  S-Lang Library C Programmer's Guide, V1.4.9
  John E. Davis, davis@space.mit.edu
  Mar 23, 2003
  ____________________________________________________________

  Table of Contents

   Preface

  1. A Brief History of S-Lang
  2. Acknowledgements
  2. Introduction
  2. Interpreter Interface
  3. Embedding the Interpreter
  4. Calling the Interpreter
     4.1 Loading Files
     4.2 Loading Strings

  5. Intrinsic Functions
     5.1 Restrictions on Intrinsic Functions
     5.2 Adding a New Intrinsic
     5.3 More Complicated Intrinsics

  6. Intrinsic Variables
  7. Aggregate Data Objects
     7.1 Arrays
     7.2 Structures
        7.2.1 Interpreter Structures
        7.2.2 Intrinsic Structures
        7.2.2 Keyboard Interface

  8. Initializing the Keyboard Interface
  9. Resetting the Keyboard Interface
  10. Initializing the SLkp Routines
  11. Setting the Interrupt Handler
  12. Reading Keyboard Input with SLang_getkey
  13. Reading Keyboard Input with SLkp_getkey
  14. Buffering Input
  15. Global Variables
  15. Screen Management
  16. Initialization
  17. Resetting SLsmg
  18. Handling Screen Resize Events
  19. SLsmg Functions
     19.1 Positioning the cursor
     19.2 Writing to the Display
     19.3 Erasing the Display
     19.4 Setting Character Attributes
     19.5 Lines and Alternate Character Sets
     19.6 Miscellaneous Functions

  20. Variables
  21. Hints for using SLsmg
  21. Signal Functions
  21. Searching Functions
  22. Regular Expressions
  23. Simple Searches
  24. Initialization
  25. SLsearch
  25. Copyright
  A. The GNU Public License
  B. The Artistic License


  ______________________________________________________________________

  1.  Preface



  S-Lang is an interpreted language that was designed from the start to
  be easily embedded into a program to provide it with a powerful
  extension language.  Examples of programs that use S-Lang as an
  extension language include the jed text editor, the slrn newsreader,
  and sldxe (unreleased), a numerical computation program.  For this
  reason, S-Lang does not exist as a separate application and many of
  the examples in this document are presented in the context of one of
  the above applications.

  S-Lang is also a programmer's library that permits a programmer to
  develop sophisticated platform-independent software.  In addition to
  providing the S-Lang extension language, the library provides
  facilities for screen management, keymaps, low-level terminal I/O,
  etc.  However, this document is concerned only with the extension
  language and does not address these other features of the S-Lang
  library.  For information about the other components of the library,
  the reader is referred to the The S-Lang Library Reference.


  1.1.  A Brief History of S-Lang



  I first began working on S-Lang sometime during the fall of 1992.  At
  that time I was writing a text editor (jed), which I wanted to endow
  with a macro language.  It occured to me that an application-
  independent language that could be embedded into the editor would
  prove more useful because I could envision embedding it into other
  programs.  As a result, S-Lang was born.

  S-Lang was originally a stack language that supported a postscript-
  like syntax.  For that reason, I named it S-Lang, where the S was
  supposed to emphasize its stack-based nature.  About a year later, I
  began to work on a preparser that would allow one to write using a
  more traditional infix syntax making it easier to use for those
  unfamiliar with stack based languages.  Currently, the syntax of the
  language resembles C, nevertheless some postscript-like features still
  remain, e.g., the `%' character is still used as a comment delimiter.



  1.2.  Acknowledgements



  Since I first released S-Lang, I have received a lot feedback about
  the library and the language from many people.  This has given me the
  opportunity and pleasure to interact with several people to make the
  library portable and easy to use.  In particular, I would like to
  thank the following individuals:

  Luchesar Ionkov <lionkov@sf.cit.bg> for his comments and criticisms of
  the syntax of the language.  He was the person who made me realize
  that the low-level byte-code engine should be totally type-
  independent.  He also improved the tokenizer and preparser and
  impressed upon me that the language needed a grammar.

  Mark Olesen <olesen@weber.me.queensu.ca> for his many patches to
  various aspects of the library and his support on AIX. He also
  contributed a lot to the pre-processing (SLprep) routines.

  John Burnell <j.burnell@irl.cri.nz> for the OS/2 port of the video and
  keyboard routines.  He also made value suggestions regarding the
  interpreter interface.

  Darrel Hankerson <hankedr@mail.auburn.edu> for cleaning up and
  unifying some of the code and the makefiles.

  Dominik Wujastyk <ucgadkw@ucl.ac.uk> who was always willing to test
  new releases of the library.

  Michael Elkins <me@muddcs.cs.hmc.edu> for his work on the curses
  emulation.

  Ulli Horlacher <framstag@belwue.de> and Oezguer Kesim <kesim@math.fu-
  berlin.de> for the S-Lang newsgroup and mailing list.

  Hunter Goatley, Andy Harper <Andy.Harper@kcl.ac.uk>, and Martin P.J.
  Zinser <zinser@decus.decus.de> for their VMS support.

  Dave Sims <sims@usa.acsys.com> and Chin Huang <cthuang@vex.net> for
  Windows 95 and Windows NT support.

  Lloyd Zusman <ljz@asfast.com> and Rich Roth <rich@on-the-net.com> for
  creating and maintaining www.s-lang.org.

  I am also grateful to many other people who send in bug-reports and
  bug-fixes, for without such community involvement, S-Lang would not be
  as well-tested and stable as it is.  Finally, I would like to thank my
  wife for her support and understanding while I spent long weekend
  hours developing the library.



  2.  Introduction



  S-Lang is a C programmer's library that includes routines for the
  rapid development of sophisticated, user friendly, multi-platform
  applications.  The S-Lang library includes the following:


  o  Low level tty input routines for reading single characters at a
     time.

  o  Keymap routines for defining keys and manipulating multiple
     keymaps.

  o  A high-level keyprocessing interface (SLkp) for handling function
     and arrow keys.

  o  High level screen management routines for manipulating both
     monochrome and color terminals.  These routines are very efficient.
     (SLsmg)

  o  Low level terminal-independent routines for manipulating the
     display of a terminal. (SLtt)

  o  Routines for reading single line input with line editing and recall
     capabilities. (SLrline)

  o  Searching functions: both ordinary searches and regular expression
     searches. (SLsearch)

  o  An embedded stack-based language interpreter with a C-like syntax.


  The library is currently available for OS/2, MSDOS, Unix, and VMS
  systems.  For the most part, the interface to library routines has
  been implemented in such a way that it appears to be platform
  independent from the point of view of the application.  In addition,
  care has been taken to ensure that the routines are ``independent'' of
  one another as much as possible.  For example, although the keymap
  routines require keyboard input, they are not tied to S-Lang's
  keyboard input routines--- one can use a different keyboard getkey
  routine if one desires.  This also means that linking to only part of
  the S-Lang library does not pull the whole library into the
  application.  Thus, S-Lang applications tend to be relatively small in
  comparison to programs that use libraries with similar capabilities.



  3.  Interpreter Interface



  The S-Lang library provides an interpreter that when embedded into an
  application, makes the application extensible.  Examples of programs
  that embed the interpreter include the jed editor and the slrn
  newsreader.

  Embedding the interpreter is easy.  The hard part is to decide what
  application specific built-in or intrinsic functions should be
  provided by the application.  The S-Lang library provides some pre-
  defined intrinsic functions, such as string processing functions, and
  simple file input-output routines.  However, the basic philosophy
  behind the interpreter is that it is not a standalone program and it
  derives much of its power from the application that embeds it.


  3.1.  Embedding the Interpreter



  Only one function needs to be called to embed the S-Lang interpreter
  into an application: SLang_init_slang.  This function initializes the
  interpreter's data structures and adds some intrinsic functions:


             if (-1 == SLang_init_slang ())
               exit (EXIT_FAILURE);



  This function does not provide file input output intrinsic nor does it
  provide mathematical functions.  To make these as well as some posix
  system calls available use


            if ((-1 == SLang_init_slang ())    /* basic interpreter functions */
                || (-1 == SLang_init_slmath ()) /* sin, cos, etc... */
                || (-1 == SLang_init_stdio ()) /* stdio file I/O */
                || (-1 == SLang_init_posix_dir ()) /* mkdir, stat, etc. */
                || (-1 == SLang_init_posix_process ()) /* getpid, umask, etc. */
               )
              exit (EXIT_FAILURE);



  If you intend to enable all intrinsic functions, then it is simpler to
  initialize the interpreter via


            if (-1 == SLang_init_all ())
              exit (EXIT_FAILURE);



  See the \slang-run-time-library for more information about the intrin-
  sic functions.



  3.2.  Calling the Interpreter



  There are several ways of calling the interpreter.  The two most
  common method is to load a file containing S-Lang code, or to load a
  string.


  3.2.1.  Loading Files

  The SLang_load_file and SLns_load_file functions may be used to
  interpret a file.  Both these functions return zero if successful, or
  -1 upon failure.  If either of these functions fail, the interpreter
  will accept no more code unless the error state is cleared.  This is
  done by calling SLang_restart function to set the interpreter to its
  default state, and setting SLang_Error to 0, e.g.,


            if (-1 == SLang_load_file ("site.sl"))
              {
                 /* Clear the error and rest the interpreter */
                 SLang_restart (1);
                 SLang_Error = 0;
              }



  When a file is loaded via SLang_load_file, any non-public variables
  and functions defined in the file will be placed into a namespace that
  is local to the file itself.  The SLns_load_file function may be used
  to load a file using a specified namespace, e.g.,


            if (-1 == SLns_load_file ("site.sl", "NS"))
              {
                SLang_restart (1);
                SLang_Error = 0;
              }



  will load site.sl into a namespace called NS.  If such a namespace
  does not exist, then it will be created.

