' function obtains the
`FILE' pointer from a currently opened Ada file.  It is then possible
to use the `Interfaces.C_Streams' package to operate on this stream, or
the stream can be passed to a C program which can operate on it
directly.  Of course the program is responsible for ensuring that only
appropriate sequences of operations are executed.

   One particular use of relevance to an Ada program is that the
`setvbuf' function can be used to control the buffering of the stream
used by an Ada file.  In the absence of such a call the standard
default buffering is used.

   The `Open' procedures in these packages open a file giving an
existing C Stream instead of a file name.  Typically this stream is
imported from a C program, allowing an Ada file to operate on an
existing C file.


File: gnat_rm.info,  Node: The GNAT Library,  Next: Interfacing to Other Languages,  Prev: The Implementation of Standard I/O,  Up: Top

The GNAT Library
****************

The GNAT library contains a number of general and special purpose
packages.  It represents functionality that the GNAT developers have
found useful, and which is made available to GNAT users.  The packages
described here are fully supported, and upwards compatibility will be
maintained in future releases, so you can use these facilities with the
confidence that the same functionality will be available in future
releases.

   The chapter here simply gives a brief summary of the facilities
available.  The full documentation is found in the spec file for the
package.  The full sources of these library packages, including both
spec and body, are provided with all GNAT releases.  For example, to
find out the full specifications of the SPITBOL pattern matching
capability, including a full tutorial and extensive examples, look in
the `g-spipat.ads' file in the library.

   For each entry here, the package name (as it would appear in a `with'
clause) is given, followed by the name of the corresponding spec file in
parentheses.  The packages are children in four hierarchies, `Ada',
`Interfaces', `System', and `GNAT', the latter being a GNAT-specific
hierarchy.

   Note that an application program should only use packages in one of
these four hierarchies if the package is defined in the Ada Reference
Manual, or is listed in this section of the GNAT Programmers Reference
Manual.  All other units should be considered internal implementation
units and should not be directly `with''ed by application code.  The
use of a `with' statement that references one of these internal
implementation units makes an application potentially dependent on
changes in versions of GNAT, and will generate a warning message.

* Menu:

* Ada.Characters.Latin_9 (a-chlat9.ads)::
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads)::
* Ada.Characters.Wide_Latin_9 (a-cwila9.ads)::
* Ada.Command_Line.Remove (a-colire.ads)::
* Ada.Direct_IO.C_Streams (a-diocst.ads)::
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)::
* Ada.Sequential_IO.C_Streams (a-siocst.ads)::
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)::
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads)::
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)::
* Ada.Text_IO.C_Streams (a-tiocst.ads)::
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)::
* GNAT.AWK (g-awk.ads)::
* GNAT.Bubble_Sort_A (g-busora.ads)::
* GNAT.Bubble_Sort_G (g-busorg.ads)::
* GNAT.Calendar (g-calend.ads)::
* GNAT.Calendar.Time_IO (g-catiio.ads)::
* GNAT.CRC32 (g-crc32.ads)::
* GNAT.Case_Util (g-casuti.ads)::
* GNAT.CGI (g-cgi.ads)::
* GNAT.CGI.Cookie (g-cgicoo.ads)::
* GNAT.CGI.Debug (g-cgideb.ads)::
* GNAT.Command_Line (g-comlin.ads)::
* GNAT.Current_Exception (g-curexc.ads)::
* GNAT.Debug_Pools (g-debpoo.ads)::
* GNAT.Debug_Utilities (g-debuti.ads)::
* GNAT.Directory_Operations (g-dirope.ads)::
* GNAT.Dynamic_Tables (g-dyntab.ads)::
* GNAT.Exception_Traces (g-exctra.ads)::
* GNAT.Expect (g-expect.ads)::
* GNAT.Float_Control (g-flocon.ads)::
* GNAT.Heap_Sort_A (g-hesora.ads)::
* GNAT.Heap_Sort_G (g-hesorg.ads)::
* GNAT.HTable (g-htable.ads)::
* GNAT.IO (g-io.ads)::
* GNAT.IO_Aux (g-io_aux.ads)::
* GNAT.Lock_Files (g-locfil.ads)::
* GNAT.MD5 (g-md5.ads)::
* GNAT.Most_Recent_Exception (g-moreex.ads)::
* GNAT.OS_Lib (g-os_lib.ads)::
* GNAT.Regexp (g-regexp.ads)::
* GNAT.Registry (g-regist.ads)::
* GNAT.Regpat (g-regpat.ads)::
* GNAT.Sockets (g-socket.ads)::
* GNAT.Source_Info (g-souinf.ads)::
* GNAT.Spell_Checker (g-speche.ads)::
* GNAT.Spitbol.Patterns (g-spipat.ads)::
* GNAT.Spitbol (g-spitbo.ads)::
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads)::
* GNAT.Spitbol.Table_Integer (g-sptain.ads)::
* GNAT.Spitbol.Table_VString (g-sptavs.ads)::
* GNAT.Table (g-table.ads)::
* GNAT.Task_Lock (g-tasloc.ads)::
* GNAT.Threads (g-thread.ads)::
* GNAT.Traceback (g-traceb.ads)::
* GNAT.Traceback.Symbolic (g-trasym.ads)::
* Interfaces.C.Extensions (i-cexten.ads)::
* Interfaces.C.Streams (i-cstrea.ads)::
* Interfaces.CPP (i-cpp.ads)::
* Interfaces.Os2lib (i-os2lib.ads)::
* Interfaces.Os2lib.Errors (i-os2err.ads)::
* Interfaces.Os2lib.Synchronization (i-os2syn.ads)::
* Interfaces.Os2lib.Threads (i-os2thr.ads)::
* Interfaces.Packed_Decimal (i-pacdec.ads)::
* Interfaces.VxWorks (i-vxwork.ads)::
* Interfaces.VxWorks.IO (i-vxwoio.ads)::
* System.Address_Image (s-addima.ads)::
* System.Assertions (s-assert.ads)::
* System.Partition_Interface (s-parint.ads)::
* System.Task_Info (s-tasinf.ads)::
* System.Wch_Cnv (s-wchcnv.ads)::
* System.Wch_Con (s-wchcon.ads)::


File: gnat_rm.info,  Node: Ada.Characters.Latin_9 (a-chlat9.ads),  Next: Ada.Characters.Wide_Latin_1 (a-cwila1.ads),  Up: The GNAT Library

`Ada.Characters.Latin_9' (`a-chlat9.ads')
=========================================

This child of `Ada.Characters' provides a set of definitions
corresponding to those in the RM-defined package
`Ada.Characters.Latin_1' but with the few modifications required for
`Latin-9' The provision of such a package is specifically authorized by
the Ada Reference Manual (RM A.3(27)).


File: gnat_rm.info,  Node: Ada.Characters.Wide_Latin_1 (a-cwila1.ads),  Next: Ada.Characters.Wide_Latin_9 (a-cwila9.ads),  Prev: Ada.Characters.Latin_9 (a-chlat9.ads),  Up: The GNAT Library

`Ada.Characters.Wide_Latin_1' (`a-cwila1.ads')
==============================================

This child of `Ada.Characters' provides a set of definitions
corresponding to those in the RM-defined package
`Ada.Characters.Latin_1' but with the types of the constants being
`Wide_Character' instead of `Character'.  The provision of such a
package is specifically authorized by the Ada Reference Manual (RM
A.3(27)).


File: gnat_rm.info,  Node: Ada.Characters.Wide_Latin_9 (a-cwila9.ads),  Next: Ada.Command_Line.Remove (a-colire.ads),  Prev: Ada.Characters.Wide_Latin_1 (a-cwila1.ads),  Up: The GNAT Library

`Ada.Characters.Wide_Latin_9' (`a-cwila1.ads')
==============================================

This child of `Ada.Characters' provides a set of definitions
corresponding to those in the GNAT defined package
`Ada.Characters.Latin_9' but with the types of the constants being
`Wide_Character' instead of `Character'.  The provision of such a
package is specifically authorized by the Ada Reference Manual (RM
A.3(27)).


File: gnat_rm.info,  Node: Ada.Command_Line.Remove (a-colire.ads),  Next: Ada.Direct_IO.C_Streams (a-diocst.ads),  Prev: Ada.Characters.Wide_Latin_9 (a-cwila9.ads),  Up: The GNAT Library

`Ada.Command_Line.Remove' (`a-colire.ads')
==========================================

This child of `Ada.Command_Line' provides a mechanism for logically
removing arguments from the argument list.  Once removed, an argument
is not visible to further calls on the subprograms in
`Ada.Command_Line' will not see the removed argument.


File: gnat_rm.info,  Node: Ada.Direct_IO.C_Streams (a-diocst.ads),  Next: Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads),  Prev: Ada.Command_Line.Remove (a-colire.ads),  Up: The GNAT Library

`Ada.Direct_IO.C_Streams' (`a-diocst.ads')
==========================================

This package provides subprograms that allow interfacing between C
streams and `Direct_IO'.  The stream identifier can be extracted from a
file opened on the Ada side, and an Ada file can be constructed from a
stream opened on the C side.


File: gnat_rm.info,  Node: Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads),  Next: Ada.Sequential_IO.C_Streams (a-siocst.ads),  Prev: Ada.Direct_IO.C_Streams (a-diocst.ads),  Up: The GNAT Library

`Ada.Exceptions.Is_Null_Occurrence' (`a-einuoc.ads')
====================================================

This child subprogram provides a way of testing for the null exception
occurrence (`Null_Occurrence') without raising an exception.


File: gnat_rm.info,  Node: Ada.Sequential_IO.C_Streams (a-siocst.ads),  Next: Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads),  Prev: Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads),  Up: The GNAT Library

`Ada.Sequential_IO.C_Streams' (`a-siocst.ads')
==============================================

This package provides subprograms that allow interfacing between C
streams and `Sequential_IO'.  The stream identifier can be extracted
from a file opened on the Ada side, and an Ada file can be constructed
from a stream opened on the C side.


File: gnat_rm.info,  Node: Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads),  Next: Ada.Strings.Unbounded.Text_IO (a-suteio.ads),  Prev: Ada.Sequential_IO.C_Streams (a-siocst.ads),  Up: The GNAT Library

`Ada.Streams.Stream_IO.C_Streams' (`a-ssicst.ads')
==================================================

This package provides subprograms that allow interfacing between C
streams and `Stream_IO'.  The stream identifier can be extracted from a
file opened on the Ada side, and an Ada file can be constructed from a
stream opened on the C side.


File: gnat_rm.info,  Node: Ada.Strings.Unbounded.Text_IO (a-suteio.ads),  Next: Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads),  Prev: Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads),  Up: The GNAT Library

`Ada.Strings.Unbounded.Text_IO' (`a-suteio.ads')
================================================

This package provides subprograms for Text_IO for unbounded strings,
avoiding the necessity for an intermediate operation with ordinary
strings.


File: gnat_rm.info,  Node: Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads),  Next: Ada.Text_IO.C_Streams (a-tiocst.ads),  Prev: Ada.Strings.Unbounded.Text_IO (a-suteio.ads),  Up: The GNAT Library

`Ada.Strings.Wide_Unbounded.Wide_Text_IO' (`a-swuwti.ads')
==========================================================

This package provides subprograms for Text_IO for unbounded wide
strings, avoiding the necessity for an intermediate operation with
ordinary wide strings.


File: gnat_rm.info,  Node: Ada.Text_IO.C_Streams (a-tiocst.ads),  Next: Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads),  Prev: Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads),  Up: The GNAT Library

`Ada.Text_IO.C_Streams' (`a-tiocst.ads')
========================================

This package provides subprograms that allow interfacing between C
streams and `Text_IO'.  The stream identifier can be extracted from a
file opened on the Ada side, and an Ada file can be constructed from a
stream opened on the C side.


File: gnat_rm.info,  Node: Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads),  Next: GNAT.AWK (g-awk.ads),  Prev: Ada.Text_IO.C_Streams (a-tiocst.ads),  Up: The GNAT Library

`Ada.Wide_Text_IO.C_Streams' (`a-wtcstr.ads')
=============================================

This package provides subprograms that allow interfacing between C
streams and `Wide_Text_IO'.  The stream identifier can be extracted
from a file opened on the Ada side, and an Ada file can be constructed
from a stream opened on the C side.


File: gnat_rm.info,  Node: GNAT.AWK (g-awk.ads),  Next: GNAT.Bubble_Sort_A (g-busora.ads),  Prev: Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads),  Up: The GNAT Library

`GNAT.AWK' (`g-awk.ads')
========================

Provides AWK-like parsing functions, with an easy interface for parsing
one or more files containing formatted data.  The file is viewed as a
database where each record is a line and a field is a data element in
this line.


File: gnat_rm.info,  Node: GNAT.Bubble_Sort_A (g-busora.ads),  Next: GNAT.Bubble_Sort_G (g-busorg.ads),  Prev: GNAT.AWK (g-awk.ads),  Up: The GNAT Library

`GNAT.Bubble_Sort_A' (`g-busora.ads')
=====================================

Provides a general implementation of bubble sort usable for sorting
arbitrary data items.  Move and comparison procedures are provided by
passing access-to-procedure values.


File: gnat_rm.info,  Node: GNAT.Bubble_Sort_G (g-busorg.ads),  Next: GNAT.Calendar (g-calend.ads),  Prev: GNAT.Bubble_Sort_A (g-busora.ads),  Up: The GNAT Library

`GNAT.Bubble_Sort_G' (`g-busorg.ads')
=====================================

Similar to `Bubble_Sort_A' except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.


File: gnat_rm.info,  Node: GNAT.Calendar (g-calend.ads),  Next: GNAT.Calendar.Time_IO (g-catiio.ads),  Prev: GNAT.Bubble_Sort_G (g-busorg.ads),  Up: The GNAT Library

`GNAT.Calendar' (`g-calend.ads')
================================

Extends the facilities provided by `Ada.Calendar' to include handling
of days of the week, an extended `Split' and `Time_Of' capability.
Also provides conversion of `Ada.Calendar.Time' values to and from the
C `timeval' format.


File: gnat_rm.info,  Node: GNAT.Calendar.Time_IO (g-catiio.ads),  Next: GNAT.CRC32 (g-crc32.ads),  Prev: GNAT.Calendar (g-calend.ads),  Up: The GNAT Library

`GNAT.Calendar.Time_IO' (`g-catiio.ads')
========================================


File: gnat_rm.info,  Node: GNAT.CRC32 (g-crc32.ads),  Next: GNAT.Case_Util (g-casuti.ads),  Prev: GNAT.Calendar.Time_IO (g-catiio.ads),  Up: The GNAT Library

`GNAT.CRC32' (`g-crc32.ads')
============================

This package implements the CRC-32 algorithm.  For a full description
of this algorithm you should have a look at: "Computation of Cyclic
Redundancy Checks via Table Look-Up", `Communications of the ACM', Vol.
31 No. 8, pp. 1008-1013, Aug. 1988.  Sarwate, D.V.