  Both the SLang_load_file and SLns_load_file functions search for files
  along an application-specified search path.  This path may be set
  using the SLpath_set_load_path function, as well as from interpeted
  code via the set_slang_load_path function.  By default, no search path
  is defined.

  Files are searched as follows: If the name begins with the equivalent
  of "./" or "../", then it is searched for with respect to the current
  directory, and not along the load-path.  If no such file exists, then
  an error will be generated. Otherwise, the file is searched for in
  each of the directories of the load-path by concatenating the path
  element with the specified file name.  The first such file found to
  exist by this process will be loaded.  If a matching file still has
  not been found, and the file name lacks an extension, then the path is
  searched with ".sl" and ".slc" appended to the filename.  If two such
  files are found (one ending with ".sl" and the other with ".slc"),
  then the more recent of the two will be used.  If no matching file has
  been found by this process, then the search will cease and an error
  generated.
  The search path is a delimiter separated list of directories that
  specify where the interpreter looks for files.  By default, the value
  of the delimiter is OS-dependent following the convention of the
  underlying OS.  For example, on Unix the delimiter is represented by a
  colon, on DOS/Windows it is a semi-colon, and on VMS it is a space.
  The SLpath_set_delimiter and SLpath_get_delimiter may be used to set
  and query the delimiter's value, respectively.


  3.2.2.  Loading Strings

  There are several other mechanisms for interacting with the
  interpreter.  For example, the SLang_load_string function loads a
  string into the interpreter and interprets it:


           if (-1 == SLang_load_string ("message (\"hello\");"))
             {
               SLang_restart (1);
               SLang_Error = 0;
             }



  Similarly, the SLns_load_string function may be used to load a string
  into a specified namespace.

  Typically, an interactive application will load a file via
  SLang_load_file and then go into a loop that consists of reading lines
  of input and sending them to the interpreter, e.g.,


             while (EOF != fgets (buf, sizeof (buf), stdin))
               {
                  if (-1 == SLang_load_string (buf))
                    SLang_restart (1);
                  SLang_Error = 0;
               }



  Finally, some applications such as jed and slrn use another method of
  interacting with the interpreter.  They read key sequences from the
  keyboard and map those key sequences to interpreter functions via the
  S-Lang keymap interface.



  3.3.  Intrinsic Functions



  An intrinsic function is simply a function that is written in C and is
  made available to the interpreter as a built-in function.  For this
  reason, the words `intrinsic' and `built-in' are often used
  interchangeably.

  Applications are expected to add application specific functions to the
  interpreter.  For example, jed adds nearly 300 editor-specific
  intrinsic functions.  The application designer should think carefully
  about what intrinsic functions to add to the interpreter.


  3.3.1.  Restrictions on Intrinsic Functions



  When implementing intrinsic functions, it is necessary to follow a few
  rules to cooperate with the interpreter.

  The C version of an intrinsic function takes only pointer arguments.
  This is because when the interpreter calls an intrinsic function, it
  passes values to the function by reference and not by value. For
  example, intrinsic with the declarations:


            int intrinsic_0 (void);
            int intrinsic_1 (char *s);
            void intrinsic_2 (char *s, int *i);
            void intrinsic_3 (int *i, double *d, double *e);



  are all valid.  However,


            int invalid_1 (char *s, int len);



  is not valid since the len parameter is not a pointer.

  The return value of an intrinsic function must be one of the following
  types: void, char, short, int, long, double, char *, as well as
  unsigned versions of the integer types.  A function such as


           int *invalid (void);



  is not permitted since int* is not a valid return-type for an intrin-
  sic function.  Any other type of value can be passed back to the
  interpreter by explicitly pushing the object onto the interpreter's
  stack via the appropriate "push" function.

  The current implementation limits the number of arguments of an
  intrinsic function to 7.  The "pop" functions can be used to allow the
  function to take an arbitrary number as seen from an interpreter
  script.

  Another restriction is that the intrinsic function should regard all
  its parameters as pointers to constant objects and make no attempt to
  modify the value to which they point.  For example,


             void truncate (char *s)
             {
                s[0] = 0;
             }



  is illegal since the function modifies the string s.

  3.3.2.  Adding a New Intrinsic



  There are two basic mechanisms for adding an intrinsic function to the
  interpreter: SLadd_intrinsic_function and SLadd_intrin_fun_table.
  Functions may be added to a specified namespace via
  SLns_add_intrinsic_function and SLns_add_intrin_fun_table functions.

  As an specific example, consider a function that will cause the
  program to exit via the exit C library function.  It is not possible
  to make this function an intrinsic because it does not meet the
  specifications for an intrinsic function that were described earlier.
  However, one can call exit from a function that is suitable, e.g.,


            void intrin_exit (int *code)
            {
               exit (*code);
            }



  This function may be made available to the interpreter as an intrinsic
  via the SLadd_intrinsic_function routine:


            if (-1 == SLadd_intrinsic_function ("exit", (FVOID_STAR) intrin_exit,
                                                SLANG_VOID_TYPE, 1,
                                                SLANG_INT_TYPE))
              exit (EXIT_FAILURE);



  This statement basically tells the interpreter that intrin_exit is a
  function that returns nothing and takes a single argument: a pointer
  to an integer (SLANG_INT_TYPE).  A user can call this function from
  within the interpreter via


            message ("Calling the exit function");
            exit (0);



  After printing a message, this will cause the intrin_exit function to
  execute, which in turn calls exit.

  The most convenient mechanism for adding new intrinsic functions is to
  create a table of SLang_Intrin_Fun_Type objects and add the table via
  the SLadd_intrin_fun_table function.  The table will look like:



      SLang_Intrin_Fun_Type My_Intrinsics [] =
      {
       /* table entries */
        MAKE_INTRINSIC_N(...),
        MAKE_INTRINSIC_N(...),
              .
              .
        MAKE_INTRINSIC_N(...),
        SLANG_END_INTRIN_FUN_TABLE
      };



  Construction of the table entries may be facilitated using a set of
  MAKE_INTRINSIC macros defined in slang.h.  The main macro is called
  MAKE_INTRINSIC_N and takes 11 arguments:


           MAKE_INTRINSIC_N(name, funct-ptr, return-type, num-args,
                            arg-1-type, arg-2-type, ... arg-7-type)



  Here name is the name of the intrinsic function that the interpreter
  is to give to the function. func-ptr is a pointer to the intrinsic
  function taking num-args and returning ret-type.  The final 7 argu-
  ments specifiy the argument types.  For example, the intrin_exit
  intrinsic described above may be added to the table using


           MAKE_INTRINSIC_N("exit", intrin_exit, SLANG_VOID_TYPE, 1,
                            SLANG_INT_TYPE, 0,0,0,0,0,0)



  While MAKE_INTRINSIC_N is the main macro for constructing table
  entries, slang.h defines other macros that may prove useful.  In
  particular, an entry for the intrin_exit function may also be created
  using any of the following forms:


           MAKE_INTRINSIC_1("exit", intrin_exit, SLANG_VOID_TYPE, SLANG_INT_TYPE)
           MAKE_INTRINSIC_I("exit", intrin_exit, SLANG_VOID_TYPE)



  See slang.h for related macros.  You are also encouraged to look at,
  e.g., slang/src/slstd.c for a more extensive examples.

  The table may be added via the SLadd_intrin_fun_table function, e.g.,


           if (-1 == SLadd_intrin_fun_table (My_Intrinsics, NULL))
             {
                /* an error occurred */
             }



  Please note that there is no need to load a given table more than
  once, and it is considered to be an error on the part of the
  application it adds the same table multiple times.  For performance
  reasons, no checking is performed by the library to see if a table has
  already been added.

  Earlier it was mentioned that intrinsics may be added to a specified
  namespace.  To this end, one must first get a pointer to the namespace
  via the SLns_create_namespace function.  The following example
  illustrates how this function is used to add the My_Intrinsics table
  to a namespace called my:


          SLang_NameSpace_Type *ns = SLns_create_namespace ("my");
          if (ns == NULL)
            return -1;

          return SLns_add_intrin_fun_table (ns, My_Intrinsics, "__MY__"));



  3.3.3.  More Complicated Intrinsics


  The intrinsic functions described in the previous example were
  functions that took a fixed number of arguments.  In this section we
  explore more complex intrinsics such as those that take a variable
  number of arguments.