Provides an extended capability for formatted output of time values with
full user control over the format.  Modeled on the GNU Date
specification.


File: gnat_rm.info,  Node: GNAT.Case_Util (g-casuti.ads),  Next: GNAT.CGI (g-cgi.ads),  Prev: GNAT.CRC32 (g-crc32.ads),  Up: The GNAT Library

`GNAT.Case_Util' (`g-casuti.ads')
=================================

A set of simple routines for handling upper and lower casing of strings
without the overhead of the full casing tables in
`Ada.Characters.Handling'.


File: gnat_rm.info,  Node: GNAT.CGI (g-cgi.ads),  Next: GNAT.CGI.Cookie (g-cgicoo.ads),  Prev: GNAT.Case_Util (g-casuti.ads),  Up: The GNAT Library

`GNAT.CGI' (`g-cgi.ads')
========================

This is a package for interfacing a GNAT program with a Web server via
the Common Gateway Interface (CGI).  Basically this package parses the
CGI parameters, which are a set of key/value pairs sent by the Web
server.  It builds a table whose index is the key and provides some
services to deal with this table.


File: gnat_rm.info,  Node: GNAT.CGI.Cookie (g-cgicoo.ads),  Next: GNAT.CGI.Debug (g-cgideb.ads),  Prev: GNAT.CGI (g-cgi.ads),  Up: The GNAT Library

`GNAT.CGI.Cookie' (`g-cgicoo.ads')
==================================

This is a package to interface a GNAT program with a Web server via the
Common Gateway Interface (CGI).  It exports services to deal with Web
cookies (piece of information kept in the Web client software).


File: gnat_rm.info,  Node: GNAT.CGI.Debug (g-cgideb.ads),  Next: GNAT.Command_Line (g-comlin.ads),  Prev: GNAT.CGI.Cookie (g-cgicoo.ads),  Up: The GNAT Library

`GNAT.CGI.Debug' (`g-cgideb.ads')
=================================

This is a package to help debugging CGI (Common Gateway Interface)
programs written in Ada.


File: gnat_rm.info,  Node: GNAT.Command_Line (g-comlin.ads),  Next: GNAT.Current_Exception (g-curexc.ads),  Prev: GNAT.CGI.Debug (g-cgideb.ads),  Up: The GNAT Library

`GNAT.Command_Line' (`g-comlin.ads')
====================================

Provides a high level interface to `Ada.Command_Line' facilities,
including the ability to scan for named switches with optional
parameters and expand file names using wild card notations.


File: gnat_rm.info,  Node: GNAT.Current_Exception (g-curexc.ads),  Next: GNAT.Debug_Pools (g-debpoo.ads),  Prev: GNAT.Command_Line (g-comlin.ads),  Up: The GNAT Library

`GNAT.Current_Exception' (`g-curexc.ads')
=========================================

Provides access to information on the current exception that has been
raised without the need for using the Ada-95 exception choice parameter
specification syntax.  This is particularly useful in simulating
typical facilities for obtaining information about exceptions provided
by Ada 83 compilers.


File: gnat_rm.info,  Node: GNAT.Debug_Pools (g-debpoo.ads),  Next: GNAT.Debug_Utilities (g-debuti.ads),  Prev: GNAT.Current_Exception (g-curexc.ads),  Up: The GNAT Library

`GNAT.Debug_Pools' (`g-debpoo.ads')
===================================

Provide a debugging storage pools that helps tracking memory corruption
problems.  See section "Finding memory problems with GNAT Debug Pool" in
the `GNAT User's Guide'.


File: gnat_rm.info,  Node: GNAT.Debug_Utilities (g-debuti.ads),  Next: GNAT.Directory_Operations (g-dirope.ads),  Prev: GNAT.Debug_Pools (g-debpoo.ads),  Up: The GNAT Library

`GNAT.Debug_Utilities' (`g-debuti.ads')
=======================================

Provides a few useful utilities for debugging purposes, including
conversion to and from string images of address values.


File: gnat_rm.info,  Node: GNAT.Directory_Operations (g-dirope.ads),  Next: GNAT.Dynamic_Tables (g-dyntab.ads),  Prev: GNAT.Debug_Utilities (g-debuti.ads),  Up: The GNAT Library

`GNAT.Directory_Operations' (g-dirope.ads)
==========================================

Provides a set of routines for manipulating directories, including
changing the current directory, making new directories, and scanning
the files in a directory.


File: gnat_rm.info,  Node: GNAT.Dynamic_Tables (g-dyntab.ads),  Next: GNAT.Exception_Traces (g-exctra.ads),  Prev: GNAT.Directory_Operations (g-dirope.ads),  Up: The GNAT Library

`GNAT.Dynamic_Tables' (`g-dyntab.ads')
======================================

A generic package providing a single dimension array abstraction where
the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Table, except
that this package declares a type that can be used to define dynamic
instances of the table, while an instantiation of GNAT.Table creates a
single instance of the table type.


File: gnat_rm.info,  Node: GNAT.Exception_Traces (g-exctra.ads),  Next: GNAT.Expect (g-expect.ads),  Prev: GNAT.Dynamic_Tables (g-dyntab.ads),  Up: The GNAT Library

`GNAT.Exception_Traces' (`g-exctra.ads')
========================================

Provides an interface allowing to control automatic output upon
exception occurrences.


File: gnat_rm.info,  Node: GNAT.Expect (g-expect.ads),  Next: GNAT.Float_Control (g-flocon.ads),  Prev: GNAT.Exception_Traces (g-exctra.ads),  Up: The GNAT Library

`GNAT.Expect' (`g-expect.ads')
==============================

Provides a set of subprograms similar to what is available with the
standard Tcl Expect tool.  It allows you to easily spawn and
communicate with an external process.  You can send commands or inputs
to the process, and compare the output with some expected regular
expression.  Currently GNAT.Expect is implemented on all native GNAT
ports except for OpenVMS.  It is not implemented for cross ports, and
in particular is not implemented for VxWorks or LynxOS.


File: gnat_rm.info,  Node: GNAT.Float_Control (g-flocon.ads),  Next: GNAT.Heap_Sort_A (g-hesora.ads),  Prev: GNAT.Expect (g-expect.ads),  Up: The GNAT Library

`GNAT.Float_Control' (`g-flocon.ads')
=====================================

Provides an interface for resetting the floating-point processor into
the mode required for correct semantic operation in Ada.  Some third
party library calls may cause this mode to be modified, and the Reset
procedure in this package can be used to reestablish the required mode.


File: gnat_rm.info,  Node: GNAT.Heap_Sort_A (g-hesora.ads),  Next: GNAT.Heap_Sort_G (g-hesorg.ads),  Prev: GNAT.Float_Control (g-flocon.ads),  Up: The GNAT Library

`GNAT.Heap_Sort_A' (`g-hesora.ads')
===================================

Provides a general implementation of heap sort usable for sorting
arbitrary data items.  Move and comparison procedures are provided by
passing access-to-procedure values.  The algorithm used is a modified
heap sort that performs approximately N*log(N) comparisons in the worst
case.


File: gnat_rm.info,  Node: GNAT.Heap_Sort_G (g-hesorg.ads),  Next: GNAT.HTable (g-htable.ads),  Prev: GNAT.Heap_Sort_A (g-hesora.ads),  Up: The GNAT Library

`GNAT.Heap_Sort_G' (`g-hesorg.ads')
===================================

Similar to `Heap_Sort_A' except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.


File: gnat_rm.info,  Node: GNAT.HTable (g-htable.ads),  Next: GNAT.IO (g-io.ads),  Prev: GNAT.Heap_Sort_G (g-hesorg.ads),  Up: The GNAT Library

`GNAT.HTable' (`g-htable.ads')
==============================

A generic implementation of hash tables that can be used to hash
arbitrary data.  Provides two approaches, one a simple static approach,
and the other allowing arbitrary dynamic hash tables.


File: gnat_rm.info,  Node: GNAT.IO (g-io.ads),  Next: GNAT.IO_Aux (g-io_aux.ads),  Prev: GNAT.HTable (g-htable.ads),  Up: The GNAT Library

`GNAT.IO' (`g-io.ads')
======================

A simple preealborable input-output package that provides a subset of
simple Text_IO functions for reading characters and strings from
Standard_Input, and writing characters, strings and integers to either
Standard_Output or Standard_Error.


File: gnat_rm.info,  Node: GNAT.IO_Aux (g-io_aux.ads),  Next: GNAT.Lock_Files (g-locfil.ads),  Prev: GNAT.IO (g-io.ads),  Up: The GNAT Library

`GNAT.IO_Aux' (`g-io_aux.ads')
==============================

   Provides some auxiliary functions for use with Text_IO, including a
test for whether a file exists, and functions for reading a line of
text.


File: gnat_rm.info,  Node: GNAT.Lock_Files (g-locfil.ads),  Next: GNAT.MD5 (g-md5.ads),  Prev: GNAT.IO_Aux (g-io_aux.ads),  Up: The GNAT Library

`GNAT.Lock_Files' (`g-locfil.ads')
==================================

Provides a general interface for using files as locks.  Can be used for
providing program level synchronization.


File: gnat_rm.info,  Node: GNAT.MD5 (g-md5.ads),  Next: GNAT.Most_Recent_Exception (g-moreex.ads),  Prev: GNAT.Lock_Files (g-locfil.ads),  Up: The GNAT Library

`GNAT.MD5' (`g-md5.ads')
========================

Implements the  MD5 Message-Digest Algorithm as described in RFC 1321.


File: gnat_rm.info,  Node: GNAT.Most_Recent_Exception (g-moreex.ads),  Next: GNAT.OS_Lib (g-os_lib.ads),  Prev: GNAT.MD5 (g-md5.ads),  Up: The GNAT Library

`GNAT.Most_Recent_Exception' (`g-moreex.ads')
=============================================

Provides access to the most recently raised exception.  Can be used for
various logging purposes, including duplicating functionality of some
Ada 83 implementation dependent extensions.


File: gnat_rm.info,  Node: GNAT.OS_Lib (g-os_lib.ads),  Next: GNAT.Regexp (g-regexp.ads),  Prev: GNAT.Most_Recent_Exception (g-moreex.ads),  Up: The GNAT Library

`GNAT.OS_Lib' (`g-os_lib.ads')
==============================

Provides a range of target independent operating system interface
functions, including time/date management, file operations, subprocess
management, including a portable spawn procedure, and access to
environment variables and error return codes.


File: gnat_rm.info,  Node: GNAT.Regexp (g-regexp.ads),  Next: GNAT.Registry (g-regist.ads),  Prev: GNAT.OS_Lib (g-os_lib.ads),  Up: The GNAT Library

`GNAT.Regexp' (`g-regexp.ads')
==============================

A simple implementation of regular expressions, using a subset of
regular expression syntax copied from familiar Unix style utilities.
This is the simples of the three pattern matching packages provided,
and is particularly suitable for "file globbing" applications.


File: gnat_rm.info,  Node: GNAT.Registry (g-regist.ads),  Next: GNAT.Regpat (g-regpat.ads),  Prev: GNAT.Regexp (g-regexp.ads),  Up: The GNAT Library

`GNAT.Registry' (`g-regist.ads')
================================

This is a high level binding to the Windows registry.  It is possible to
do simple things like reading a key value, creating a new key.  For full
registry API, but at a lower level of abstraction, refer to the
Win32.Winreg package provided with the Win32Ada binding


File: gnat_rm.info,  Node: GNAT.Regpat (g-regpat.ads),  Next: GNAT.Sockets (g-socket.ads),  Prev: GNAT.Registry (g-regist.ads),  Up: The GNAT Library

`GNAT.Regpat' (`g-regpat.ads')
==============================

A complete implementation of Unix-style regular expression matching,
copied from the original V7 style regular expression library written in
C by Henry Spencer (and binary compatible with this C library).


File: gnat_rm.info,  Node: GNAT.Sockets (g-socket.ads),  Next: GNAT.Source_Info (g-souinf.ads),  Prev: GNAT.Regpat (g-regpat.ads),  Up: The GNAT Library

`GNAT.Sockets' (`g-socket.ads')
===============================

A high level and portable interface to develop sockets based
applications.  This package is based on the sockets thin binding found
in GNAT.Sockets.Thin.  Currently GNAT.Sockets is implemented on all
native GNAT ports except for OpenVMS.  It is not implemented for the
LynxOS cross port.


File: gnat_rm.info,  Node: GNAT.Source_Info (g-souinf.ads),  Next: GNAT.Spell_Checker (g-speche.ads),  Prev: GNAT.Sockets (g-socket.ads),  Up: The GNAT Library

`GNAT.Source_Info' (`g-souinf.ads')
===================================

Provides subprograms that give access to source code information known
at compile time, such as the current file name and line number.


File: gnat_rm.info,  Node: GNAT.Spell_Checker (g-speche.ads),  Next: GNAT.Spitbol.Patterns (g-spipat.ads),  Prev: GNAT.Source_Info (g-souinf.ads),  Up: The GNAT Library

`GNAT.Spell_Checker' (`g-speche.ads')
=====================================

Provides a function for determining whether one string is a plausible
near misspelling of another string.


File: gnat_rm.info,  Node: GNAT.Spitbol.Patterns (g-spipat.ads),  Next: GNAT.Spitbol (g-spitbo.ads),  Prev: GNAT.Spell_Checker (g-speche.ads),  Up: The GNAT Library

`GNAT.Spitbol.Patterns' (`g-spipat.ads')
========================================

A complete implementation of SNOBOL4 style pattern matching.  This is
the most elaborate of the pattern matching packages provided.  It fully
duplicates the SNOBOL4 dynamic pattern construction and matching
capabilities, using the efficient algorithm developed by Robert Dewar
for the SPITBOL system.


File: gnat_rm.info,  Node: GNAT.Spitbol (g-spitbo.ads),  Next: GNAT.Spitbol.Table_Boolean (g-sptabo.ads),  Prev: GNAT.Spitbol.Patterns (g-spipat.ads),  Up: The GNAT Library

`GNAT.Spitbol' (`g-spitbo.ads')
===============================

The top level package of the collection of SPITBOL-style functionality,
this package provides basic SNOBOL4 string manipulation functions, such
as Pad, Reverse, Trim, Substr capability, as well as a generic table
function useful for constructing arbitrary mappings from strings in the
style of the SNOBOL4 TABLE function.


File: gnat_rm.info,  Node: GNAT.Spitbol.Table_Boolean (g-sptabo.ads),  Next: GNAT.Spitbol.Table_Integer (g-sptain.ads),  Prev: GNAT.Spitbol (g-spitbo.ads),  Up: The GNAT Library

`GNAT.Spitbol.Table_Boolean' (`g-sptabo.ads')
=============================================

A library level of instantiation of `GNAT.Spitbol.Patterns.Table' for
type `Standard.Boolean', giving an implementation of sets of string
values.