  Consider a function that takes two double precision numbers and
  returns the lesser:


            double intrin_min (double *a, double *b)
            {
               if (*a < *b) return *a;
               return *b;
            }



  This function may be added to a table of intrinsics using


           MAKE_INTRINSIC_2("vmin", intrin_min, SLANG_DOUBLE_TYPE,
                            SLANG_DOUBLE_TYPE, SLANG_DOUBLE_TYPE)



  It is useful to extend this function to take an arbitray number of
  arguments and return the lesser.  Consider the following variant:



      double intrin_min_n (int *num_ptr)
      {
         double min_value, x;
         unsigned int num = (unsigned int) *num_ptr;

         if (-1 == SLang_pop_double (&min_value, NULL, NULL))
           return 0.0;
         num--;

         while (num > 0)
           {
              num--;
              if (-1 == SLang_pop_double (&x, NULL, NULL))
                return 0.0;
              if (x < min_value) min_value = x;
           }
         return min_value;
      }



  Here the number to compare is passed to the function and the actual
  numbers are removed from the stack via the SLang_pop_double function.
  A suitable table entry for it is


           MAKE_INTRINSIC_I("vmin", intrin_min_n, SLANG_DOUBLE_TYPE)



  This function would be used in an interpreter script via a statement
  such as


             variable xmin = vmin (x0, x1, x2, x3, x4, 5);



  which computes the smallest of 5 values.

  The problem with this intrinsic function is that the user must
  explicitly specify how many numbers to compare.  It would be more
  convenient to simply use


             variable xmin = vmin (x0, x1, x2, x3, x4);



  An intrinsic function can query the value of the variable
  SLang_Num_Function_Args to obtain the necessary information:



      double intrin_min (void)
      {
         double min_value, x;

         unsigned int num = SLang_Num_Function_Args;

         if (-1 == SLang_pop_double (&min_value, NULL, NULL))
           return 0.0;
         num--;

         while (num > 0)
           {
              num--;
              if (-1 == SLang_pop_double (&x, NULL, NULL))
                return 0.0;
              if (x < min_value) min_value = x;
           }
         return min_value;
      }



  This may be declared as an intrinsic using:


           MAKE_INTRINSIC_0("vmin", intrin_min, SLANG_DOUBLE_TYPE)



  3.4.  Intrinsic Variables



  It is possible to access an application's global variables from within
  the interpreter.  The current implementation supports the access of
  variables of type int, char *, and double.

  There are two basic methods of making an intrinsic variable available
  to the interpreter.  The most straight forward method is to use the
  function SLadd_intrinsic_variable:


            int SLadd_intrinsic_variable (char *name, VOID_STAR addr,
                                          unsigned char data_type,
                                          int read_only);



  For example, suppose that I is an integer variable, e.g.,


            int I;



  One can make it known to the interpreter as I_Variable via a statement
  such as


            if (-1 == SLadd_intrinsic_variable ("I_Variable", &I,
                                                 SLANG_INT_TYPE, 0))
              exit (EXIT_FAILURE);



  Similarly, if S is declared as


           char *S;



  then


            if (-1 == SLadd_intrinsic_variable ("S_Variable", &S,
                                                 SLANG_STRING_TYPE, 1))
              exit (EXIT_FAILURE);



  makes S available as a read-only variable with the name S_Variable.
  Note that if a pointer variable is made available to the interpreter,
  its value is managed by the interpreter and not the application.  For
  this reason, it is recommended that such variables be declared as
  read-only.

  It is important to note that if S were declared as an array of
  characters, e.g.,


            char S[256];



  then it would not be possible to make it directly available to the
  interpreter.  However, one could create a pointer to it, i.e.,


            char *S_Ptr = S;



  and make S_Ptr available as a read-only variable.

  One should not make the mistake of trying to use the same address for
  different variables as the following example illustrates:



       int do_not_try_this (void)
       {
          static char *names[3] = {"larry", "curly", "moe"};
          unsigned int i;

          for (i = 0; i < 3; i++)
            {
               int value;
               if (-1 == SLadd_intrinsic_variable (names[i], (VOID_STAR) &value,
                                                   SLANG_INT_TYPE, 1))
                 return -1;
            }
          return 0;
       }



  Not only does this piece of code create intrinsic variables that use
  the same address, it also uses the address of a local variable that
  will go out of scope.

  The most convenient method for adding many intrinsic variables to the
  interpreter is to create an array of SLang_Intrin_Var_Type objects and
  then add the array via SLadd_intrin_var_table.  For example, the array


           static SLang_Intrin_Var_Type Intrin_Vars [] =
           {
              MAKE_VARIABLE("I_Variable", &I, SLANG_INT_TYPE, 0),
              MAKE_VARIABLE("S_Variable", &S_Ptr, SLANG_STRING_TYPE, 1),
              SLANG_END_TABLE
           };



  may be added via


           if (-1 == SLadd_intrin_var_table (Intrin_Vars, NULL))
             exit (EXIT_FAILURE);



  It should be rather obvious that the arguments to the MAKE_VARIABLE
  macro correspond to the parameters of the SLadd_intrinsic_variable
  function.

  Finally, variables may be added to a specific namespace via the
  SLns_add_intrin_var_table and SLns_add_intrinsic_variable functions.



  3.5.  Aggregate Data Objects


  An aggregate data object is an object that can contain more than one
  data value.  The S-Lang interpreter supports several such objects:
  arrays, structure, and associative arrays.  In the following sections,
  information about interacting with these objects is given.



  3.5.1.  Arrays


  An intrinsic function may interact with an array in several different
  ways.  For example, an intrinsic may create an array and return it.
  The basic functions for manipulating arrays include:


          SLang_create_array
          SLang_pop_array_of_type
          SLang_push_array
          SLang_free_array
          SLang_get_array_element
          SLang_set_array_element



  The use of these functions will be illustrated via a few simple exam-
  ples.

  The first example shows how to create an return an array of strings to
  the interpreter.  In particular, the names of the four seasons of the
  year will be returned:


           void months_of_the_year (void)
           {
              static char *seasons[4] =
                {
                   "Spring", "Summer", "Autumn", "Winter"
                };
              SLang_Array_Type *at;
              int i, four;

              four = 4;
              at = SLang_create_array (SLANG_STRING_TYPE, 0, NULL, &four, 1);
              if (at == NULL)
                return;

              /* Now set the elements of the array */
              for (i = 0; i < 4; i++)
                {
                  if (-1 == SLang_set_array_element (at, &i, &seasons[i]))
                    {
                       SLang_free_array (at);
                       return;
                    }
                }

             (void) SLang_push_array (at, 0);
             SLang_free_array (at);
           }



  This example illustrates several points.  First of all, the SLang_cre-
  ate_array function was used to create a 1 dimensional array of 4
  strings.  Since this function could fail, its return value was
  checked.  Then the SLang_set_array_element function was used to set
  the elements of the newly created array.  Note that the address con-
  taining the value of the array element was passed and not the value of
  the array element itself.  That is,


      SLang_set_array_element (at, &i, seasons[i])



  was not used.  The return value from this function was also checked
  because it too could also fail.  Finally, the array was pushed onto
  the interpreter's stack and then it was freed.  It is important to
  understand why it was freed.  This is because arrays are reference-
  counted.  When the array was created, it was returned with a reference
  count of 1.  When it was pushed, the reference count was bumped up to
  2.  Then since it was nolonger needed by the function,
  SLang_free_array was called to decrement the reference count back to
  1.  For convenience, the second argument to SLang_push_array deter-
  mines whether or not it is to also free the array.  So, instead of the
  two function calls:


          (void) SLang_push_array (at, 0);
          SLang_free_array (at);



  it is preferable to combine them as


          (void) SLang_push_array (at, 1);



  The second example returns a diagonal array of a specified size to the
  stack.  A diagonal array is a 2-d array with all elements zero except
  for those along the diagonal, which have a value of one:


          void make_diagonal_array (int n)
          {
             SLang_Array_Type *at;
             int dims[2];
             int i, one;

             dims[0] = dims[1] = n;
             at = SLang_create_array (SLANG_INT_TYPE, 0, NULL, dims, 2);
             if (at == NULL)
               return;

             one = 1;
             for (i = 0; i < n; i++)
               {
                  dims[0] = dims[1] = i;
                  if (-1 == SLang_set_array_element (at, dims, &one))
                    {
                       SLang_free_array (at);
                       return;
                    }
               }

             (void) SLang_push_array (at, 1);
          }



  In this example, only the diagonal elements of the array were set.
  This is bacause when the array was created, all its elements were set
  to zero.

  Now consider an example that acts upon an existing array.  In
  particular, consider one that computes the trace of a 2-d matrix,
  i.e., the sum of the diagonal elements:


          double compute_trace (void)
          {
             SLang_Array_Type *at;
             double trace;
             int dims[2];

             if (-1 == SLang_pop_array_of_type (&at, SLANG_DOUBLE_TYPE))
               return 0.0;

             /* We want a 2-d square matrix.  If the matrix is 1-d and has only one
                element, then return that element. */
             trace = 0.0;
             if (((at->num_dims == 1) && (at->dims[0] == 1))
                 || ((at->num_dims == 2) && (at->dims[0] == at->dims[1])))
               {
                  double dtrace;
                  int n = at->dims[0];

                  for (i = 0; i < n; i++)
                    {
                       dims[0] = dims[1] = i;
                       (void) SLang_get_array_element (at, &dims, &dtrace);
                       trace += dtrace;
                    }
               }
            else SLang_verror (SL_TYPE_MISMATCH, "Expecting a square matrix");

            SLang_free_array (at);
            return trace;
          }



  In this example, SLang_pop_array_of_type was used to pop an array of
  doubles from the stack.  This function will make implicit typecasts in
  order to return an array of the requested type.