File: gnat_rm.info,  Node: GNAT.Spitbol.Table_Integer (g-sptain.ads),  Next: GNAT.Spitbol.Table_VString (g-sptavs.ads),  Prev: GNAT.Spitbol.Table_Boolean (g-sptabo.ads),  Up: The GNAT Library

`GNAT.Spitbol.Table_Integer' (`g-sptain.ads')
=============================================

A library level of instantiation of `GNAT.Spitbol.Patterns.Table' for
type `Standard.Integer', giving an implementation of maps from string
to integer values.


File: gnat_rm.info,  Node: GNAT.Spitbol.Table_VString (g-sptavs.ads),  Next: GNAT.Table (g-table.ads),  Prev: GNAT.Spitbol.Table_Integer (g-sptain.ads),  Up: The GNAT Library

`GNAT.Spitbol.Table_VString' (`g-sptavs.ads')
=============================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for a
variable length string type, giving an implementation of general maps
from strings to strings.


File: gnat_rm.info,  Node: GNAT.Table (g-table.ads),  Next: GNAT.Task_Lock (g-tasloc.ads),  Prev: GNAT.Spitbol.Table_VString (g-sptavs.ads),  Up: The GNAT Library

`GNAT.Table' (`g-table.ads')
============================

A generic package providing a single dimension array abstraction where
the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Dynamic_Tables,
except that this package declares a single instance of the table type,
while an instantiation of GNAT.Dynamic_Tables creates a type that can be
used to define dynamic instances of the table.


File: gnat_rm.info,  Node: GNAT.Task_Lock (g-tasloc.ads),  Next: GNAT.Threads (g-thread.ads),  Prev: GNAT.Table (g-table.ads),  Up: The GNAT Library

`GNAT.Task_Lock' (`g-tasloc.ads')
=================================

A very simple facility for locking and unlocking sections of code using
a single global task lock.  Appropriate for use in situations where
contention between tasks is very rarely expected.


File: gnat_rm.info,  Node: GNAT.Threads (g-thread.ads),  Next: GNAT.Traceback (g-traceb.ads),  Prev: GNAT.Task_Lock (g-tasloc.ads),  Up: The GNAT Library

`GNAT.Threads' (`g-thread.ads')
===============================

Provides facilities for creating and destroying threads with explicit
calls.  These threads are known to the GNAT run-time system.  These
subprograms are exported C-convention procedures intended to be called
from foreign code.  By using these primitives rather than directly
calling operating systems routines, compatibility with the Ada tasking
runt-time is provided.


File: gnat_rm.info,  Node: GNAT.Traceback (g-traceb.ads),  Next: GNAT.Traceback.Symbolic (g-trasym.ads),  Prev: GNAT.Threads (g-thread.ads),  Up: The GNAT Library

`GNAT.Traceback' (`g-traceb.ads')
=================================

Provides a facility for obtaining non-symbolic traceback information,
useful in various debugging situations.


File: gnat_rm.info,  Node: GNAT.Traceback.Symbolic (g-trasym.ads),  Next: Interfaces.C.Extensions (i-cexten.ads),  Prev: GNAT.Traceback (g-traceb.ads),  Up: The GNAT Library

`GNAT.Traceback.Symbolic' (`g-trasym.ads')
==========================================

Provides symbolic traceback information that includes the subprogram
name and line number information.


File: gnat_rm.info,  Node: Interfaces.C.Extensions (i-cexten.ads),  Next: Interfaces.C.Streams (i-cstrea.ads),  Prev: GNAT.Traceback.Symbolic (g-trasym.ads),  Up: The GNAT Library

`Interfaces.C.Extensions' (`i-cexten.ads')
==========================================

This package contains additional C-related definitions, intended for
use with either manually or automatically generated bindings to C
libraries.


File: gnat_rm.info,  Node: Interfaces.C.Streams (i-cstrea.ads),  Next: Interfaces.CPP (i-cpp.ads),  Prev: Interfaces.C.Extensions (i-cexten.ads),  Up: The GNAT Library

`Interfaces.C.Streams' (`i-cstrea.ads')
=======================================

This package is a binding for the most commonly used operations on C
streams.


File: gnat_rm.info,  Node: Interfaces.CPP (i-cpp.ads),  Next: Interfaces.Os2lib (i-os2lib.ads),  Prev: Interfaces.C.Streams (i-cstrea.ads),  Up: The GNAT Library

`Interfaces.CPP' (`i-cpp.ads')
==============================

This package provides facilities for use in interfacing to C++.  It is
primarily intended to be used in connection with automated tools for
the generation of C++ interfaces.


File: gnat_rm.info,  Node: Interfaces.Os2lib (i-os2lib.ads),  Next: Interfaces.Os2lib.Errors (i-os2err.ads),  Prev: Interfaces.CPP (i-cpp.ads),  Up: The GNAT Library

`Interfaces.Os2lib' (`i-os2lib.ads')
====================================

This package provides interface definitions to the OS/2 library.  It is
a thin binding which is a direct translation of the various `<bse.h>'
files.


File: gnat_rm.info,  Node: Interfaces.Os2lib.Errors (i-os2err.ads),  Next: Interfaces.Os2lib.Synchronization (i-os2syn.ads),  Prev: Interfaces.Os2lib (i-os2lib.ads),  Up: The GNAT Library

`Interfaces.Os2lib.Errors' (`i-os2err.ads')
===========================================

This package provides definitions of the OS/2 error codes.


File: gnat_rm.info,  Node: Interfaces.Os2lib.Synchronization (i-os2syn.ads),  Next: Interfaces.Os2lib.Threads (i-os2thr.ads),  Prev: Interfaces.Os2lib.Errors (i-os2err.ads),  Up: The GNAT Library

`Interfaces.Os2lib.Synchronization' (`i-os2syn.ads')
====================================================

This is a child package that provides definitions for interfacing to
the `OS/2' synchronization primitives.


File: gnat_rm.info,  Node: Interfaces.Os2lib.Threads (i-os2thr.ads),  Next: Interfaces.Packed_Decimal (i-pacdec.ads),  Prev: Interfaces.Os2lib.Synchronization (i-os2syn.ads),  Up: The GNAT Library

`Interfaces.Os2lib.Threads' (`i-os2thr.ads')
============================================

This is a child package that provides definitions for interfacing to
the `OS/2' thread primitives.


File: gnat_rm.info,  Node: Interfaces.Packed_Decimal (i-pacdec.ads),  Next: Interfaces.VxWorks (i-vxwork.ads),  Prev: Interfaces.Os2lib.Threads (i-os2thr.ads),  Up: The GNAT Library

`Interfaces.Packed_Decimal' (`i-pacdec.ads')
============================================

This package provides a set of routines for conversions to and from a
packed decimal format compatible with that used on IBM mainframes.


File: gnat_rm.info,  Node: Interfaces.VxWorks (i-vxwork.ads),  Next: Interfaces.VxWorks.IO (i-vxwoio.ads),  Prev: Interfaces.Packed_Decimal (i-pacdec.ads),  Up: The GNAT Library

`Interfaces.VxWorks' (`i-vxwork.ads')
=====================================

This package provides a limited binding to the VxWorks API.  In
particular, it interfaces with the VxWorks hardware interrupt
facilities.


File: gnat_rm.info,  Node: Interfaces.VxWorks.IO (i-vxwoio.ads),  Next: System.Address_Image (s-addima.ads),  Prev: Interfaces.VxWorks (i-vxwork.ads),  Up: The GNAT Library

`Interfaces.VxWorks.IO' (`i-vxwoio.ads')
========================================

This package provides a limited binding to the VxWorks' I/O API.  In
particular, it provides procedures that enable the use of Get_Immediate
under VxWorks.


File: gnat_rm.info,  Node: System.Address_Image (s-addima.ads),  Next: System.Assertions (s-assert.ads),  Prev: Interfaces.VxWorks.IO (i-vxwoio.ads),  Up: The GNAT Library

`System.Address_Image' (`s-addima.ads')
=======================================

This function provides a useful debugging function that gives an
(implementation dependent) string which identifies an address.


File: gnat_rm.info,  Node: System.Assertions (s-assert.ads),  Next: System.Partition_Interface (s-parint.ads),  Prev: System.Address_Image (s-addima.ads),  Up: The GNAT Library

`System.Assertions' (`s-assert.ads')
====================================

This package provides the declaration of the exception raised by an
run-time assertion failure, as well as the routine that is used
internally to raise this assertion.


File: gnat_rm.info,  Node: System.Partition_Interface (s-parint.ads),  Next: System.Task_Info (s-tasinf.ads),  Prev: System.Assertions (s-assert.ads),  Up: The GNAT Library

`System.Partition_Interface' (`s-parint.ads')
=============================================

This package provides facilities for partition interfacing.  It is used
primarily in a distribution context when using Annex E with `GLADE'.


File: gnat_rm.info,  Node: System.Task_Info (s-tasinf.ads),  Next: System.Wch_Cnv (s-wchcnv.ads),  Prev: System.Partition_Interface (s-parint.ads),  Up: The GNAT Library

`System.Task_Info' (`s-tasinf.ads')
===================================

This package provides target dependent functionality that is used to
support the `Task_Info' pragma


File: gnat_rm.info,  Node: System.Wch_Cnv (s-wchcnv.ads),  Next: System.Wch_Con (s-wchcon.ads),  Prev: System.Task_Info (s-tasinf.ads),  Up: The GNAT Library

`System.Wch_Cnv' (`s-wchcnv.ads')
=================================

This package provides routines for converting between wide characters
and a representation as a value of type `Standard.String', using a
specified wide character encoding method.  It uses definitions in
package `System.Wch_Con'.


File: gnat_rm.info,  Node: System.Wch_Con (s-wchcon.ads),  Prev: System.Wch_Cnv (s-wchcnv.ads),  Up: The GNAT Library

`System.Wch_Con' (`s-wchcon.ads')
=================================

This package provides definitions and descriptions of the various
methods used for encoding wide characters in ordinary strings.  These
definitions are used by the package `System.Wch_Cnv'.


File: gnat_rm.info,  Node: Interfacing to Other Languages,  Next: Machine Code Insertions,  Prev: The GNAT Library,  Up: Top

Interfacing to Other Languages
******************************

The facilities in annex B of the Ada 95 Reference Manual are fully
implemented in GNAT, and in addition, a full interface to C++ is
provided.

* Menu:

* Interfacing to C::
* Interfacing to C++::
* Interfacing to COBOL::
* Interfacing to Fortran::
* Interfacing to non-GNAT Ada code::


File: gnat_rm.info,  Node: Interfacing to C,  Next: Interfacing to C++,  Up: Interfacing to Other Languages

Interfacing to C
================

Interfacing to C with GNAT can use one of two approaches:

  1. The types in the package `Interfaces.C' may be used.

  2. Standard Ada types may be used directly.  This may be less
     portable to other compilers, but will work on all GNAT compilers,
     which guarantee correspondence between the C and Ada types.

Pragma `Convention C' maybe applied to Ada types, but mostly has no
effect, since this is the default.  The following table shows the
correspondence between Ada scalar types and the corresponding C types.

`Integer'
     `int'

`Short_Integer'
     `short'

`Short_Short_Integer'
     `signed char'

`Long_Integer'
     `long'

`Long_Long_Integer'
     `long long'

`Short_Float'
     `float'

`Float'
     `float'

`Long_Float'
     `double'

`Long_Long_Float'
     This is the longest floating-point type supported by the hardware.

   * Ada enumeration types map to C enumeration types directly if pragma
     `Convention C' is specified, which causes them to have int length.
     Without pragma `Convention C', Ada enumeration types map to 8,
     16, or 32 bits (i.e. C types `signed char', `short', `int',
     respectively) depending on the number of values passed.  This is
     the only case in which pragma `Convention C' affects the
     representation of an Ada type.

   * Ada access types map to C pointers, except for the case of
     pointers to unconstrained types in Ada, which have no direct C
     equivalent.

   * Ada arrays map directly to C arrays.

   * Ada records map directly to C structures.

   * Packed Ada records map to C structures where all members are bit
     fields of the length corresponding to the `TYPE'Size' value in Ada.


File: gnat_rm.info,  Node: Interfacing to C++,  Next: Interfacing to COBOL,  Prev: Interfacing to C,  Up: Interfacing to Other Languages

Interfacing to C++
==================

The interface to C++ makes use of the following pragmas, which are
primarily intended to be constructed automatically using a binding
generator tool, although it is possible to construct them by hand.  No
suitable binding generator tool is supplied with GNAT though.

   Using these pragmas it is possible to achieve complete
inter-operability between Ada tagged types and C class definitions.
See *Note Implementation Defined Pragmas:: for more details.

`pragma CPP_Class ([Entity =>] LOCAL_NAME)'
     The argument denotes an entity in the current declarative region
     that is declared as a tagged or untagged record type.  It
     indicates that the type corresponds to an externally declared C++
     class type, and is to be laid out the same way that C++ would lay
     out the type.

`pragma CPP_Constructor ([Entity =>] LOCAL_NAME)'
     This pragma identifies an imported function (imported in the usual
     way with pragma `Import') as corresponding to a C++ constructor.

`pragma CPP_Vtable ...'
     One `CPP_Vtable' pragma can be present for each component of type
     `CPP.Interfaces.Vtable_Ptr' in a record to which pragma `CPP_Class'
     applies.


File: gnat_rm.info,  Node: Interfacing to COBOL,  Next: Interfacing to Fortran,  Prev: Interfacing to C++,  Up: Interfacing to Other Languages

Interfacing to COBOL
====================

Interfacing to COBOL is achieved as described in section B.4 of the Ada
95 reference manual.


File: gnat_rm.info,  Node: Interfacing to Fortran,  Next: Interfacing to non-GNAT Ada code,  Prev: Interfacing to COBOL,  Up: Interfacing to Other Languages

Interfacing to Fortran
======================

Interfacing to Fortran is achieved as described in section B.5 of the
reference manual.  The pragma `Convention Fortran', applied to a
multi-dimensional array causes the array to be stored in column-major
order as required for convenient interface to Fortran.


File: gnat_rm.info,  Node: Interfacing to non-GNAT Ada code,  Prev: Interfacing to Fortran,  Up: Interfacing to Other Languages

Interfacing to non-GNAT Ada code
================================

   It is possible to specify the convention `Ada' in a pragma `Import'
or pragma `Export'.  However this refers to the calling conventions used
by GNAT, which may or may not be similar enough to those used by some
other Ada 83 or Ada 95 compiler to allow interoperation.

   If arguments types are kept simple, and if the foreign compiler
generally follows system calling conventions, then it may be possible
to integrate files compiled by other Ada compilers, provided that the
elaboration issues are adequately addressed (for example by eliminating
the need for any load time elaboration).

   In particular, GNAT running on VMS is designed to be highly
compatible with the DEC Ada 83 compiler, so this is one case in which
it is possible to import foreign units of this type, provided that the
data items passed are restricted to simple scalar values or simple
record types without variants, or simple array types with fixed bounds.


File: gnat_rm.info,  Node: Machine Code Insertions,  Next: GNAT Implementation of Tasking,  Prev: Interfacing to Other Languages,  Up: Top

Machine Code Insertions
***********************

Package `Machine_Code' provides machine code support as described in
the Ada 95 Reference Manual in two separate forms:
   * Machine code statements, consisting of qualified expressions that
     fit the requirements of RM section 13.8.

   * An intrinsic callable procedure, providing an alternative
     mechanism of including machine instructions in a subprogram.

   The two features are similar, and both closely related to the
mechanism provided by the asm instruction in the GNU C compiler.  Full
understanding and use of the facilities in this package requires
understanding the asm instruction as described in `Using and Porting
the GNU Compiler Collection (GCC)' by Richard Stallman.  Calls to the
function `Asm' and the procedure `Asm' have identical semantic
restrictions and effects as described below.  Both are provided so that
the procedure call can be used as a statement, and the function call
can be used to form a code_statement.

   The first example given in the GCC documentation is the C `asm'
instruction:
        asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));

The equivalent can be written for GNAT as:

     Asm ("fsinx %1 %0",
          My_Float'Asm_Output ("=f", result),
          My_Float'Asm_Input  ("f",  angle));

   The first argument to `Asm' is the assembler template, and is
identical to what is used in GNU C.  This string must be a static
expression.  The second argument is the output operand list.  It is
either a single `Asm_Output' attribute reference, or a list of such
references enclosed in parentheses (technically an array aggregate of
such references).

   The `Asm_Output' attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix.  The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g. what kind of register is required).
The second argument is the variable to be updated with the result.  The
possible values for constraint are the same as those used in the RTL,
and are dependent on the configuration file used to build the GCC back
end.  If there are no output operands, then this argument may either be
omitted, or explicitly given as `No_Output_Operands'.

   The second argument of `MY_FLOAT'Asm_Output' functions as though it
were an `out' parameter, which is a little curious, but all names have
the form of expressions, so there is no syntactic irregularity, even
though normally functions would not be permitted `out' parameters.  The
third argument is the list of input operands.  It is either a single
`Asm_Input' attribute reference, or a list of such references enclosed
in parentheses (technically an array aggregate of such references).

   The `Asm_Input' attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix.  The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g. what kind of register is required).  The second argument is the
value to be used as the input argument.  The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.

   If there are no input operands, this argument may either be omitted,
or explicitly given as `No_Input_Operands'.  The fourth argument, not
present in the above example, is a list of register names, called the
"clobber" argument.  This argument, if given, must be a static string
expression, and is a space or comma separated list of names of registers
that must be considered destroyed as a result of the `Asm' call.  If
this argument is the null string (the default value), then the code
generator assumes that no additional registers are destroyed.

   The fifth argument, not present in the above example, called the
"volatile" argument, is by default `False'.  It can be set to the
literal value `True' to indicate to the code generator that all
optimizations with respect to the instruction specified should be
suppressed, and that in particular, for an instruction that has outputs,
the instruction will still be generated, even if none of the outputs are
used.  See the full description in the GCC manual for further details.

   The `Asm' subprograms may be used in two ways.  First the procedure
forms can be used anywhere a procedure call would be valid, and
correspond to what the RM calls "intrinsic" routines.  Such calls can
be used to intersperse machine instructions with other Ada statements.
Second, the function forms, which return a dummy value of the limited
private type `Asm_Insn', can be used in code statements, and indeed
this is the only context where such calls are allowed.  Code statements
appear as aggregates of the form:

     Asm_Insn'(Asm (...));
     Asm_Insn'(Asm_Volatile (...));

   In accordance with RM rules, such code statements are allowed only
within subprograms whose entire body consists of such statements.  It is
not permissible to intermix such statements with other Ada statements.

   Typically the form using intrinsic procedure calls is more convenient
and more flexible.  The code statement form is provided to meet the RM
suggestion that such a facility should be made available.  The following
is the exact syntax of the call to `Asm' (of course if named notation is
used, the arguments may be given in arbitrary order, following the
normal rules for use of positional and named arguments)

     ASM_CALL ::= Asm (
                      [Template =>] static_string_EXPRESSION
                    [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
                    [,[Inputs   =>] INPUT_OPERAND_LIST       ]
                    [,[Clobber  =>] static_string_EXPRESSION ]
                    [,[Volatile =>] static_boolean_EXPRESSION] )
     OUTPUT_OPERAND_LIST ::=
       No_Output_Operands
     | OUTPUT_OPERAND_ATTRIBUTE
     | (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})
     OUTPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)
     INPUT_OPERAND_LIST ::=
       No_Input_Operands
     | INPUT_OPERAND_ATTRIBUTE
     | (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})
     INPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)


File: gnat_rm.info,  Node: GNAT Implementation of Tasking,  Next: Code generation for array aggregates,  Prev: Machine Code Insertions,  Up: Top

GNAT Implementation of Tasking
******************************

* Menu:

* Mapping Ada Tasks onto the Underlying Kernel Threads::
* Ensuring Compliance with the Real-Time Annex::


File: gnat_rm.info,  Node: Mapping Ada Tasks onto the Underlying Kernel Threads,  Next: Ensuring Compliance with the Real-Time Annex,  Up: GNAT Implementation of Tasking

Mapping Ada Tasks onto the Underlying Kernel Threads
====================================================

   GNAT run-time system comprises two layers:

   * GNARL (GNAT Run-time  Layer)

   * GNULL (GNAT Low-level Library)

   In GNAT, Ada's tasking services rely on a platform and OS independent
layer known as GNARL.  This code is responsible for implementing the
correct semantics of Ada's task creation, rendezvous, protected
operations etc.

   GNARL decomposes Ada's tasking semantics into simpler lower level
operations such as create a thread, set the priority of a thread,
yield, create a lock, lock/unlock, etc.  The spec for these low-level
operations constitutes GNULLI, the GNULL Interface.  This interface is
directly inspired from the POSIX real-time API.

   If the underlying executive or OS implements the POSIX standard
faithfully, the GNULL Interface maps as is to the services offered by
the underlying kernel.  Otherwise, some target dependent glue code maps
the services offered by the underlying kernel to the semantics expected
by GNARL.

   Whatever the underlying OS (VxWorks, UNIX, OS/2, Windows NT, etc.)
the key point is that each Ada task is mapped on a thread in the
underlying kernel.  For example, in the case of VxWorks, one Ada task =
one VxWorks task.

   In addition Ada task priorities map onto the underlying thread
priorities.  Mapping Ada tasks onto the underlying kernel threads has
several advantages:

  1. The underlying scheduler is used to schedule the Ada tasks.  This
     makes Ada tasks as efficient as kernel threads from a scheduling
     standpoint.

  2. Interaction with code written in C containing threads is eased
     since at the lowest level Ada tasks and C threads map onto the same
     underlying kernel concept.

  3. When an Ada task is blocked during I/O the remaining Ada tasks are
     able to proceed.

  4. On multi-processor systems Ada Tasks can execute in parallel.


File: gnat_rm.info,  Node: Ensuring Compliance with the Real-Time Annex,  Prev: Mapping Ada Tasks onto the Underlying Kernel Threads,  Up: GNAT Implementation of Tasking

Ensuring Compliance with the Real-Time Annex
============================================

   The reader will be quick to notice that while mapping Ada tasks onto
the underlying threads has significant advantages, it does create some
complications when it comes to respecting the scheduling semantics
specified in the real-time annex (Annex D).

   For instance Annex D requires that for the FIFO_Within_Priorities
scheduling policy we have:

     When the active priority of a ready task that is not running
     changes, or the setting of its base priority takes effect, the
     task is removed from the ready queue for its old active priority
     and is added at the tail of the ready queue for its new active
     priority, except in the case where the active priority is lowered
     due to the loss of inherited priority, in which case the task is
     added at the head of the ready queue for its new active priority.

   While most kernels do put tasks at the end of the priority queue when
a task changes its priority, (which respects the main
FIFO_Within_Priorities requirement), almost none keep a thread at the
beginning of its priority queue when its priority drops from the loss
of inherited priority.

   As a result most vendors have provided incomplete Annex D
implementations.

   The GNAT run-time, has a nice cooperative solution to this problem
which ensures that accurate FIFO_Within_Priorities semantics are
respected.

   The principle is as follows.  When an Ada task T is about to start
running, it checks whether some other Ada task R with the same priority
as T has been suspended due to the loss of priority inheritance.  If
this is the case, T yields and is placed at the end of its priority
queue.  When R arrives at the front of the queue it executes.

   Note that this simple scheme preserves the relative order of the
tasks that were ready to execute in the priority queue where R has been
placed at the end.


File: gnat_rm.info,  Node: Code generation for array aggregates,  Next: Specialized Needs Annexes,  Prev: GNAT Implementation of Tasking,  Up: Top

Code generation for array aggregates
************************************

* Menu:

* Static constant aggregates with static bounds::
* Constant aggregates with an unconstrained nominal types::
* Aggregates with static bounds::
* Aggregates with non-static bounds::
* Aggregates in assignments statements::

   Aggregate have a rich syntax and allow the user to specify the
values of complex data structures by means of a single construct.  As a
result, the code generated for aggregates can be quite complex and
involve loops, case statements and multiple assignments.  In the
simplest cases, however, the compiler will recognize aggregates whose
components and constraints are fully static, and in those cases the
compiler will generate little or no executable code.  The following is
an outline of the code that GNAT generates for various aggregate
constructs.  For further details, the user will find it useful to
examine the output produced by the -gnatG flag to see the expanded
source that is input to the code generator.  The user will also want to
examine the assembly code generated at various levels of optimization.

   The code generated for aggregates depends on the context, the
component values, and the type.  In the context of an object
declaration the code generated is generally simpler than in the case of
an assignment.  As a general rule, static component values and static
subtypes also lead to simpler code.


File: gnat_rm.info,  Node: Static constant aggregates with static bounds,  Next: Constant aggregates with an unconstrained nominal types,  Up: Code generation for array aggregates

Static constant aggregates with static bounds
=============================================

   For the declarations:
         type One_Dim is array (1..10) of integer;
         ar0 : constant One_Dim := ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 0);

   GNAT generates no executable code: the constant ar0 is placed in
static memory.  The same is true for constant aggregates with named
associations:

         Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1=> 1);
         Cr3 : constant One_Dim := (others => 7777);

   The same is true for multidimensional constant arrays such as:

         type two_dim is array (1..3, 1..3) of integer;
         Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));

   The same is true for arrays of one-dimensional arrays: the following
are static:

     type ar1b is array (1..3) of boolean;
     type ar_ar is array (1..3) of ar1b;
     None : constant ar1b := (others => false);      --  fully static
     None2 : constant ar_ar := (1..3 => None);       --  fully static

   However, for multidimensional aggregates with named associations,
GNAT will generate assignments and loops, even if all associations are
static.  The following two declarations generate a loop for the first
dimension, and individual component assignments for the second
dimension:

     Zero1: constant two_dim := (1..3 => (1..3 => 0));
     Zero2: constant two_dim := (others => (others => 0));


File: gnat_rm.info,  Node: Constant aggregates with an unconstrained nominal types,  Next: Aggregates with static bounds,  Prev: Static constant aggregates with static bounds,  Up: Code generation for array aggregates

Constant aggregates with an unconstrained nominal types
=======================================================

   In such cases the aggregate itself establishes the subtype, so that
associations with `others' cannot be used.  GNAT determines the bounds
for the actual subtype of the aggregate, and allocates the aggregate
statically as well.  No code is generated for the following:

         type One_Unc is array (natural range <>) of integer;
         Cr_Unc : constant One_Unc := (12,24,36);


File: gnat_rm.info,  Node: Aggregates with static bounds,  Next: Aggregates with non-static bounds,  Prev: Constant aggregates with an unconstrained nominal types,  Up: Code generation for array aggregates

Aggregates with static bounds
=============================

   In all previous examples the aggregate was the initial (and
immutable) value of a constant.  If the aggregate initializes a
variable, then code is generated for it as a combination of individual
assignments and loops over the target object.  The declarations

            Cr_Var1 : One_Dim := (2, 5, 7, 11);
            Cr_Var2 : One_Dim := (others > -1);

   generate the equivalent of

            Cr_Var1 (1) := 2;
            Cr_Var1 (2) := 3;
            Cr_Var1 (3) := 5;
            Cr_Var1 (4) := 11;
     
            for I in Cr_Var2'range loop
               Cr_Var2 (I) := =-1;
            end loop;


File: gnat_rm.info,  Node: Aggregates with non-static bounds,  Next: Aggregates in assignments statements,  Prev: Aggregates with static bounds,  Up: Code generation for array aggregates

Aggregates with non-static bounds
=================================

   If the bounds of the aggregate are not statically compatible with
the bounds of the nominal subtype  of the target, then constraint
checks have to be generated on the bounds.  For a multidimensional
array, constraint checks may have to be applied to sub-arrays
individually, if they do not have statically compatible subtypes.


File: gnat_rm.info,  Node: Aggregates in assignments statements,  Prev: Aggregates with non-static bounds,  Up: Code generation for array aggregates

Aggregates in assignments statements
====================================

   In general, aggregate assignment requires the construction of a
temporary, and a copy from the temporary to the target of the
assignment.  This is because it is not always possible to convert the
assignment into a series of individual component assignments.  For
example, consider the simple case:

           A := (A(2), A(1));

   This cannot be converted into:

             A(1) := A(2);
             A(2) := A(1);

   So the aggregate has to be built first in a separate location, and
then copied into the target.  GNAT recognizes simple cases where this
intermediate step is not required, and the assignments can be performed
in place, directly into the target.  The following sufficient criteria
are applied:

  1. The bounds of the aggregate are static, and the associations are
     static.

  2. The components of the aggregate are static constants, names of
     simple variables that are not renamings, or expressions not
     involving     indexed components whose operands obey these rules.

   If any of these conditions are violated, the aggregate will be built
in a temporary (created either by the front-end or the code generator)
and then that temporary will be copied onto the target.


File: gnat_rm.info,  Node: Specialized Needs Annexes,  Next: Compatibility Guide,  Prev: Code generation for array aggregates,  Up: Top

Specialized Needs Annexes
*************************

Ada 95 defines a number of specialized needs annexes, which are not
required in all implementations.  However, as described in this chapter,
GNAT implements all of these special needs annexes:

Systems Programming (Annex C)
     The Systems Programming Annex is fully implemented.

Real-Time Systems (Annex D)
     The Real-Time Systems Annex is fully implemented.

Distributed Systems (Annex E)
     Stub generation is fully implemented in the GNAT compiler.  In
     addition, a complete compatible PCS is available as part of the
     GLADE system, a separate product.  When the two products are used
     in conjunction, this annex is fully implemented.

Information Systems (Annex F)
     The Information Systems annex is fully implemented.

Numerics (Annex G)
     The Numerics Annex is fully implemented.

Safety and Security (Annex H)
     The Safety and Security annex is fully implemented.