  3.5.2.  Structures



  For the purposes of this section, we shall differentiate structures
  according to whether or not they correspond to an application defined
  C structure.  Those that do are called intrinsic structures, and those
  do not are called S-Lang interpreter structures.


  3.5.2.1.  Interpreter Structures


  The following simple example shows one method that may be used to
  create and return a structure with a string and integer field to the
  interpreter's stack:

      int push_struct_example (char *string_value, int int_value)
      {
         char *field_names[2];
         unsigned char field_types[2];
         VOID_STAR field_values[2];

         field_names[0] = "string_field";
         field_types[0] = SLANG_STRING_TYPE;
         field_values[0] = &string_value;

         field_names[1] = "int_field";
         field_types[1] = SLANG_INT_TYPE;
         field_values[1] = &int_value;

         if (-1 == SLstruct_create_struct (2, field_names,
                                              field_types, field_values))
           return -1;
         return 0;
      }



  Here, SLstruct_create_struct is used to push a structure with the
  specified field names and values onto the interpreter's stack.

  A simpler mechanism exists provided that one has already defined a C
  structure with a description of how the structure is laid out.  For
  example, consider a C structure defined by


           typedef struct
           {
              char *s;
              int i;
           }
           SI_Type;



  Its layout may be specified via a table of SLang_CStruct_Field_Type
  entries:


           SLang_CStruct_Field_Type SI_Type_Layout [] =
           {
             MAKE_CSTRUCT_FIELD(SI_Type, s, "string_field", SLANG_STRING_TYPE, 0),
             MAKE_CSTRUCT_FIELD(SI_Type, i, "int_field", SLANG_INT_TYPE, 0),
             SLANG_END_CSTRUCT_TABLE
           };



  Here, MAKE_CSTRUCT_FIELD is a macro taking 5 arguments:


           MAKE_CSTRUCT_FIELD(C-structure-type,
                              C-field-name,
                              slang-field-name,
                              slang-data-type,
                              is-read-only)



  The first argument is the structure type, the second is the name of a
  field of the structure, the third is a string that specifies the name
  of the corresponding field of the S-Lang structure, the fourth argu-
  ment specifies the field's type, and the last argument specifies
  whether or not the field should be regarded as read-only.

  Once the layout of the structure has been specified, pushing a S-Lang
  version of the structure is trival:


           int push_struct_example (char *string_value, int int_value)
           {
              SI_Type si;

              si.s = string_value;
              si.i = int_value;
              return SLang_push_cstruct ((VOID_STAR)&si, SI_Type_Layout);
           }



  This mechanism of structure creation also permits a S-Lang structure
  to be passed to an intrinsic function through the use of the
  SLang_pop_cstruct routine, e.g.,


            void print_si_struct (void)
            {
               SI_Type si;
               if (-1 == SLang_pop_cstruct ((VOID_STAR)&si, SI_Type_Layout))
                 return;
               printf ("si.i=%d", si.i);
               printf ("si.s=%s", si.s);
               SLang_free_cstruct ((VOID_STAR)&si, SI_Type_Layout);
            }



  Assuming print_si_struct exists as an intrinsic function, the S-Lang
  code


            variable s = struct {string_field, int_field};
            s.string_field = "hello";
            s.int_field = 20;
            print_si_struct (s);



  would result in the display of


            si.i=20;
            si.s=hello



  Note that the SLang_free_cstruct function was called after the con-
  tents of si were nolonger needed.  This was necessary because
  SLang_pop_cstruct allocated memory to set the char *s field of si.
  Calling SLang_free_cstruct frees up such memory.

  Now consider the following:


           typedef struct
           {
              pid_t pid;
              gid_t group;
           }
           X_t;



  How should the layout of this structure be defined?  One might be
  tempted to use:


           SLang_CStruct_Field_Type X_t_Layout [] =
           {
             MAKE_CSTRUCT_FIELD(X_t, pid, "pid", SLANG_INT_TYPE, 0),
             MAKE_CSTRUCT_FIELD(X_t, group, "group", SLANG_INT_TYPE, 0),
             SLANG_END_CSTRUCT_TABLE
           };



  However, this assumes pid_t and gid_t have been typedefed as ints.
  But what if gid_t is a short?  In such a case, using


             MAKE_CSTRUCT_FIELD(X_t, group, "group", SLANG_SHORT_TYPE, 0),



  would be the appropriate entry for the group field.  Of course, one
  has no way of knowing how gid_t is declared on other systems.  For
  this reason, it is preferable to use the MAKE_CSTRUCT_INT_FIELD macro
  in cases involving integer valued fields, e.g.,


           SLang_CStruct_Field_Type X_t_Layout [] =
           {
             MAKE_CSTRUCT_INT_FIELD(X_t, pid, "pid", 0),
             MAKE_CSTRUCT_INT_FIELD(X_t, group, "group", 0),
             SLANG_END_CSTRUCT_TABLE
           };



  Before leaving this section, it is important to mention that access to
  character array fields is not permitted via this interface.  That is,
  a structure such as


            typedef struct
            {
               char name[32];
            }
            Name_Type;



  is not supported since char name[32] is not a SLANG_STRING_TYPE
  object.  Always keep in mind that a SLANG_STRING_TYPE object is a char
  *.


  3.5.2.2.  Intrinsic Structures


  Here we show how to make intrinsic structures available to the
  interpreter.

  The simplest interface is to structure pointers and not to the actual
  structures themselves.  The latter would require the interpreter to be
  involved with the creation and destruction of the structures.  Dealing
  with the pointers themselves is far simpler.

  As an example, consider an object such as


           typedef struct _Window_Type
           {
              char *title;
              int row;
              int col;
              int width;
              int height;
           } Window_Type;



  which defines a window object with a title, size (width, height), and
  location (row, col).

  We can make variables of type Window_Type available to the interpreter
  via a table as follows:


          static SLang_IStruct_Field_Type Window_Type_Field_Table [] =
          {
            MAKE_ISTRUCT_FIELD(Window_Type, title, "title", SLANG_STRING_TYPE, 1),
            MAKE_ISTRUCT_FIELD(Window_Type, row, "row", SLANG_INT_TYPE, 0),
            MAKE_ISTRUCT_FIELD(Window_Type, col, "col", SLANG_INT_TYPE, 0),
            MAKE_ISTRUCT_FIELD(Window_Type, width, "width", SLANG_INT_TYPE, 0),
            MAKE_ISTRUCT_FIELD(Window_Type, height, "height", SLANG_INT_TYPE, 0),
            SLANG_END_ISTRUCT_TABLE
          };



  More precisely, this defines the layout of the Window_Type structure.
  Here, the title has been declared as a read-only field.  Using


            MAKE_ISTRUCT_FIELD(Window_Type, title, "title", SLANG_STRING_TYPE, 0),



  would allow read-write access.

  Now suppose that My_Window is a pointer to a Window_Type object, i.e.,



      Window_Type *My_Window;



  We can make this variable available to the interpreter via the
  SLadd_istruct_table function:


           if (-1 == SLadd_istruct_table (Window_Type_Field_Table,
                                          (VOID_STAR) &My_Window,
                                          "My_Win"))
             exit (1);



  This creates a S-Lang interpreter variable called My_Win whose value
  corresponds to the My_Win structure.  This would permit one to access
  the fields of My_Window via S-Lang statements such as


            define set_width_and_height (w,h)
            {
                My_Win.width = w;
                My_Win.height = h;
            }



  It is extremely important to understand that the interface described
  in this section does not allow the interpreter to create new instances
  of Window_Type objects.  The interface merely defines an association
  or correspondence between an intrinsic structure pointer and a S-Lang
  variable.  For example, if the value of My_Window is NULL, then My_Win
  would also be NULL.

  One should be careful in allowing read/write access to character
  string fields.  If read/write access is allowed, then the application
  should always use the SLang_create_slstring and SLang_free_slstring
  functions to set the character string field of the structure.



  4.  Keyboard Interface



  S-Lang's keyboard interface has been designed to allow an application
  to read keyboard input from the user in a system-independent manner.
  The interface consists of a set of low routines for reading single
  character data as well as a higher level interface (SLkp) which
  utilize S-Lang's keymap facility for reading multi-character
  sequences.

  To initialize the interface, one must first call the function
  SLang_init_tty. Before exiting the program, the function
  SLang_reset_tty must be called to restore the keyboard interface to
  its original state.  Once initialized, the low-level SLang_getkey
  function may be used to read simgle keyboard characters from the
  terminal.  An application using the higher-level SLkp interface will
  read charcters using the SLkp_getkey function.

  In addition to these basic functions, there are also functions to
  ``unget'' keyboard characters, flush the input, detect pending-input
  with a timeout, etc. These functions are defined below.



  4.1.  Initializing the Keyboard Interface



  The function SLang_init_tty must be called to initialize the terminal
  for single character input.  This puts the terminal in a mode usually
  referred to as ``raw'' mode.

  The prototype for the function is:


             int SLang_init_tty (int abort_char, int flow_ctrl, int opost);



  It takes three parameters that are used to specify how the terminal is
  to be initialized.

  The first parameter, abort_char, is used to specify the interrupt
  character (SIGINT).  Under MSDOS, this value corresponds to the scan
  code of the character that will be used to generate the interrupt.
  For example, under MSDOS, 34 should be used to make Ctrl-G generate an
  interrupt signal since 34 is the scan code for G.  On other systems,
  the value of abort_char will simply be the ascii value of the control
  character that will be used to generate the interrupt signal, e.g., 7
  for Ctrl-G.  If -1 is passed, the interrupt character will not be
  changed.