File: gnat_rm.info,  Node: Compatibility Guide,  Next: GNU Free Documentation License,  Prev: Specialized Needs Annexes,  Up: Top

Compatibility Guide
*******************

This chapter contains sections that describe compatibility issues
between GNAT and other Ada 83 and Ada 95 compilation systems, to aid in
porting applications developed in other Ada environments.

* Menu:

* Compatibility with Ada 83::
* Compatibility with DEC Ada 83::
* Compatibility with Other Ada 95 Systems::
* Representation Clauses::


File: gnat_rm.info,  Node: Compatibility with Ada 83,  Next: Compatibility with DEC Ada 83,  Up: Compatibility Guide

Compatibility with Ada 83
=========================

Ada 95 is designed to be highly upwards compatible with Ada 83.  In
particular, the design intention is that the difficulties associated
with moving from Ada 83 to Ada 95 should be no greater than those that
occur when moving from one Ada 83 system to another.

   However, there are a number of points at which there are minor
incompatibilities.  The Ada 95 Annotated Reference Manual contains full
details of these issues, and should be consulted for a complete
treatment.  In practice the following are the most likely issues to be
encountered.

Character range
     The range of `Standard.Character' is now the full 256 characters
     of Latin-1, whereas in most Ada 83 implementations it was
     restricted to 128 characters.  This may show up as compile time or
     runtime errors.  The desirable fix is to adapt the program to
     accommodate the full character set, but in some cases it may be
     convenient to define a subtype or derived type of Character that
     covers only the restricted range.

New reserved words
     The identifiers `abstract', `aliased', `protected', `requeue',
     `tagged', and `until' are reserved in Ada 95.  Existing Ada 83
     code using any of these identifiers must be edited to use some
     alternative name.

Freezing rules
     The rules in Ada 95 are slightly different with regard to the
     point at which entities are frozen, and representation pragmas and
     clauses are not permitted past the freeze point.  This shows up
     most typically in the form of an error message complaining that a
     representation item appears too late, and the appropriate
     corrective action is to move the item nearer to the declaration of
     the entity to which it refers.

     A particular case is that representation pragmas (including the
     extended DEC Ada 83 compatibility pragmas such as
     `Export_Procedure'), cannot be applied to a subprogram body.  If
     necessary, a separate subprogram declaration must be introduced to
     which the pragma can be applied.

Optional bodies for library packages
     In Ada 83, a package that did not require a package body was
     nevertheless allowed to have one.  This lead to certain surprises
     in compiling large systems (situations in which the body could be
     unexpectedly ignored).  In Ada 95, if a package does not require a
     body then it is not permitted to have a body.  To fix this
     problem, simply remove a redundant body if it is empty, or, if it
     is non-empty, introduce a dummy declaration into the spec that
     makes the body required.  One approach is to add a private part to
     the package declaration (if necessary), and define a parameterless
     procedure called Requires_Body, which must then be given a dummy
     procedure body in the package body, which then becomes required.

`Numeric_Error' is now the same as `Constraint_Error'
     In Ada 95, the exception `Numeric_Error' is a renaming of
     `Constraint_Error'.  This means that it is illegal to have
     separate exception handlers for the two exceptions.  The fix is
     simply to remove the handler for the `Numeric_Error' case (since
     even in Ada 83, a compiler was free to raise `Constraint_Error' in
     place of `Numeric_Error' in all cases).

Indefinite subtypes in generics
     In Ada 83, it was permissible to pass an indefinite type (e.g.
     `String') as the actual for a generic formal private type, but
     then the instantiation would be illegal if there were any
     instances of declarations of variables of this type in the generic
     body.  In Ada 95, to avoid this clear violation of the contract
     model, the generic declaration clearly indicates whether or not
     such instantiations are permitted.  If a generic formal parameter
     has explicit unknown discriminants, indicated by using `(<>)'
     after the type name, then it can be instantiated with indefinite
     types, but no variables can be declared of this type.  Any attempt
     to declare a variable will result in an illegality at the time the
     generic is declared.  If the `(<>)' notation is not used, then it
     is illegal to instantiate the generic with an indefinite type.
     This will show up as a compile time error, and the fix is usually
     simply to add the `(<>)' to the generic declaration.

   All implementations of GNAT provide a switch that causes GNAT to
operate in Ada 83 mode.  In this mode, some but not all compatibility
problems of the type described above are handled automatically.  For
example, the new Ada 95 protected keywords are not recognized in this
mode.  However, in practice, it is usually advisable to make the
necessary modifications to the program to remove the need for using
this switch.


File: gnat_rm.info,  Node: Compatibility with Other Ada 95 Systems,  Next: Representation Clauses,  Prev: Compatibility with DEC Ada 83,  Up: Compatibility Guide

Compatibility with Other Ada 95 Systems
=======================================

Providing that programs avoid the use of implementation dependent and
implementation defined features of Ada 95, as documented in the Ada 95
reference manual, there should be a high degree of portability between
GNAT and other Ada 95 systems.  The following are specific items which
have proved troublesome in moving GNAT programs to other Ada 95
compilers, but do not affect porting code to GNAT.

Ada 83 Pragmas and Attributes
     Ada 95 compilers are allowed, but not required, to implement the
     missing Ada 83 pragmas and attributes that are no longer defined
     in Ada 95.  GNAT implements all such pragmas and attributes,
     eliminating this as a compatibility concern, but some other Ada 95
     compilers reject these pragmas and attributes.

Special-needs Annexes
     GNAT implements the full set of special needs annexes.  At the
     current time, it is the only Ada 95 compiler to do so.  This means
     that programs making use of these features may not be portable to
     other Ada 95 compilation systems.

Representation Clauses
     Some other Ada 95 compilers implement only the minimal set of
     representation clauses required by the Ada 95 reference manual.
     GNAT goes far beyond this minimal set, as described in the next
     section.


File: gnat_rm.info,  Node: Representation Clauses,  Prev: Compatibility with Other Ada 95 Systems,  Up: Compatibility Guide

Representation Clauses
======================

The Ada 83 reference manual was quite vague in describing both the
minimal required implementation of representation clauses, and also
their precise effects.  The Ada 95 reference manual is much more
explicit, but the minimal set of capabilities required in Ada 95 is
quite limited.

   GNAT implements the full required set of capabilities described in
the Ada 95 reference manual, but also goes much beyond this, and in
particular an effort has been made to be compatible with existing Ada
83 usage to the greatest extent possible.

   A few cases exist in which Ada 83 compiler behavior is incompatible
with requirements in the Ada 95 reference manual.  These are instances
of intentional or accidental dependence on specific implementation
dependent characteristics of these Ada 83 compilers.  The following is
a list of the cases most likely to arise in existing legacy Ada 83 code.

Implicit Packing
     Some Ada 83 compilers allowed a Size specification to cause
     implicit packing of an array or record.  This could cause
     expensive implicit conversions for change of representation in the
     presence of derived types, and the Ada design intends to avoid
     this possibility.  Subsequent AI's were issued to make it clear
     that such implicit change of representation in response to a Size
     clause is inadvisable, and this recommendation is represented
     explicitly in the Ada 95 RM as implementation advice that is
     followed by GNAT.  The problem will show up as an error message
     rejecting the size clause.  The fix is simply to provide the
     explicit pragma `Pack', or for more fine tuned control, provide a
     Component_Size clause.

Meaning of Size Attribute
     The Size attribute in Ada 95 for discrete types is defined as
     being the minimal number of bits required to hold values of the
     type.  For example, on a 32-bit machine, the size of Natural will
     typically be 31 and not 32 (since no sign bit is required).  Some
     Ada 83 compilers gave 31, and some 32 in this situation.  This
     problem will usually show up as a compile time error, but not
     always.  It is a good idea to check all uses of the 'Size
     attribute when porting Ada 83 code.  The GNAT specific attribute
     Object_Size can provide a useful way of duplicating the behavior of
     some Ada 83 compiler systems.

Size of Access Types
     A common assumption in Ada 83 code is that an access type is in
     fact a pointer, and that therefore it will be the same size as a
     System.Address value.  This assumption is true for GNAT in most
     cases with one exception.  For the case of a pointer to an
     unconstrained array type (where the bounds may vary from one value
     of the access type to another), the default is to use a "fat
     pointer", which is represented as two separate pointers, one to
     the bounds, and one to the array.  This representation has a
     number of advantages, including improved efficiency.  However, it
     may cause some difficulties in porting existing Ada 83 code which
     makes the assumption that, for example, pointers fit in 32 bits on
     a machine with 32-bit addressing.

     To get around this problem, GNAT also permits the use of "thin
     pointers" for access types in this case (where the designated type
     is an unconstrained array type).  These thin pointers are indeed
     the same size as a System.Address value.  To specify a thin
     pointer, use a size clause for the type, for example:

          type X is access all String;
          for X'Size use Standard'Address_Size;

     which will cause the type X to be represented using a single
     pointer.  When using this representation, the bounds are right
     behind the array.  This representation is slightly less efficient,
     and does not allow quite such flexibility in the use of foreign
     pointers or in using the Unrestricted_Access attribute to create
     pointers to non-aliased objects.  But for any standard portable
     use of the access type it will work in a functionally correct
     manner and allow porting of existing code.  Note that another way
     of forcing a thin pointer representation is to use a component
     size clause for the element size in an array, or a record
     representation clause for an access field in a record.


File: gnat_rm.info,  Node: Compatibility with DEC Ada 83,  Next: Compatibility with Other Ada 95 Systems,  Prev: Compatibility with Ada 83,  Up: Compatibility Guide

Compatibility with DEC Ada 83
=============================

The VMS version of GNAT fully implements all the pragmas and attributes
provided by DEC Ada 83, as well as providing the standard DEC Ada 83
libraries, including Starlet.  In addition, data layouts and parameter
passing conventions are highly compatible.  This means that porting
existing DEC Ada 83 code to GNAT in VMS systems should be easier than
most other porting efforts.  The following are some of the most
significant differences between GNAT and DEC Ada 83.

Default floating-point representation
     In GNAT, the default floating-point format is IEEE, whereas in DEC
     Ada 83, it is VMS format.  GNAT does implement the necessary
     pragmas (Long_Float, Float_Representation) for changing this
     default.

System
     The package System in GNAT exactly corresponds to the definition
     in the Ada 95 reference manual, which means that it excludes many
     of the DEC Ada 83 extensions.  However, a separate package Aux_DEC
     is provided that contains the additional definitions, and a
     special pragma, Extend_System allows this package to be treated
     transparently as an extension of package System.

To_Address
     The definitions provided by Aux_DEC are exactly compatible with
     those in the DEC Ada 83 version of System, with one exception.
     DEC Ada provides the following declarations:

          TO_ADDRESS(INTEGER)
          TO_ADDRESS(UNSIGNED_LONGWORD)
          TO_ADDRESS(universal_integer)

     The version of TO_ADDRESS taking a universal integer argument is
     in fact an extension to Ada 83 not strictly compatible with the
     reference manual.  In GNAT, we are constrained to be exactly
     compatible with the standard, and this means we cannot provide
     this capability.  In DEC Ada 83, the point of this definition is
     to deal with a call like:

             TO_ADDRESS (16#12777#);

     Normally, according to the Ada 83 standard, one would expect this
     to be ambiguous, since it matches both the INTEGER and
     UNSIGNED_LONGWORD forms of TO_ADDRESS.  However, in DEC Ada 83,
     there is no ambiguity, since the definition using
     universal_integer takes precedence.

     In GNAT, since the version with universal_integer cannot be
     supplied, it is not possible to be 100% compatible.  Since there
     are many programs using numeric constants for the argument to
     TO_ADDRESS, the decision in GNAT was to change the name of the
     function in the UNSIGNED_LONGWORD case, so the declarations
     provided in the GNAT version of AUX_Dec are:

          function To_Address (X : Integer) return Address;
          pragma Pure_Function (To_Address);
          
          function To_Address_Long (X : Unsigned_Longword)
           return Address;
          pragma Pure_Function (To_Address_Long);

     This means that programs using TO_ADDRESS for UNSIGNED_LONGWORD
     must change the name to TO_ADDRESS_LONG.

Task_Id values
     The Task_Id values assigned will be different in the two systems,
     and GNAT does not provide a specified value for the Task_Id of the
     environment task, which in GNAT is treated like any other declared
     task.

   For full details on these and other less significant compatibility
issues, see appendix E of the Digital publication entitled `DEC Ada,
Technical Overview and Comparison on DIGITAL Platforms'.

   For GNAT running on other than VMS systems, all the DEC Ada 83
pragmas and attributes are recognized, although only a subset of them
can sensibly be implemented.  The description of pragmas in this
reference manual indicates whether or not they are applicable to
non-VMS systems.


File: gnat_rm.info,  Node: GNU Free Documentation License,  Next: Index,  Prev: Compatibility Guide,  Up: Top

GNU Free Documentation License
******************************

                      Version 1.2, November 2002
     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307, USA
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

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     This License is a kind of "copyleft", which means that derivative
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     We have designed this License in order to use it for manuals for
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     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in
     the notice that says that the Document is released under this
     License.  If a section does not fit the above definition of
     Secondary then it is not allowed to be designated as Invariant.
     The Document may contain zero Invariant Sections.  If the Document
     does not identify any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images
     composed of pixels) generic paint programs or (for drawings) some
     widely available drawing editor, and that is suitable for input to
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     markup, has been arranged to thwart or discourage subsequent
     modification by readers is not Transparent.  An image format is
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     copy that is not "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
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     produced by some word processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow
     the conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the
     title equally prominent and visible.  You may add other material
     on the covers in addition.  Copying with changes limited to the
     covers, as long as they preserve the title of the Document and
     satisfy these conditions, can be treated as verbatim copying in
     other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a
     machine-readable Transparent copy along with each Opaque copy, or
     state in or with each Opaque copy a computer-network location from
     which the general network-using public has access to download
     using public-standard network protocols a complete Transparent
     copy of the Document, free of added material.  If you use the
     latter option, you must take reasonably prudent steps, when you
     begin distribution of Opaque copies in quantity, to ensure that
     this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you
     distribute an Opaque copy (directly or through your agents or
     retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of
     copies, to give them a chance to provide you with an updated
     version of the Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with
     the Modified Version filling the role of the Document, thus
     licensing distribution and modification of the Modified Version to
     whoever possesses a copy of it.  In addition, you must do these
     things in the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of
          previous versions (which should, if there were any, be listed
          in the History section of the Document).  You may use the
          same title as a previous version if the original publisher of
          that version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on
          the Title Page.  If there is no section Entitled "History" in
          the Document, create one stating the title, year, authors,
          and publisher of the Document as given on its Title Page,
          then add an item describing the Modified Version as stated in
          the previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in
          the "History" section.  You may omit a network location for a
          work that was published at least four years before the
          Document itself, or if the original publisher of the version
          it refers to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the
          section all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document,
          unaltered in their text and in their titles.  Section numbers
          or the equivalent are not considered part of the section
          titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end
     of the list of Cover Texts in the Modified Version.  Only one
     passage of Front-Cover Text and one of Back-Cover Text may be
     added by (or through arrangements made by) any one entity.  If the
     Document already includes a cover text for the same cover,
     previously added by you or by arrangement made by the same entity
     you are acting on behalf of, you may not add another; but you may
     replace the old one, on explicit permission from the previous
     publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination
     all of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the
     documents in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow
     this License in all other respects regarding verbatim copying of
     that document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of
     a storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warrany Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     `http://www.gnu.org/copyleft/'.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If
     the Document does not specify a version number of this License,
     you may choose any version ever published (not as a draft) by the
     Free Software Foundation.

ADDENDUM: How to use this License for your documents
====================================================

   To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
       A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.


File: gnat_rm.info,  Node: Index,  Prev: GNU Free Documentation License,  Up: Top

Index
*****

* Menu:

* -gnatR switch:                         Determining the Representations chosen by GNAT.
* Abort_Defer:                           Implementation Defined Pragmas.
* Abort_Signal:                          Implementation Defined Attributes.
* Access, unrestricted:                  Implementation Defined Attributes.
* Accuracy requirements:                 Implementation Advice.
* Accuracy, complex arithmetic:          Implementation Advice.
* Ada 83 attributes:                     Implementation Defined Attributes.
* Ada 95 ISO/ANSI Standard:              What This Reference Manual Contains.
* Ada.Characters.Handling:               Implementation Advice.
* Ada.Characters.Latin_9 (a-chlat9.ads): Ada.Characters.Latin_9 (a-chlat9.ads).
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads): Ada.Characters.Wide_Latin_1 (a-cwila1.ads).
* Ada.Characters.Wide_Latin_9 (a-cwila1.ads): Ada.Characters.Wide_Latin_9 (a-cwila9.ads).
* Ada.Command_Line.Remove (a-colire.ads): Ada.Command_Line.Remove (a-colire.ads).
* Ada.Direct_IO.C_Streams (a-diocst.ads): Ada.Direct_IO.C_Streams (a-diocst.ads).
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads): Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads).
* Ada.Sequential_IO.C_Streams (a-siocst.ads): Ada.Sequential_IO.C_Streams (a-siocst.ads).
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads): Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads).
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads): Ada.Strings.Unbounded.Text_IO (a-suteio.ads).
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads): Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads).
* Ada.Text_IO.C_Streams (a-tiocst.ads):  Ada.Text_IO.C_Streams (a-tiocst.ads).
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads): Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads).
* Ada_83:                                Implementation Defined Pragmas.
* Ada_95:                                Implementation Defined Pragmas.
* Address Clause:                        Address Clauses.
* Address clauses:                       Implementation Advice.
* Address image:                         System.Address_Image (s-addima.ads).
* Address of subprogram code:            Implementation Defined Attributes.
* Address, as private type:              Implementation Advice.
* Address, operations of:                Implementation Advice.
* Address_Size:                          Implementation Defined Attributes.
* Alignment Clause:                      Alignment Clauses.
* Alignment clauses:                     Implementation Advice.
* Alignment, default:                    Alignment Clauses.
* Alignment, maximum:                    Implementation Defined Attributes.
* Alignments of components:              Implementation Defined Pragmas.
* Alternative Character Sets:            Implementation Advice.
* Annotate:                              Implementation Defined Pragmas.
* Argument passing mechanisms:           Implementation Defined Pragmas.
* Arrays, extendable <1>:                GNAT.Table (g-table.ads).
* Arrays, extendable:                    GNAT.Dynamic_Tables (g-dyntab.ads).
* Arrays, multidimensional:              Implementation Advice.
* Asm_Input:                             Implementation Defined Attributes.
* Asm_Output:                            Implementation Defined Attributes.
* Assert:                                Implementation Defined Pragmas.
* Assert_Failure, exception:             System.Assertions (s-assert.ads).
* Assertions:                            System.Assertions (s-assert.ads).
* Ast_Entry:                             Implementation Defined Pragmas.
* AST_Entry:                             Implementation Defined Attributes.
* Attribute:                             Address Clauses.
* AWK:                                   GNAT.AWK (g-awk.ads).
* Biased representation:                 Biased Representation.
* Big endian:                            Implementation Defined Attributes.
* Bit:                                   Implementation Defined Attributes.
* Bit ordering:                          Implementation Advice.
* bit ordering:                          Bit_Order Clauses.
* Bit_Order Clause:                      Bit_Order Clauses.
* Bit_Position:                          Implementation Defined Attributes.
* Boolean_Entry_Barriers:                Implementation Defined Characteristics.
* Bounded errors:                        Implementation Advice.
* Bounded-length strings:                Implementation Advice.
* Bubble sort <1>:                       GNAT.Bubble_Sort_G (g-busorg.ads).
* Bubble sort:                           GNAT.Bubble_Sort_A (g-busora.ads).
* byte ordering:                         Effect of Bit_Order on Byte Ordering.
* C streams, interfacing:                Interfaces.C.Streams (i-cstrea.ads).
* C Streams, Interfacing with Direct_IO: Ada.Direct_IO.C_Streams (a-diocst.ads).
* C Streams, Interfacing with Sequential_IO: Ada.Sequential_IO.C_Streams (a-siocst.ads).
* C Streams, Interfacing with Stream_IO: Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads).
* C Streams, Interfacing with Text_IO:   Ada.Text_IO.C_Streams (a-tiocst.ads).
* C Streams, Interfacing with Wide_Text_IO: Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads).
* C++ interfacing:                       Interfaces.CPP (i-cpp.ads).
* C, interfacing with:                   Implementation Advice.
* C_Pass_By_Copy:                        Implementation Defined Pragmas.
* Calendar <1>:                          GNAT.Calendar (g-calend.ads).
* Calendar:                              GNAT.Calendar.Time_IO (g-catiio.ads).
* Casing of External names:              Implementation Defined Pragmas.
* Casing utilities:                      GNAT.Case_Util (g-casuti.ads).
* CGI (Common Gateway Interface):        GNAT.CGI (g-cgi.ads).
* CGI (Common Gateway Interface) cookie support: GNAT.CGI.Cookie (g-cgicoo.ads).
* CGI (Common Gateway Interface) debugging: GNAT.CGI.Debug (g-cgideb.ads).
* Character handling (GNAT.Case_Util):   GNAT.Case_Util (g-casuti.ads).
* Character Sets:                        Implementation Advice.
* Checks, suppression of:                Implementation Advice.
* Child Units:                           Implementation Advice.
* COBOL support:                         Implementation Advice.
* COBOL, interfacing with:               Implementation Advice.
* Code_Address:                          Implementation Defined Attributes.
* Command line:                          GNAT.Command_Line (g-comlin.ads).
* Command line, argument removal:        Ada.Command_Line.Remove (a-colire.ads).
* Comment:                               Implementation Defined Pragmas.
* Common_Object:                         Implementation Defined Pragmas.
* Compatibility (between Ada 83 and Ada 95): Compatibility with Ada 83.
* Complex arithmetic accuracy:           Implementation Advice.
* Complex elementary functions:          Implementation Advice.
* Complex types:                         Implementation Advice.
* Complex_Representation:                Implementation Defined Pragmas.
* Component Clause:                      Record Representation Clauses.
* Component_Alignment:                   Implementation Defined Pragmas.
* Component_Size:                        Implementation Defined Pragmas.
* Component_Size Clause:                 Component_Size Clauses.
* Component_Size clauses:                Implementation Advice.
* Component_Size_4:                      Implementation Defined Pragmas.
* Convention, effect on representation:  Effect of Convention on Representation.
* Convention_Identifier:                 Implementation Defined Pragmas.
* Conventions, synonyms:                 Implementation Defined Pragmas.
* Conventions, typographical:            Conventions.
* Cookie support in CGI:                 GNAT.CGI.Cookie (g-cgicoo.ads).
* CPP_Class:                             Implementation Defined Pragmas.
* CPP_Constructor:                       Implementation Defined Pragmas.
* CPP_Virtual:                           Implementation Defined Pragmas.
* CPP_Vtable:                            Implementation Defined Pragmas.
* CRC32:                                 GNAT.CRC32 (g-crc32.ads).
* Current exception:                     GNAT.Current_Exception (g-curexc.ads).
* Cyclic Redundancy Check:               GNAT.CRC32 (g-crc32.ads).
* Debug:                                 Implementation Defined Pragmas.
* Debug pools:                           GNAT.Debug_Pools (g-debpoo.ads).
* Debugging <1>:                         GNAT.Exception_Traces (g-exctra.ads).
* Debugging <2>:                         GNAT.Debug_Utilities (g-debuti.ads).
* Debugging:                             GNAT.Debug_Pools (g-debpoo.ads).
* debugging with Initialize_Scalars:     Implementation Defined Pragmas.
* Dec Ada 83:                            Implementation Defined Pragmas.
* Dec Ada 83 casing compatibility:       Implementation Defined Pragmas.
* Decimal radix support:                 Implementation Advice.
* Default_Bit_Order:                     Implementation Defined Attributes.
* Deferring aborts:                      Implementation Defined Pragmas.
* Directory operations:                  GNAT.Directory_Operations (g-dirope.ads).
* Discriminants, testing for:            Implementation Defined Attributes.
* Duration'Small:                        Implementation Advice.
* Elab_Body:                             Implementation Defined Attributes.
* Elab_Spec:                             Implementation Defined Attributes.
* Elaborated:                            Implementation Defined Attributes.
* Elaboration control:                   Implementation Defined Pragmas.
* Elaboration_Checks:                    Implementation Defined Pragmas.
* Eliminate:                             Implementation Defined Pragmas.
* Elimination of unused subprograms:     Implementation Defined Pragmas.
* Emax:                                  Implementation Defined Attributes.
* Enclosing_Entity:                      Enclosing_Entity.
* Entry queuing policies:                Implementation Advice.
* Enum_Rep:                              Implementation Defined Attributes.
* Enumeration representation clauses:    Implementation Advice.
* Enumeration values:                    Implementation Advice.
* Epsilon:                               Implementation Defined Attributes.
* Error detection:                       Implementation Advice.
* Exception information:                 Implementation Advice.
* Exception retrieval:                   GNAT.Current_Exception (g-curexc.ads).
* Exception traces:                      GNAT.Exception_Traces (g-exctra.ads).
* Exception, obtaining most recent:      GNAT.Most_Recent_Exception (g-moreex.ads).
* Exception_Information':                Exception_Information.
* Exception_Message:                     Exception_Message.
* Exception_Name:                        Exception_Name.
* Export <1>:                            Address Clauses.
* Export:                                Implementation Advice.
* Export_Exception:                      Implementation Defined Pragmas.
* Export_Function:                       Implementation Defined Pragmas.
* Export_Object:                         Implementation Defined Pragmas.
* Export_Procedure:                      Implementation Defined Pragmas.
* Export_Valued_Procedure:               Implementation Defined Pragmas.
* Extend_System:                         Implementation Defined Pragmas.
* External:                              Implementation Defined Pragmas.
* External Names, casing:                Implementation Defined Pragmas.
* External_Name_Casing:                  Implementation Defined Pragmas.
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
* File:                                  File.
* File locking:                          GNAT.Lock_Files (g-locfil.ads).
* Finalize_Storage_Only:                 Implementation Defined Pragmas.
* Fixed_Value:                           Implementation Defined Attributes.
* Float types:                           Implementation Advice.
* Float_Representation:                  Implementation Defined Pragmas.
* Floating-Point Processor:              GNAT.Float_Control (g-flocon.ads).
* Foreign threads:                       GNAT.Threads (g-thread.ads).
* Fortran, interfacing with:             Implementation Advice.
* Get_Immediate <1>:                     Get_Immediate.
* Get_Immediate:                         Implementation Advice.
* GNAT.AWK (g-awk.ads):                  GNAT.AWK (g-awk.ads).
* GNAT.Bubble_Sort_A (g-busora.ads):     GNAT.Bubble_Sort_A (g-busora.ads).
* GNAT.Bubble_Sort_G (g-busorg.ads):     GNAT.Bubble_Sort_G (g-busorg.ads).
* GNAT.Calendar (g-calend.ads):          GNAT.Calendar (g-calend.ads).
* GNAT.Calendar.Time_IO (g-catiio.ads):  GNAT.Calendar.Time_IO (g-catiio.ads).
* GNAT.Case_Util (g-casuti.ads):         GNAT.Case_Util (g-casuti.ads).
* GNAT.CGI (g-cgi.ads):                  GNAT.CGI (g-cgi.ads).
* GNAT.CGI.Cookie (g-cgicoo.ads):        GNAT.CGI.Cookie (g-cgicoo.ads).
* GNAT.CGI.Debug (g-cgideb.ads):         GNAT.CGI.Debug (g-cgideb.ads).
* GNAT.Command_Line (g-comlin.ads):      GNAT.Command_Line (g-comlin.ads).
* GNAT.CRC32 (g-crc32.ads):              GNAT.CRC32 (g-crc32.ads).
* GNAT.Current_Exception (g-curexc.ads): GNAT.Current_Exception (g-curexc.ads).
* GNAT.Debug_Pools (g-debpoo.ads):       GNAT.Debug_Pools (g-debpoo.ads).
* GNAT.Debug_Utilities (g-debuti.ads):   GNAT.Debug_Utilities (g-debuti.ads).
* GNAT.Directory_Operations (g-dirope.ads): GNAT.Directory_Operations (g-dirope.ads).
* GNAT.Dynamic_Tables (g-dyntab.ads):    GNAT.Dynamic_Tables (g-dyntab.ads).
* GNAT.Exception_Traces (g-exctra.ads):  GNAT.Exception_Traces (g-exctra.ads).
* GNAT.Expect (g-expect.ads):            GNAT.Expect (g-expect.ads).
* GNAT.Float_Control (g-flocon.ads):     GNAT.Float_Control (g-flocon.ads).
* GNAT.Heap_Sort_A (g-hesora.ads):       GNAT.Heap_Sort_A (g-hesora.ads).
* GNAT.Heap_Sort_G (g-hesorg.ads):       GNAT.Heap_Sort_G (g-hesorg.ads).
* GNAT.HTable (g-htable.ads):            GNAT.HTable (g-htable.ads).
* GNAT.IO (g-io.ads):                    GNAT.IO (g-io.ads).
* GNAT.IO_Aux (g-io_aux.ads):            GNAT.IO_Aux (g-io_aux.ads).
* GNAT.Lock_Files (g-locfil.ads):        GNAT.Lock_Files (g-locfil.ads).
* GNAT.MD5 (g-md5.ads):                  GNAT.MD5 (g-md5.ads).
* GNAT.Most_Recent_Exception (g-moreex.ads): GNAT.Most_Recent_Exception (g-moreex.ads).
* GNAT.OS_Lib (g-os_lib.ads):            GNAT.OS_Lib (g-os_lib.ads).
* GNAT.Regexp (g-regexp.ads):            GNAT.Regexp (g-regexp.ads).
* GNAT.Registry (g-regist.ads):          GNAT.Registry (g-regist.ads).
* GNAT.Regpat (g-regpat.ads):            GNAT.Regpat (g-regpat.ads).
* GNAT.Sockets (g-socket.ads):           GNAT.Sockets (g-socket.ads).
* GNAT.Source_Info (g-souinf.ads):       GNAT.Source_Info (g-souinf.ads).
* GNAT.Spell_Checker (g-speche.ads):     GNAT.Spell_Checker (g-speche.ads).
* GNAT.Spitbol (g-spitbo.ads):           GNAT.Spitbol (g-spitbo.ads).
* GNAT.Spitbol.Patterns (g-spipat.ads):  GNAT.Spitbol.Patterns (g-spipat.ads).
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads): GNAT.Spitbol.Table_Boolean (g-sptabo.ads).
* GNAT.Spitbol.Table_Integer (g-sptain.ads): GNAT.Spitbol.Table_Integer (g-sptain.ads).
* GNAT.Spitbol.Table_VString (g-sptavs.ads): GNAT.Spitbol.Table_VString (g-sptavs.ads).
* GNAT.Table (g-table.ads):              GNAT.Table (g-table.ads).
* GNAT.Task_Lock (g-tasloc.ads):         GNAT.Task_Lock (g-tasloc.ads).
* GNAT.Threads (g-thread.ads):           GNAT.Threads (g-thread.ads).
* GNAT.Traceback (g-traceb.ads):         GNAT.Traceback (g-traceb.ads).
* GNAT.Traceback.Symbolic (g-trasym.ads): GNAT.Traceback.Symbolic (g-trasym.ads).
* Has_Discriminants:                     Implementation Defined Attributes.
* Hash tables:                           GNAT.HTable (g-htable.ads).
* Heap usage, implicit:                  Implementation Advice.
* IBM Packed Format:                     Interfaces.Packed_Decimal (i-pacdec.ads).
* Ident:                                 Implementation Defined Pragmas.
* Image, of an address:                  System.Address_Image (s-addima.ads).
* Img:                                   Implementation Defined Attributes.
* Implementation-dependent features:     About This Guide.
* Import:                                Address Clauses.
* Import_Exception:                      Implementation Defined Pragmas.
* Import_Function:                       Implementation Defined Pragmas.
* Import_Object:                         Implementation Defined Pragmas.
* Import_Procedure:                      Implementation Defined Pragmas.
* Import_Valued_Procedure:               Implementation Defined Pragmas.
* Initialization, suppression of:        Implementation Defined Pragmas.
* Initialize_Scalars:                    Implementation Defined Pragmas.
* Inline_Always:                         Implementation Defined Pragmas.
* Inline_Generic:                        Implementation Defined Pragmas.
* Input/Output facilities <1>:           GNAT.IO_Aux (g-io_aux.ads).
* Input/Output facilities:               GNAT.IO (g-io.ads).
* Integer maps:                          GNAT.Spitbol.Table_Integer (g-sptain.ads).
* Integer types:                         Implementation Advice.
* Integer_Value:                         Implementation Defined Attributes.
* Interface:                             Implementation Defined Pragmas.
* Interface_Name:                        Implementation Defined Pragmas.
* Interfaces:                            Implementation Advice.
* Interfaces.C.Extensions (i-cexten.ads): Interfaces.C.Extensions (i-cexten.ads).
* Interfaces.C.Streams (i-cstrea.ads):   Interfaces.C.Streams (i-cstrea.ads).
* Interfaces.CPP (i-cpp.ads):            Interfaces.CPP (i-cpp.ads).
* Interfaces.Os2lib (i-os2lib.ads):      Interfaces.Os2lib (i-os2lib.ads).
* Interfaces.Os2lib.Errors (i-os2err.ads): Interfaces.Os2lib.Errors (i-os2err.ads).
* Interfaces.Os2lib.Synchronization (i-os2syn.ads): Interfaces.Os2lib.Synchronization (i-os2syn.ads).
* Interfaces.Os2lib.Threads (i-os2thr.ads): Interfaces.Os2lib.Threads (i-os2thr.ads).
* Interfaces.Packed_Decimal (i-pacdec.ads): Interfaces.Packed_Decimal (i-pacdec.ads).
* Interfaces.VxWorks (i-vxwork.ads):     Interfaces.VxWorks (i-vxwork.ads).
* Interfaces.VxWorks.IO (i-vxwoio.ads):  Interfaces.VxWorks.IO (i-vxwoio.ads).
* Interfacing to C++:                    Implementation Defined Pragmas.
* Interfacing to VxWorks:                Interfaces.VxWorks (i-vxwork.ads).
* Interfacing to VxWorks' I/O:           Interfaces.VxWorks.IO (i-vxwoio.ads).
* Interfacing with C++:                  Implementation Defined Pragmas.