  Pressing the interrupt character specified by the first argument will
  generate a signal (SIGINT) that may or not be caught by the
  application.  It is up to the application to catch this signal.  S-
  Lang provides the function Slang_set_abort_signal to make it easy to
  facilitate this task.

  The second parameter is used to specify whether or not flow control
  should be used.  If this parameter is zero, flow control is enabled
  otherwise it is disabled.  Disabling flow control is necessary to pass
  certain characters to the application (e.g., Ctrl-S and Ctrl-Q).  For
  some systems such as MSDOS, this parameter is meaningless.

  The third parameter, opost, is used to turn output processing on or
  off.  If opost is zero, output processing is not turned on otherwise,
  output processing is turned on.

  The SLang_init_tty function returns -1 upon failure.  In addition,
  after it returns, the S-Lang global variable SLang_TT_Baud_Rate will
  be set to the baud rate of the terminal if this value can be
  determined.

  Example:


             if (-1 == SLang_init_tty (7, 0, 0))  /* For MSDOS, use 34 as scan code */
               {
                 fprintf (stderr, "Unable to initialize the terminal.\n");
                 exit (1);
               }
             SLang_set_abort_signal (NULL);



  Here the terminal is initialized such that flow control and output
  processing are turned off.  In addition, the character Ctrl-G (-- For
  MSDOS systems, use the scan code 34 instead of 7 for Ctrl-G--) has
  been specified to be the interrupt character.  The function
  SLang_set_abort_signal is used to install the default S-Lang interrupt
  signal handler.



  4.2.  Resetting the Keyboard Interface



  The function SLang_reset_tty must be called to reset the terminal to
  the state it was in before the call to SLang_init_tty.  The prototype
  for this function is:


             void SLang_reset_tty (void);



  Usually this function is only called before the program exits.  How-
  ever, if the program is suspended it should also be called just before
  suspension.



  4.3.  Initializing the SLkp  Routines



  Extra initialization of the higher-level SLkp functions are required
  because they are layered on top of the lower level routines.  Since
  the SLkp_getkey function is able to process function and arrow keys in
  a terminal independent manner, it is necessary to call the
  SLtt_get_terminfo function to get information about the escape
  character sequences that the terminal's function keys send.  Once that
  information is available, the SLkp_init function can construct the
  proper keymaps to process the escape sequences.

  This part of the initialization process for an application using this
  interface will look something like:



             SLtt_get_terminfo ();
             if (-1 == SLkp_init ())
               {
                  SLang_doerror ("SLkp_init failed.");
                  exit (1);
               }
             if (-1 == SLang_init_tty (-1, 0, 1))
               {
                  SLang_doerror ("SLang_init_tty failed.");
                  exit (1);
               }



  It is important to check the return status of the SLkp_init function
  which can failed if it cannot allocate enough memory for the keymap.



  4.4.  Setting the Interrupt Handler



  The function SLang_set_abort_signal may be used to associate an
  interrupt handler with the interrupt character that was previously
  specified by the SLang_init_tty function call.  The prototype for this
  function is:


             void SLang_set_abort_signal (void (*)(int));



  This function returns nothing and takes a single parameter which is a
  pointer to a function taking an integer value and returning void.  If
  a NULL pointer is passed, the default S-Lang interrupt handler will be
  used. The S-Lang default interrupt handler under Unix looks like:


             static void default_sigint (int sig)
             {
               SLsignal_intr (SIGINT, default_sigint);
               SLKeyBoard_Quit = 1;
               if (SLang_Ignore_User_Abort == 0) SLang_Error = USER_BREAK;
             }



  It simply sets the global variable SLKeyBoard_Quit to one and if the
  variable SLang_Ignore_User_Abort is non-zero, SLang_Error is set to
  indicate a user break condition.  (The function SLsignal_intr is simi-
  lar to the standard C signal function except that it will interrupt
  system calls.  Some may not like this behavior and may wish to call
  this SLang_set_abort_signal with a different handler.)
  Although the function expressed above is specific to Unix, the
  analogous routines for other operating systems are equivalent in
  functionality even though the details of the implementation may vary
  drastically (e.g., under MSDOS, the hardware keyboard interrupt int 9h
  is hooked).



  4.5.  Reading Keyboard Input with SLang_getkey



  After initializing the keyboard via SLang_init_tty, the S-Lang
  function SLang_getkey may be used to read characters from the terminal
  interface.  In addition, the function SLang_input_pending may be used
  to determine whether or not keyboard input is available to be read.

  These functions have prototypes:


             unsigned int SLang_getkey (void);
             int SLang_input_pending (int tsecs);



  The SLang_getkey function returns a single character from the termi-
  nal.  Upon failure, it returns 0xFFFF.  If the interrupt character
  specified by the SLang_init_tty function is pressed while this func-
  tion is called, the function will return the value of the interrupt
  character and set the S-Lang global variable SLKeyBoard_Quit to a non-
  zero value.  In addition, if the default S-Lang interrupt handler has
  been specified by a NULL argument to the SLang_set_abort_signal func-
  tion, the global variable SLang_Error will be set to USER_BREAK unless
  the variable SLang_Ignore_User_Abort is non-zero.

  The SLang_getkey function waits until input is available to be read.
  The SLang_input_pending function may be used to determine whether or
  not input is ready.  It takes a single parameter that indicates the
  amount of time to wait for input before returning with information
  regarding the availability of input.  This parameter has units of one
  tenth (1/10) of a second, i.e., to wait one second, the value of the
  parameter should be 10.  Passing a value of zero causes the function
  to return right away.  SLang_input_pending returns a positive integer
  if input is available or zero if input is not available.  It will
  return -1 if an error occurs.

  Here is a simple example that reads keys from the terminal until one
  presses Ctrl-G or until 5 seconds have gone by with no input:



        #include <stdio.h>
        #include "slang.h"
        int main ()
        {
           int abort_char = 7;  /* For MSDOS, use 34 as scan code */
           unsigned int ch;

           if (-1 == SLang_init_tty (abort_char, 0, 1))
             {
                fprintf (stderr, "Unable to initialize the terminal.\n");
                exit (-1);
             }
           SLang_set_abort_signal (NULL);
           while (1)
             {
                fputs ("\nPress any key.  To quit, press Ctrl-G: ", stdout);
                fflush (stdout);
                if (SLang_input_pending (50) == 0)  /* 50/10 seconds */
                  {
                     fputs ("Waited too long! Bye\n", stdout);
                     break;
                  }

                ch = SLang_getkey ();
                if (SLang_Error == USER_BREAK)
                  {
                     fputs ("Ctrl-G pressed!  Bye\n", stdout);
                     break;
                  }
                putc ((int) ch, stdout);
             }
           SLang_reset_tty ();
           return 0;
        }



  4.6.  Reading Keyboard Input with SLkp_getkey



  Unlike the low-level function SLang_getkey, the SLkp_getkey function
  can read a multi-character sequence associated with function keys.
  The SLkp_getkey function uses SLang_getkey and S-Lang's keymap
  facility to process escape sequences.  It returns a single integer
  which describes the key that was pressed:


             int SLkp_getkey (void);



  That is, the SLkp_getkey function simple provides a mapping between
  keys and integers.  In this context the integers are called keysyms.

  For single character input such as generated by the a key on the
  keyboard, the function returns the character that was generated, e.g.,
  'a'.  For single characters, SLkp_getkey will always return an keysym
  whose value ranges from 0 to 256. For keys that generate multiple
  character sequences, e.g., a function or arrow key, the function
  returns an keysym whose value is greater that 256.  The actual values
  of these keysyms are represented as macros defined in the slang.h
  include file.  For example, the up arrow key corresponds to the keysym
  whose value is SL_KEY_UP.

  Since it is possible for the user to enter a character sequence that
  does not correspond to any key.  If this happens, the special keysym
  SL_KEY_ERR will be returned.

  Here is an example of how SLkp_getkey may be used by a file viewer:


             switch (SLkp_getkey ())
               {
                  case ' ':
                  case SL_KEY_NPAGE:
                     next_page ();
                     break;
                  case 'b':
                  case SL_KEY_PPAGE:
                     previous_page ();
                     break;
                  case '\r':
                  case SL_KEY_DOWN:
                     next_line ();
                     break;
                      .
                      .
                  case SL_KEY_ERR:
                  default:
                     SLtt_beep ();
               }



  Unlike its lower-level counterpart, SLang_getkey, there do not yet
  exist any functions in the library that are capable of ``ungetting''
  keysyms.  In particular, the SLang_ungetkey function will not work.



  4.7.  Buffering Input



  S-Lang has several functions pushing characters back onto the input
  stream to be read again later by SLang_getkey.  It should be noted
  that none of the above functions are designed to push back keysyms
  read by the SLkp_getkey function.  These functions are declared as
  follows:


             void SLang_ungetkey (unsigned char ch);
             void SLang_ungetkey_string (unsigned char *buf, int buflen);
             void SLang_buffer_keystring (unsigned char *buf, int buflen);



  SLang_ungetkey is the most simple of the three functions.  It takes a
  single character a pushes it back on to the input stream.  The next
  call to SLang_getkey will return this character.  This function may be
  used to peek at the character to be read by first reading it and then
  putting it back.
  SLang_ungetkey_string has the same function as SLang_ungetkey except
  that it is able to push more than one character back onto the input
  stream.  Since this function can push back null (ascii 0) characters,
  the number of characters to push is required as one of the parameters.

  The last of these three functions, SLang_buffer_keystring can handle
  more than one charater but unlike the other two, it places the
  characters at the end of the keyboard buffer instead of at the
  beginning.

  Note that the use of each of these three functions will cause
  SLang_input_pending to return right away with a non-zero value.

  Finally, the S-Lang keyboard interface includes the function
  SLang_flush_input with prototype


             void SLang_flush_input (void);



  It may be used to discard all input.

  Here is a simple example that looks to see what the next key to be
  read is if one is available:


             int peek_key ()
             {
                int ch;
                if (SLang_input_pending (0) == 0) return -1;
                ch = SLang_getkey ();
                SLang_ungetkey (ch);
                return ch;
             }



  4.8.  Global Variables


  Although the following S-Lang global variables have already been
  mentioned earlier, they are gathered together here for completeness.

  int SLang_Ignore_User_Abort; If non-zero, pressing the interrupt
  character will not result in SLang_Error being set to USER_BREAK.

  volatile int SLKeyBoard_Quit; This variable is set to a non-zero value
  when the interrupt character is pressed. If the interrupt character is
  pressed when SLang_getkey is called, the interrupt character will be
  returned from SLang_getkey.

  int SLang_TT_Baud_Rate; On systems which support it, this variable is
  set to the value of the terminal's baud rate after the call to
  SLang_init_tty.



  5.  Screen Management



  The S-Lang library provides two interfaces to terminal independent
  routines for manipulating the display on a terminal.  The highest
  level interface, known as the SLsmg interface is discussed in this
  section.  It provides high level screen management functions more
  manipulating the display in an optimal manner and is similar in spirit
  to the curses library.  The lowest level interface, or the SLtt
  interface, is used by the SLsmg routines to actually perform the task
  of writing to the display.  This interface is discussed in another
  section.  Like the keyboard routines, the SLsmg routines are platform
  independent and work the same on MSDOS, OS/2, Unix, and VMS.

  The screen management, or SLsmg, routines are initialized by function
  SLsmg_init_smg.  Once initialized, the application uses various SLsmg
  functions to write to a virtual display.  This does not cause the
  physical terminal display to be updated immediately.  The physical
  display is updated to look like the virtual display only after a call
  to the function SLsmg_refresh.  Before exiting, the application using
  these routines is required to call SLsmg_reset_smg to reset the
  display system.

  The following subsections explore S-Lang's screen management system in
  greater detail.


  5.1.  Initialization


  The function SLsmg_init_smg must be called before any other SLsmg
  function can be used.  It has the simple prototype:


             int SLsmg_init_smg (void);



  It returns zero if successful or -1 if it cannot allocate space for
  the virtual display.

  For this routine to properly initialize the virtual display, the
  capabilities of the terminal must be known as well as the size of the
  physical display.  For these reasons, the lower level SLtt routines
  come into play.  In particular, before the first call to
  SLsmg_init_smg, the application is required to call the function
  SLtt_get_terminfo before calling SLsmg_init_smg.

  The SLtt_get_terminfo function sets the global variables
  SLtt_Screen_Rows and SLtt_Screen_Cols to the values appropriate for
  the terminal.  It does this by calling the SLtt_get_screen_size
  function to query the terminal driver for the appropriate values for
  these variables.  From this point on, it is up to the application to
  maintain the correct values for these variables by calling the
  SLtt_get_screen_size function whenever the display size changes, e.g.,
  in response to a SIGWINCH signal. Finally, if the application is going
  to read characters from the keyboard, it is also a good idea to
  initialize the keyboard routines at this point as well.


  5.2.  Resetting SLsmg



  Before the program exits or suspends, the function SLsmg_reset_tty
  should be called to shutdown the display system.  This function has
  the prototype


             void SLsmg_reset_smg (void);



  This will deallocate any memory allocated for the virtual screen and
  reset the terminal's display.

  Basically, a program that uses the SLsmg screen management functions
  and S-Lang's keyboard interface will look something like:


             #include "slang.h"
             int main ()
             {
                SLtt_get_terminfo ();
                SLang_init_tty (-1, 0, 0);
                SLsmg_init_smg ();

                /* do stuff .... */

                SLsmg_reset_smg ();
                SLang_reset_tty ();
                return 0;
             }



  If this program is compiled and run, all it will do is clear the
  screen and position the cursor at the bottom of the display.  In the
  following sections, other SLsmg functions will be introduced which may
  be used to make this simple program do much more.


  5.3.  Handling Screen Resize Events

  The function SLsmg_reinit_smg is designed to be used in conjunction
  with resize events.

  Under Unix-like operating systems, when the size of the display
  changes, the application will be sent a SIGWINCH signal.  To properly
  handle this signal, the SLsmg routines must be reinitialized to use
  the new display size.  This may be accomplished by calling
  SLtt_get_screen_size to get the new size, followed by SLsmg_reinit_smg
  to reinitialize the SLsmg interface to use the new size.  Keep in mind
  that these routines should not be called from within the signal
  handler.  The following code illustrates the main ideas involved in
  handling such events:



       static volatile int Screen_Size_Changed;
       static sigwinch_handler (int sig)
       {
          Screen_Size_Changed = 1;
          SLsignal (SIGWINCH, sigwinch_handler);
       }

       int main (int argc, char **argv)
       {
          SLsignal (SIGWINCH, sigwinch_handler);
          SLsmg_init_smg ();
            .
            .
          /* Now enter main loop */
          while (not_done)
            {
               if (Screen_Size_Changed)
                 {
                    SLtt_get_screen_size ();
                    SLsmg_reinit_smg ();
                    redraw_display ();
                 }
               .
               .
            }
         return 0;
       }



  5.4.  SLsmg Functions



  In the previous sections, functions for initializing and shutting down
  the SLsmg routines were discussed.  In this section, the rest of the
  SLsmg functions are presented.  These functions act only on the
  virtual display.  The physical display is updated when the
  SLsmg_refresh function is called and not until that time.  This
  function has the simple prototype:


            void SLsmg_refresh (void);



  5.4.1.  Positioning the cursor


  The SLsmg_gotorc function is used to position the cursor at a given
  row and column.  The prototype for this function is:


             void SLsmg_gotorc (int row, int col);



  The origin of the screen is at the top left corner and is given the
  coordinate (0, 0), i.e., the top row of the screen corresponds to row
  = 0 and the first column corresponds to col = 0.  The last row of the
  screen is given by row = SLtt_Screen_Rows - 1.

  It is possible to change the origin of the coordinate system by using
  the function SLsmg_set_screen_start with prototype:


            void SLsmg_set_screen_start (int *r, int *c);



  This function takes pointers to the new values of the first row and
  first column.  It returns the previous values by modifying the values
  of the integers at the addresses specified by the parameter list.  A
  NULL pointer may be passed to indicate that the origin is to be set to
  its initial value of 0.  For example,


             int r = 10;
             SLsmg_set_screen_start (&r, NULL);



  sets the origin to (10, 0) and after the function returns, the vari-
  able r will have the value of the previous row origin.


  5.4.2.  Writing to the Display


  SLsmg has several routines for outputting text to the virtual display.
  The following points should be understood:

  o  The text is output at the position of the cursor of the virtual
     display and the cursor is advanced to the position that corresponds
     to the end of the text.


  o  Text does not wrap at the boundary of the display--- it is
     trucated.  This behavior seems to be more useful in practice since
     most programs that would use screen management tend to be line
     oriented.


  o  Control characters are displayed in a two character sequence
     representation with ^ as the first character.  That is, Ctrl-X is
     output as ^X.


  o  The newline character does not cause the cursor to advance to the
     next row.  Instead, when a newline character is encountered when
     outputting text, the output routine will return.  That is,
     outputting a string containing a newline character will only
     display the contents of the string up to the newline character.


  Although the some of the above items might appear to be too
  restrictive, in practice this is not seem to be the case.  In fact,
  the design of the output routines was influenced by their actual use
  and modified to simplify the code of the application utilizing them.

  void SLsmg_write_char (char ch); Write a single character to the
  virtual display.


  void SLsmg_write_nchars (char *str, int len); Write len characters
  pointed to by str to the virtual display.

  void SLsmg_write_string (char *str); Write the null terminated string
  given by pointer str to the virtual display.  This function is a
  wrapper around SLsmg_write_nchars.

  void SLsmg_write_nstring (char *str, int n); Write the null terminated
  string given by pointer str to the virtual display.  At most, only n
  characters are written.  If the length of the string is less than n,
  then the string will be padded with blanks.  This function is a
  wrapper around SLsmg_write_nchars.

  void SLsmg_printf (char *fmt, ...); This function is similar to printf
  except that it writes to the SLsmg virtual display.

  void SLsmg_vprintf (char *, va_list); Like SLsmg_printf but uses a
  variable argument list.


  5.4.3.  Erasing the Display


  The following functions may be used to fill portions of the display
  with blank characters.  The attributes of blank character are the
  current attributes.  (See below for a discussion of character
  attributes)

  void SLsmg_erase_eol (void); Erase line from current position to the
  end of the line.

  void SLsmg_erase_eos (void); Erase from the current position to the
  end of the screen.

  void SLsmg_cls (void); Clear the entire virtual display.


  5.4.4.  Setting Character Attributes


  Character attributes define the visual characteristics the character
  possesses when it is displayed.  Visual characteristics include the
  foreground and background colors as well as other attributes such as
  blinking, bold, and so on.  Since SLsmg takes a different approach to
  this problem than other screen management libraries an explanation of
  this approach is given here.  This approach has been motivated by
  experience with programs that require some sort of screen management.

  Most programs that use SLsmg are composed of specific textual objects
  or objects made up of line drawing characters. For example, consider
  an application with a menu bar with drop down menus.  The menus might
  be enclosed by some sort of frame or perhaps a shadow.  The basic idea
  is to associate an integer to each of the objects (e.g., menu bar,
  shadow, current menu item, etc.) and create a mapping from the integer
  to the set of attributes.  In the terminology of SLsmg, the integer is
  simply called an object.

  For example, the menu bar might be associated with the object 1, the
  drop down menu could be object 2, the shadow could be object 3, and so
  on.

  The range of values for the object integer is restricted from 0 up to
  and including 255 on all systems except MSDOS where the maximum
  allowed integer is 15 (-- This difference is due to memory constraints
  imposed by MSDOS.  This restriction might be removed in a future
  version of the library.--) . The object numbered zero should not be
  regarding as an object at all.  Rather it should be regarded as all
  other objects that have not explicitly been given an object number.
  SLsmg, or more precisely SLtt, refers to the attributes of this
  special object as the default or normal attributes.

  The SLsmg routines know nothing about the mapping of the color to the
  attributes associated with the color.  The actual mapping takes place
  at a lower level in the SLtt routines.  Hence, to map an object to the
  actual set of attributes requires a call to any of the following SLtt
  routines:


            void SLtt_set_color (int obj, char *name, char *fg, char *bg);
            void SLtt_set_color_object (int obj, SLtt_Char_Type attr);
            void SLtt_set_mono (int obj, char *, SLtt_Char_Type attr);



  Only the first of these routines will be discussed briefly here.  The
  latter two functions allow more fine control over the object to
  attribute mapping (such as assigning a ``blink'' attribute to the
  object).  For a more full explanation on all of these routines see the
  section about the SLtt interface.

  The SLtt_set_color function takes four parameters.  The first
  parameter, obj, is simply the integer of the object for which
  attributes are to be assigned.  The second parameter is currently
  unused by these routines.  The third and forth parameters, fg and bg,
  are the names of the foreground and background color to be used
  associated with the object.  The strings that one can use for the
  third and fourth parameters can be any one of the 16 colors:


            "black"                "gray"
            "red"                  "brightred"
            "green"                "brightgreen"
            "brown"                "yellow"
            "blue"                 "brightblue"
            "magenta"              "brightmagenta"
            "cyan"                 "brightcyan"
            "lightgray"            "white"



  The value of the foreground parameter fg can be anyone of these six-
  teen colors.   However, on most terminals, the background color will
  can only be one of the colors listed in the first column (-- This is
  also true on the Linux console.  However, it need not be the case and
  hopefully the designers of Linux will someday remove this restric-
  tion.--) .

  Of course not all terminals are color terminals.  If the S-Lang global
  variable SLtt_Use_Ansi_Colors is non-zero, the terminal is assumed to
  be a color terminal.  The SLtt_get_terminfo will try to determine
  whether or not the terminal supports colors and set this variable
  accordingly.  It does this by looking for the capability in the
  terminfo/termcap database.  Unfortunately many Unix databases lack
  this information and so the SLtt_get_terminfo routine will check
  whether or not the environment variable COLORTERM exists.  If it
  exists, the terminal will be assumed to support ANSI colors and
  SLtt_Use_Ansi_Colors will be set to one.  Nevertheless, the
  application should provide some other mechanism to set this variable,
  e.g., via a command line parameter.

  When the SLtt_Use_Ansi_Colors variable is zero, all objects with
  numbers greater than one will be displayed in inverse video (-- This
  behavior can be modified by using the SLtt_set_mono function call.--)
  .

  With this background, the SLsmg functions for setting the character
  attributes can now be defined.  These functions simply set the object
  attributes that are to be assigned to subsequent characters written to
  the virtual display.  For this reason, the new attribute is called the
  current attribute.

  void SLsmg_set_color (int obj); Set the current attribute to those of
  object obj.

  void SLsmg_normal_video (void); This function is equivalent to
  SLsmg_set_color (0).

  void SLsmg_reverse_video (void); This function is equivalent to
  SLsmg_set_color (1).  On monochrome terminals, it is equivalent to
  setting the subsequent character attributes to inverse video.

  Unfortunately there does not seem to be a standard way for the
  application or, in particular, the library to determine which color
  will be used by the terminal for the default background.  Such
  information would be useful in initializing the foreground and
  background colors associated with the default color object (0).  FOr
  this reason, it is up to the application to provide some means for the
  user to indicate what these colors are for the particular terminal
  setup. To facilitate this, the SLtt_get_terminfo function checks for
  the existence of the COLORFGBG environment variable.  If this variable
  exists, its value will be used to initialize the colors associated
  with the default color object.  Specifically, the value is assumed to
  consist of a foreground color name and a background color name
  separated by a semicolon.  For example, if the value of COLORTERM is
  lightgray;blue, the default color object will be initialized to
  represent a lightgray foreground upon a blue background.


  5.4.5.  Lines and Alternate Character Sets

  The S-Lang screen management library also includes routines for
  turning on and turning off alternate character sets.  This is
  especially useful for drawing horizontal and vertical lines.

  void SLsmg_set_char_set (int flag); If flag is non-zero, subsequent
  write functions will use characters from the alternate character set.
  If flag is zero, the default, or, ordinary character set will be used.

  void SLsmg_draw_hline (int len); Draw a horizontal line from the
  current position to the column that is len characters to the right.

  void SLsmg_draw_vline (int len); Draw a horizontal line from the
  current position to the row that is len rows below.

  void SLsmg_draw_box (int r, int c, int dr, int dc); Draw a box whose
  upper right corner is at row r and column c.  The box spans dr rows
  and dc columns.  The current position will be left at row r and column
  c.


  5.4.6.  Miscellaneous Functions


  void SLsmg_touch_lines (int r, int n); Mark screen rows numbered r, r
  + 1, ... r + (n - 1) as modified.  When SLsmg_refresh is called, these
  rows will be completely redrawn.
  unsigned short SLsmg_char_at(void); Returns the character and its
  attributes object number at the current cursor position.  The
  character itself occupies the lower byte and the object attributes
  number forms the upper byte.  The object returned by this function
  call should not be written back out via any of the functions that
  write characters or character strings.



  5.5.  Variables



  The following S-Lang global variables are used by the SLsmg interface.
  Some of these have been previously discussed.

  int SLtt_Screen_Rows; int SLtt_Screen_Cols; The number of rows and
  columns of the physical display.  If either of these numbers changes,
  the functions SLsmg_reset_smg and SLsmg_init_smg should be called
  again so that the SLsmg routines can re-adjust to the new size.

  int SLsmg_Tab_Width; Set this variable to the tab width that will be
  used when expanding tab characters.  The default is 8.

  int SLsmg_Display_Eight_Bit This variable determines how characters
  with the high bit set are to be output.  Specifically, a character
  with the high bit set with a value greater than or equal to this value
  is output as is; otherwise, it will be output in a 7-bit
  representation.  The default value for this variable is 128 for MSDOS
  and 160 for other systems (ISO-Latin).

  int SLtt_Use_Ansi_Colors; If this value is non-zero, the terminal is
  assumed to support ANSI colors otherwise it is assumed to be
  monochrome.  The default is 0.

  int SLtt_Term_Cannot_Scroll; If this value is zero, the SLsmg will
  attempt to scroll the physical display to optimize the update.  If it
  is non-zero, the screen management routines will not perform this
  optimization.  For some applications, this variable should be set to
  zero.  The default value is set by the SLtt_get_terminfo function.



  5.6.  Hints for using SLsmg


  This section discusses some general design issues that one must face
  when writing an application that requires some sort of screen
  management.



  6.  Signal Functions



  Almost all non-trivial programs must worry about signals.  This is
  especially true for programs that use the S-Lang terminal input/output
  and screen management routines.  Unfortunately, there is no fixed way
  to handle signals; otherwise, the Unix kernel would take care of all
  issues regarding signals and the application programmer would never
  have to worry about them.  For this reason, none of the routines in
  the S-Lang library catch signals; however, some of the routines block
  the delivery of signals during crucial moments.  It is up to the
  application programmer to install handlers for the various signals of
  interest.

  For the interpreter, the most important signal to worry about is
  SIGINT.  This signal is usually generated when the user presses Ctrl-C
  at the keyboard.  The interpreter checks the value of the SLang_Error
  variable to determine whether or not it should abort the interpreting
  process and return control back to the application.  This means that
  if SIGINT is to be used to abort the interpreter, a signal handler for
  SIGINT should be installed.  The handler should set the value of
  SLang_Error to SL_USER_BREAK.

  Applications that use the tty getkey routines or the screen management
  routines must worry about signals such as:


            SIGINT                interrupt
            SIGTSTP               stop
            SIGQUIT               quit
            SIGTTOU               background write
            SIGTTIN               background read
            SIGWINCH              window resize



  It is important that handlers be established for these signals while
  the either the SLsmg routines or the getkey routines are initialized.
  The SLang_init_tty, SLang_reset_tty, SLsmg_init_smg, and
  SLsmg_reset_smg functions block these signals from occuring while they
  are being called.

  Since a signal can be delivered at any time, it is important for the
  signal handler to call only functions that can be called from a signal
  handler.  This usually means that such function must be re-entrant. In
  particular, the SLsmg routines are not re-entrant; hence, they should
  not be called when a signal is being processed unless the application
  can ensure that the signal was not delivered while an SLsmg function
  was called.  This statement applies to many other functions such as
  malloc, or, more generally, any function that calls malloc.  The
  upshot is that the signal handler should not attempt to do too much
  except set a global variable for the application to look at while not
  in a signal handler.

  The S-Lang library provides two functions for blocking and unblocking
  the above signals:


           int SLsig_block_signals (void);
           int SLsig_unblock_signals (void);



  It should be noted that for every call to SLsig_block_signals, a cor-
  responding call should be made to SLsig_unblock_signals, e.g.,


           void update_screen ()
           {
              SLsig_block_signals ();

              /* Call SLsmg functions */
                  .
                  .
              SLsig_unblock_signals ();
           }



  See demo/pager.c for examples.



  7.  Searching Functions



  The S-Lang library incorporates two types of searches: Regular
  expression pattern matching and ordinary searching.


  7.1.  Regular Expressions



  !!! No documentation available yet !!!



  7.2.  Simple Searches


  The routines for ordinary searching are defined in the slsearch.c
  file.  To use these routines, simply include "slang.h" in your program
  and simply call the appropriate routines.

  The searches can go in either a forward or backward direction and can
  either be case or case insensitive.  The region that is searched may
  contain null characters (ASCII 0) however, the search string cannot in
  the current implementation.  In addition the length of the string to
  be found is currently limited to 256 characters.

  Before searching, the function SLsearch_init must first be called to
  `preprocess' the search string.



  7.3.  Initialization


  The function SLsearch_init must be called before a search can take
  place.  Its prototype is:


           int SLsearch_init (char *key, int dir, int case_sens, SLsearch_Type *st);



  Here key is the string to be searched for.  dir specifies the direc-
  tion of the search: a value greater than zero is used for searching
  forward and a value less than zero is used for searching backward.
  The parameter case_sens specifies whether the search is case sensitive
  or not.  A non-zero value indicates that case is important.  st is a
  pointer to a structure of type SLsearch_Type defined in "slang.h".
  This structure is initialized by this routine and must be passed to
  SLsearch when the search is actually performed.

  This routine returns the length of the string to be searched for.



  7.4.  SLsearch



             Prototype: unsigned char *SLsearch (unsigned char *pmin,
                                                 unsigned char *pmax,
                                                 SLsearch_Type *st);



  This function performs the search defined by a previous call to
  SLsearch_init over a region specified by the pointers pmin and pmax.

  It returns a pointer to the start of the match if successful or it
  will return NULL if a match was not found.



  A.  Copyright

  The S-Lang library is distributed under two copyrights: the GNU Genral
  Public License, and the Artistic License.  Any program that uses the
  interpreter must adhere to rules of one of these licenses.


  A.1.  The GNU Public License



                          GNU GENERAL PUBLIC LICENSE
                             Version 2, June 1991

        Copyright (C) 1989, 1991 Free Software Foundation, Inc.
                                 675 Mass Ave, Cambridge, MA 02139, USA
        Everyone is permitted to copy and distribute verbatim copies
        of this license document, but changing it is not allowed.

                                   Preamble



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  We protect your rights with two steps: (1) copyright the software, and
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  special exception, the source code distributed need not include any-
  thing that is normally distributed (in either source or binary form)
  with the major components (compiler, kernel, and so on) of the operat-
  ing system on which the executable runs, unless that component itself
  accompanies the executable.

  If distribution of executable or object code is made by offering
  access to copy from a designated place, then offering equivalent
  access to copy the source code from the same place counts as
  distribution of the source code, even though third parties are not
  compelled to copy the source along with the object code.

  4. You may not copy, modify, sublicense, or distribute the Program
  except as expressly provided under this License.  Any attempt
  otherwise to copy, modify, sublicense or distribute the Program 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.

  5. You are not required to accept this License, since you have not
  signed it.  However, nothing else grants you permission to modify or
  distribute the Program or its derivative works.  These actions are
  prohibited by law if you do not accept this License.  Therefore, by
  modifying or distributing the Program (or any work based on the
  Program), you indicate your acceptance of this License to do so, and
  all its terms and conditions for copying, distributing or modifying
  the Program or works based on it.

  6. Each time you redistribute the Program (or any work based on the
  Program), the recipient automatically receives a license from the
  original licensor to copy, distribute or modify the Program subject to
  these terms and conditions.  You may not impose any further
  restrictions on the recipients' exercise of the rights granted herein.
  You are not responsible for enforcing compliance by third parties to
  this License.

  7. If, as a consequence of a court judgment or allegation of patent
  infringement or for any other reason (not limited to patent issues),
  conditions are imposed on you (whether by court order, agreement or
  otherwise) that contradict the conditions of this License, they do not
  excuse you from the conditions of this License.  If you cannot
  distribute so as to satisfy simultaneously your obligations under this
  License and any other pertinent obligations, then as a consequence you
  may not distribute the Program at all.  For example, if a patent
  license would not permit royalty-free redistribution of the Program by
  all those who receive copies directly or indirectly through you, then
  the only way you could satisfy both it and this License would be to
  refrain entirely from distribution of the Program.

  If any portion of this section is held invalid or unenforceable under
  any particular circumstance, the balance of the section is intended to
  apply and the section as a whole is intended to apply in other
  circumstances.

  It is not the purpose of this section to induce you to infringe any
  patents or other property right claims or to contest validity of any
  such claims; this section has the sole purpose of protecting the
  integrity of the free software distribution system, which is
  implemented by public license practices.  Many people have made
  generous contributions to the wide range of software distributed
  through that system in reliance on consistent application of that
  system; it is up to the author/donor to decide if he or she is willing
  to distribute software through any other system and a licensee cannot
  impose that choice.

  This section is intended to make thoroughly clear what is believed to
  be a consequence of the rest of this License.

  8. If the distribution and/or use of the Program is restricted in
  certain countries either by patents or by copyrighted interfaces, the
  original copyright holder who places the Program under this License
  may add an explicit geographical distribution limitation excluding
  those countries, so that distribution is permitted only in or among
  countries not thus excluded.  In such case, this License incorporates
  the limitation as if written in the body of this License.

  9. The Free Software Foundation may publish revised and/or new
  versions of the General Public 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.

  Each version is given a distinguishing version number.  If the Program
  specifies a version number of this License which applies to it and
  "any later version", you have the option of following the terms and
  conditions either of that version or of any later version published by
  the Free Software Foundation.  If the Program does not specify a
  version number of this License, you may choose any version ever
  published by the Free Software Foundation.

  10. If you wish to incorporate parts of the Program into other free
  programs whose distribution conditions are different, write to the
  author to ask for permission.  For software which is copyrighted by
  the Free Software Foundation, write to the Free Software Foundation;
  we sometimes make exceptions for this.  Our decision will be guided by
  the two goals of preserving the free status of all derivatives of our
  free software and of promoting the sharing and reuse of software
  generally.


                                   NO WARRANTY

         11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
       FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
       OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
       PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
       OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
       MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
       TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
       PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
       REPAIR OR CORRECTION.

         12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
       WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
       REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
       INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
       OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
       TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
       YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
       PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
       POSSIBILITY OF SUCH DAMAGES.

                            END OF TERMS AND CONDITIONS



  Appendix: How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
  possible use to the public, the best way to achieve this is to make it
  free software which everyone can redistribute and change under these
  terms.

  To do so, attach the following notices to the program.  It is safest
  to attach them to the start of each source file to most effectively
  convey the exclusion of warranty; and each file should have at least
  the "copyright" line and a pointer to where the full notice is found.


           <one line to give the program's name and a brief idea of what it does.>
           Copyright (C) 19yy  <name of author>

           This program is free software; you can redistribute it and/or modify
           it under the terms of the GNU General Public License as published by
           the Free Software Foundation; either version 2 of the License, or
           (at your option) any later version.

           This program is distributed in the hope that it will be useful,
           but WITHOUT ANY WARRANTY; without even the implied warranty of
           MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
           GNU General Public License for more details.

           You should have received a copy of the GNU General Public License
           along with this program; if not, write to the Free Software
           Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.



  Also add information on how to contact you by electronic and paper
  mail.

  If the program is interactive, make it output a short notice like this
  when it starts in an interactive mode:


           Gnomovision version 69, Copyright (C) 19yy name of author
           Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
           This is free software, and you are welcome to redistribute it
           under certain conditions; type `show c' for details.



  The hypothetical commands `show w' and `show c' should show the appro-
  priate parts of the General Public License.  Of course, the commands
  you use may be called something other than `show w' and `show c'; they
  could even be mouse-clicks or menu items--whatever suits your program.

  You should also get your employer (if you work as a programmer) or
  your school, if any, to sign a "copyright disclaimer" for the program,
  if necessary.  