* Interfacing, to C++:                   Interfaces.CPP (i-cpp.ads).
* Interfacing, to OS/2 <1>:              Interfaces.Os2lib.Errors (i-os2err.ads).
* Interfacing, to OS/2 <2>:              Interfaces.Os2lib.Threads (i-os2thr.ads).
* Interfacing, to OS/2 <3>:              Interfaces.Os2lib.Synchronization (i-os2syn.ads).
* Interfacing, to OS/2:                  Interfaces.Os2lib (i-os2lib.ads).
* Interrupt priority, maximum:           Implementation Defined Attributes.
* Interrupt support:                     Implementation Advice.
* Interrupts:                            Implementation Advice.
* Intrinsic operator:                    Intrinsic Operators.
* Intrinsic Subprograms:                 Intrinsic Subprograms.
* Large:                                 Implementation Defined Attributes.
* Latin-1:                               Compatibility with Ada 83.
* Latin_1 constants for Wide_Character:  Ada.Characters.Wide_Latin_1 (a-cwila1.ads).
* Latin_9 constants for Character:       Ada.Characters.Latin_9 (a-chlat9.ads).
* Latin_9 constants for Wide_Character:  Ada.Characters.Wide_Latin_9 (a-cwila9.ads).
* License:                               Implementation Defined Pragmas.
* License checking:                      Implementation Defined Pragmas.
* Line:                                  Line.
* Link_With:                             Implementation Defined Pragmas.
* Linker_Alias:                          Implementation Defined Pragmas.
* Linker_Section:                        Implementation Defined Pragmas.
* Little endian:                         Implementation Defined Attributes.
* Locking:                               GNAT.Task_Lock (g-tasloc.ads).
* Locking Policies:                      Implementation Advice.
* Locking using files:                   GNAT.Lock_Files (g-locfil.ads).
* Long_Float:                            Implementation Defined Pragmas.
* Machine operations:                    Implementation Advice.
* Machine_Attribute:                     Implementation Defined Pragmas.
* Machine_Size:                          Implementation Defined Attributes.
* Main_Storage:                          Implementation Defined Pragmas.
* Mantissa:                              Implementation Defined Attributes.
* Maps <1>:                              GNAT.Spitbol.Table_VString (g-sptavs.ads).
* Maps:                                  GNAT.Spitbol.Table_Integer (g-sptain.ads).
* Max_Entry_Queue_Depth:                 Implementation Defined Characteristics.
* Max_Interrupt_Priority:                Implementation Defined Attributes.
* Max_Priority:                          Implementation Defined Attributes.
* Maximum_Alignment:                     Implementation Defined Attributes.
* Mechanism_Code:                        Implementation Defined Attributes.
* Memory corruption debugging:           GNAT.Debug_Pools (g-debpoo.ads).
* Message Digest MD5:                    GNAT.MD5 (g-md5.ads).
* Multidimensional arrays:               Implementation Advice.
* Named numbers, representation of:      Implementation Defined Attributes.
* No_Calendar:                           Implementation Defined Characteristics.
* No_Dynamic_Interrupts:                 Implementation Defined Characteristics.
* No_Elaboration_Code:                   Implementation Defined Characteristics.
* No_Entry_Calls_In_Elaboration_Code:    Implementation Defined Characteristics.
* No_Entry_Queue:                        Implementation Defined Characteristics.
* No_Enumeration_Maps:                   Implementation Defined Characteristics.
* No_Exception_Handlers:                 Implementation Defined Characteristics.
* No_Implementation_Attributes:          Implementation Defined Characteristics.
* No_Implementation_Pragmas:             Implementation Defined Characteristics.
* No_Implementation_Restrictions:        Implementation Defined Characteristics.
* No_Implicit_Conditionals:              Implementation Defined Characteristics.
* No_Implicit_Loops:                     Implementation Defined Characteristics.
* No_Local_Protected_Objects:            Implementation Defined Characteristics.
* No_Protected_Type_Allocators:          Implementation Defined Characteristics.
* No_Return:                             Implementation Defined Pragmas.
* No_Run_Time:                           Implementation Defined Pragmas.
* No_Secondary_Stack:                    Implementation Defined Characteristics.
* No_Select_Statements:                  Implementation Defined Characteristics.
* No_Standard_Storage_Pools:             Implementation Defined Characteristics.
* No_Streams:                            Implementation Defined Characteristics.
* No_Task_Attributes:                    Implementation Defined Characteristics.
* No_Task_Termination:                   Implementation Defined Characteristics.
* No_Tasking:                            Implementation Defined Characteristics.
* No_Wide_Characters:                    Implementation Defined Characteristics.
* Normalize_Scalars:                     Implementation Defined Pragmas.
* Null_Occurrence, testing for:          Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads).
* Null_Parameter:                        Implementation Defined Attributes.
* Numerics:                              Implementation Advice.
* Object_Size <1>:                       Implementation Defined Attributes.
* Object_Size:                           Value_Size and Object_Size Clauses.
* OpenVMS <1>:                           Implementation Defined Pragmas.
* OpenVMS <2>:                           Implementation Defined Attributes.
* OpenVMS:                               Implementation Defined Pragmas.
* Operating System interface:            GNAT.OS_Lib (g-os_lib.ads).
* Operations, on Address:                Implementation Advice.
* ordering, of bits:                     Bit_Order Clauses.
* ordering, of bytes:                    Effect of Bit_Order on Byte Ordering.
* OS/2 Error codes:                      Interfaces.Os2lib.Errors (i-os2err.ads).
* OS/2 interfacing <1>:                  Interfaces.Os2lib.Errors (i-os2err.ads).
* OS/2 interfacing:                      Interfaces.Os2lib (i-os2lib.ads).
* OS/2 synchronization primitives:       Interfaces.Os2lib.Synchronization (i-os2syn.ads).
* OS/2 thread interfacing:               Interfaces.Os2lib.Threads (i-os2thr.ads).
* Package Interfaces:                    Implementation Advice.
* Package Interrupts:                    Implementation Advice.
* Package Task_Attributes:               Implementation Advice.
* Packed Decimal:                        Interfaces.Packed_Decimal (i-pacdec.ads).
* Packed types:                          Implementation Advice.
* Parameters, passing mechanism:         Implementation Defined Attributes.
* Parameters, when passed by reference:  Implementation Defined Attributes.
* Parsing:                               GNAT.AWK (g-awk.ads).
* Partition communication subsystem:     Implementation Advice.
* Partition intefacing functions:        System.Partition_Interface (s-parint.ads).
* Passed_By_Reference:                   Implementation Defined Attributes.
* Passing by copy:                       Implementation Defined Pragmas.
* Passing by descriptor:                 Implementation Defined Pragmas.
* Passive:                               Implementation Defined Pragmas.
* Pattern matching <1>:                  GNAT.Spitbol.Patterns (g-spipat.ads).
* Pattern matching <2>:                  GNAT.Regpat (g-regpat.ads).
* Pattern matching:                      GNAT.Regexp (g-regexp.ads).
* PCS:                                   Implementation Advice.
* Polling:                               Implementation Defined Pragmas.
* Portability:                           About This Guide.
* Pragma Pack (for arrays):              Pragma Pack for Arrays.
* Pragma Pack (for records):             Pragma Pack for Records.
* Pragma, representation:                Representation Clauses and Pragmas.
* Pragmas:                               Implementation Advice.
* Pre-elaboration requirements:          Implementation Advice.
* Preemptive abort:                      Implementation Advice.
* Priority, maximum:                     Implementation Defined Attributes.
* Propagate_Exceptions:                  Implementation Defined Pragmas.
* Protected procedure handlers:          Implementation Advice.
* Psect_Object:                          Implementation Defined Pragmas.
* Pure:                                  Implementation Defined Pragmas.
* Pure_Function:                         Implementation Defined Pragmas.
* Random number generation:              Implementation Advice.
* Range_Length:                          Implementation Defined Attributes.
* Ravenscar:                             Implementation Defined Pragmas.
* Record Representation Clause:          Record Representation Clauses.
* Record representation clauses:         Implementation Advice.
* Regular expressions <1>:               GNAT.Regpat (g-regpat.ads).
* Regular expressions:                   GNAT.Regexp (g-regexp.ads).
* Removing command line arguments:       Ada.Command_Line.Remove (a-colire.ads).
* Representation Clause:                 Representation Clauses and Pragmas.
* Representation clauses:                Implementation Advice.
* Representation Clauses:                Representation Clauses and Pragmas.
* Representation clauses, enumeration:   Implementation Advice.
* Representation clauses, records:       Implementation Advice.
* Representation of enums:               Implementation Defined Attributes.
* Representation of wide characters:     System.Wch_Cnv (s-wchcnv.ads).
* Representation Pragma:                 Representation Clauses and Pragmas.
* Representation, determination of:      Determining the Representations chosen by GNAT.
* Restricted_Run_Time:                   Implementation Defined Pragmas.
* Return values, passing mechanism:      Implementation Defined Attributes.
* Rotate_Left:                           Rotate_Left.
* Rotate_Right:                          Rotate_Right.
* Safe_Emax:                             Implementation Defined Attributes.
* Safe_Large:                            Implementation Defined Attributes.
* Sets of strings:                       GNAT.Spitbol.Table_Boolean (g-sptabo.ads).
* Share_Generic:                         Implementation Defined Pragmas.
* Shift_Left:                            Shift_Left.
* Shift_Right:                           Shift_Right.
* Shift_Right_Arithmetic:                Shift_Right_Arithmetic.
* Simple I/O:                            GNAT.IO (g-io.ads).
* Size Clause:                           Size Clauses.
* Size clauses:                          Implementation Advice.
* Size for biased representation:        Biased Representation.
* Size of Address:                       Implementation Defined Attributes.
* Size, of objects:                      Value_Size and Object_Size Clauses.
* Size, setting for not-first subtype:   Implementation Defined Attributes.
* Size, used for objects:                Implementation Defined Attributes.
* Size, VADS compatibility <1>:          Implementation Defined Attributes.
* Size, VADS compatibility:              Implementation Defined Pragmas.
* Size, variant record objects:          Size of Variant Record Objects.
* Small:                                 Implementation Defined Attributes.
* Sockets:                               GNAT.Sockets (g-socket.ads).
* Sorting <1>:                           GNAT.Heap_Sort_G (g-hesorg.ads).
* Sorting <2>:                           GNAT.Bubble_Sort_A (g-busora.ads).
* Sorting <3>:                           GNAT.Bubble_Sort_G (g-busorg.ads).
* Sorting:                               GNAT.Heap_Sort_A (g-hesora.ads).
* Source Information:                    GNAT.Source_Info (g-souinf.ads).
* Source_File_Name:                      Implementation Defined Pragmas.
* Source_Location:                       Source_Location.
* Source_Reference:                      Implementation Defined Pragmas.
* Spawn capability:                      GNAT.OS_Lib (g-os_lib.ads).
* Spell checking:                        GNAT.Spell_Checker (g-speche.ads).
* SPITBOL interface:                     GNAT.Spitbol (g-spitbo.ads).
* SPITBOL pattern matching:              GNAT.Spitbol.Patterns (g-spipat.ads).
* SPITBOL Tables <1>:                    GNAT.Spitbol.Table_Boolean (g-sptabo.ads).
* SPITBOL Tables <2>:                    GNAT.Spitbol.Table_VString (g-sptavs.ads).
* SPITBOL Tables:                        GNAT.Spitbol.Table_Integer (g-sptain.ads).
* Static_Priorities:                     Implementation Defined Characteristics.
* Static_Storage_Size:                   Implementation Defined Characteristics.
* Storage place attributes:              Implementation Advice.
* Storage_Size Clause:                   Storage_Size Clauses.
* Storage_Unit <1>:                      Implementation Defined Pragmas.
* Storage_Unit:                          Implementation Defined Attributes.
* Stream files:                          Treating Text_IO Files as Streams.
* Stream oriented attributes:            Implementation Advice.
* Stream_Convert:                        Implementation Defined Pragmas.
* String maps:                           GNAT.Spitbol.Table_VString (g-sptavs.ads).
* Style_Checks:                          Implementation Defined Pragmas.
* Subprogram address:                    Implementation Defined Attributes.
* Subtitle:                              Implementation Defined Pragmas.
* Suppress_All:                          Implementation Defined Pragmas.
* Suppress_Initialization:               Implementation Defined Pragmas.
* Suppressing initialization:            Implementation Defined Pragmas.
* Suppression of checks:                 Implementation Advice.
* Synchronization, OS/2:                 Interfaces.Os2lib.Synchronization (i-os2syn.ads).
* system, extending:                     Implementation Defined Pragmas.
* System.Address_Image (s-addima.ads):   System.Address_Image (s-addima.ads).
* System.Assertions (s-assert.ads):      System.Assertions (s-assert.ads).
* System.Partition_Interface (s-parint.ads): System.Partition_Interface (s-parint.ads).
* System.Task_Info (s-tasinf.ads):       System.Task_Info (s-tasinf.ads).
* System.Wch_Cnv (s-wchcnv.ads):         System.Wch_Cnv (s-wchcnv.ads).
* System.Wch_Con (s-wchcon.ads):         System.Wch_Con (s-wchcon.ads).
* Table implementation <1>:              GNAT.Table (g-table.ads).
* Table implementation:                  GNAT.Dynamic_Tables (g-dyntab.ads).
* Task locking:                          GNAT.Task_Lock (g-tasloc.ads).
* Task synchronization:                  GNAT.Task_Lock (g-tasloc.ads).
* Task_Attributes:                       Implementation Advice.
* Task_Info:                             Implementation Defined Pragmas.
* Task_Info pragma:                      System.Task_Info (s-tasinf.ads).
* Task_Name:                             Implementation Defined Pragmas.
* Task_Storage:                          Implementation Defined Pragmas.
* Tasking restrictions:                  Implementation Advice.
* Text_IO:                               GNAT.IO_Aux (g-io_aux.ads).
* Text_IO extensions:                    Text_IO Extensions.
* Text_IO for unbounded strings:         Text_IO Facilities for Unbounded Strings.
* Text_IO, extensions for unbounded strings: Ada.Strings.Unbounded.Text_IO (a-suteio.ads).
* Text_IO, extensions for unbounded wide strings: Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads).
* Thread control, OS/2:                  Interfaces.Os2lib.Threads (i-os2thr.ads).
* Threads, foreign:                      GNAT.Threads (g-thread.ads).
* Tick:                                  Implementation Defined Attributes.
* Time:                                  GNAT.Calendar.Time_IO (g-catiio.ads).
* Time, monotonic:                       Implementation Advice.
* Time_Slice:                            Implementation Defined Pragmas.
* Title:                                 Implementation Defined Pragmas.
* To_Address <1>:                        Address Clauses.
* To_Address:                            Implementation Defined Attributes.
* Trace back facilities <1>:             GNAT.Traceback.Symbolic (g-trasym.ads).
* Trace back facilities:                 GNAT.Traceback (g-traceb.ads).
* Type_Class:                            Implementation Defined Attributes.
* Typographical conventions:             Conventions.
* UET_Address:                           Implementation Defined Attributes.
* Unbounded_String, IO support:          Ada.Strings.Unbounded.Text_IO (a-suteio.ads).
* Unbounded_String, Text_IO operations:  Text_IO Facilities for Unbounded Strings.
* Unbounded_Wide_String, IO support:     Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads).
* Unchecked conversion:                  Implementation Advice.
* Unchecked deallocation:                Implementation Advice.
* Unchecked_Union:                       Implementation Defined Pragmas.
* Unimplemented_Unit:                    Implementation Defined Pragmas.
* Unions in C:                           Implementation Defined Pragmas.
* Universal_Literal_String:              Implementation Defined Attributes.
* Unreferenced:                          Implementation Defined Pragmas.
* Unreserve_All_Interrupts:              Implementation Defined Pragmas.
* Unrestricted_Access:                   Implementation Defined Attributes.
* Unsuppress:                            Implementation Defined Pragmas.
* Use_VADS_Size:                         Implementation Defined Pragmas.
* VADS_Size:                             Implementation Defined Attributes.
* Validity_Checks:                       Implementation Defined Pragmas.
* Value_Size <1>:                        Implementation Defined Attributes.
* Value_Size:                            Value_Size and Object_Size Clauses.
* Variant record objects, size:          Size of Variant Record Objects.
* Volatile:                              Implementation Defined Pragmas.
* VxWorks, Get_Immediate:                Interfaces.VxWorks.IO (i-vxwoio.ads).
* VxWorks, I/O interfacing:              Interfaces.VxWorks.IO (i-vxwoio.ads).
* VxWorks, interfacing:                  Interfaces.VxWorks (i-vxwork.ads).
* Warnings:                              Implementation Defined Pragmas.
* Warnings, unreferenced:                Implementation Defined Pragmas.
* Wchar_T_Size:                          Implementation Defined Attributes.
* Weak_External:                         Implementation Defined Pragmas.
* Wide Character, Representation:        System.Wch_Cnv (s-wchcnv.ads).
* Wide String, Conversion:               System.Wch_Cnv (s-wchcnv.ads).
* Windows Registry:                      GNAT.Registry (g-regist.ads).
* Word_Size:                             Implementation Defined Attributes.
* Zero address, passing:                 Implementation Defined Attributes.
* Zero Cost Exceptions:                  Implementation Defined Pragmas.



Tag Table:
Node: Top228
Node: About This Guide5850
Node: What This Reference Manual Contains6923
Node: Conventions9219
Node: Related Information10155
Node: Implementation Defined Pragmas11183
Node: Implementation Defined Attributes109042
Node: Implementation Advice133113
Node: Implementation Defined Characteristics177748
Node: Intrinsic Subprograms220488
Node: Intrinsic Operators221623
Node: Enclosing_Entity222713
Node: Exception_Information223251
Node: Exception_Message223796
Node: Exception_Name224313
Node: File224788
Node: Line225203
Node: Rotate_Left225624
Node: Rotate_Right226503
Node: Shift_Left226794
Node: Shift_Right227079
Node: Shift_Right_Arithmetic227377
Node: Source_Location227713
Node: Representation Clauses and Pragmas228171
Node: Alignment Clauses229369
Node: Size Clauses232853
Node: Storage_Size Clauses235409
Node: Size of Variant Record Objects238109
Node: Biased Representation241393
Node: Value_Size and Object_Size Clauses242609
Node: Component_Size Clauses249320
Node: Bit_Order Clauses250456
Node: Effect of Bit_Order on Byte Ordering254163
Node: Pragma Pack for Arrays264112
Node: Pragma Pack for Records266194
Node: Record Representation Clauses268703
Node: Enumeration Clauses270512
Node: Address Clauses272099
Node: Effect of Convention on Representation278801
Node: Determining the Representations chosen by GNAT281532
Node: Standard Library Routines286347
Node: The Implementation of Standard I/O303855
Node: Standard I/O Packages305956
Node: FORM Strings306836
Node: Direct_IO307375
Node: Sequential_IO308256
Node: Text_IO310194
Node: Text_IO Stream Pointer Positioning312729
Node: Text_IO Reading and Writing Non-Regular Files314266
Node: Get_Immediate316138
Node: Treating Text_IO Files as Streams316971
Node: Text_IO Extensions317636
Node: Text_IO Facilities for Unbounded Strings318475
Node: Wide_Text_IO319881
Node: Wide_Text_IO Stream Pointer Positioning324738
Node: Wide_Text_IO Reading and Writing Non-Regular Files325764
Node: Stream_IO326299
Node: Shared Files326920
Node: Open Modes329756
Node: Operations on C Streams331067
Node: Interfacing to C Streams338032
Node: The GNAT Library341121
Node: Ada.Characters.Latin_9 (a-chlat9.ads)345796
Node: Ada.Characters.Wide_Latin_1 (a-cwila1.ads)346314
Node: Ada.Characters.Wide_Latin_9 (a-cwila9.ads)346923
Node: Ada.Command_Line.Remove (a-colire.ads)347535
Node: Ada.Direct_IO.C_Streams (a-diocst.ads)348059
Node: Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)348582
Node: Ada.Sequential_IO.C_Streams (a-siocst.ads)349022
Node: Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)349569
Node: Ada.Strings.Unbounded.Text_IO (a-suteio.ads)350116
Node: Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)350577
Node: Ada.Text_IO.C_Streams (a-tiocst.ads)351058
Node: Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)351582
Node: GNAT.AWK (g-awk.ads)352087
Node: GNAT.Bubble_Sort_A (g-busora.ads)352528
Node: GNAT.Bubble_Sort_G (g-busorg.ads)352938
Node: GNAT.Calendar (g-calend.ads)353424
Node: GNAT.Calendar.Time_IO (g-catiio.ads)353889
Node: GNAT.CRC32 (g-crc32.ads)354132
Node: GNAT.Case_Util (g-casuti.ads)354763
Node: GNAT.CGI (g-cgi.ads)355127
Node: GNAT.CGI.Cookie (g-cgicoo.ads)355641
Node: GNAT.CGI.Debug (g-cgideb.ads)356070
Node: GNAT.Command_Line (g-comlin.ads)356395
Node: GNAT.Current_Exception (g-curexc.ads)356830
Node: GNAT.Debug_Pools (g-debpoo.ads)357387
Node: GNAT.Debug_Utilities (g-debuti.ads)357806
Node: GNAT.Directory_Operations (g-dirope.ads)358188
Node: GNAT.Dynamic_Tables (g-dyntab.ads)358619
Node: GNAT.Exception_Traces (g-exctra.ads)359252
Node: GNAT.Expect (g-expect.ads)359591
Node: GNAT.Float_Control (g-flocon.ads)360283
Node: GNAT.Heap_Sort_A (g-hesora.ads)360804
Node: GNAT.Heap_Sort_G (g-hesorg.ads)361329
Node: GNAT.HTable (g-htable.ads)361803
Node: GNAT.IO (g-io.ads)362205
Node: GNAT.IO_Aux (g-io_aux.ads)362636
Node: GNAT.Lock_Files (g-locfil.ads)362991
Node: GNAT.MD5 (g-md5.ads)363324
Node: GNAT.Most_Recent_Exception (g-moreex.ads)363610
Node: GNAT.OS_Lib (g-os_lib.ads)364049
Node: GNAT.Regexp (g-regexp.ads)364525
Node: GNAT.Registry (g-regist.ads)365008
Node: GNAT.Regpat (g-regpat.ads)365494
Node: GNAT.Sockets (g-socket.ads)365916
Node: GNAT.Source_Info (g-souinf.ads)366426
Node: GNAT.Spell_Checker (g-speche.ads)366798
Node: GNAT.Spitbol.Patterns (g-spipat.ads)367154
Node: GNAT.Spitbol (g-spitbo.ads)367707
Node: GNAT.Spitbol.Table_Boolean (g-sptabo.ads)368271
Node: GNAT.Spitbol.Table_Integer (g-sptain.ads)368692
Node: GNAT.Spitbol.Table_VString (g-sptavs.ads)369140
Node: GNAT.Table (g-table.ads)369577
Node: GNAT.Task_Lock (g-tasloc.ads)370192
Node: GNAT.Threads (g-thread.ads)370604
Node: GNAT.Traceback (g-traceb.ads)371197
Node: GNAT.Traceback.Symbolic (g-trasym.ads)371543
Node: Interfaces.C.Extensions (i-cexten.ads)371911
Node: Interfaces.C.Streams (i-cstrea.ads)372328
Node: Interfaces.CPP (i-cpp.ads)372659
Node: Interfaces.Os2lib (i-os2lib.ads)373062
Node: Interfaces.Os2lib.Errors (i-os2err.ads)373456
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Node: Interfaces.Packed_Decimal (i-pacdec.ads)374602
Node: Interfaces.VxWorks (i-vxwork.ads)375016
Node: Interfaces.VxWorks.IO (i-vxwoio.ads)375413
Node: System.Address_Image (s-addima.ads)375829
Node: System.Assertions (s-assert.ads)376214
Node: System.Partition_Interface (s-parint.ads)376638
Node: System.Task_Info (s-tasinf.ads)377049
Node: System.Wch_Cnv (s-wchcnv.ads)377396
Node: System.Wch_Con (s-wchcon.ads)377856
Node: Interfacing to Other Languages378237
Node: Interfacing to C378714
Node: Interfacing to C++380546
Node: Interfacing to COBOL381896
Node: Interfacing to Fortran382179
Node: Interfacing to non-GNAT Ada code382647
Node: Machine Code Insertions383782
Node: GNAT Implementation of Tasking390378
Node: Mapping Ada Tasks onto the Underlying Kernel Threads390705
Node: Ensuring Compliance with the Real-Time Annex392823
Node: Code generation for array aggregates394943
Node: Static constant aggregates with static bounds396527
Node: Constant aggregates with an unconstrained nominal types398122
Node: Aggregates with static bounds398842
Node: Aggregates with non-static bounds399728
Node: Aggregates in assignments statements400318
Node: Specialized Needs Annexes401754
Node: Compatibility Guide402847
Node: Compatibility with Ada 83403363
Node: Compatibility with Other Ada 95 Systems408299
Node: Representation Clauses409821
Node: Compatibility with DEC Ada 83414345
Node: GNU Free Documentation License418206
Node: Index440624

End Tag Table
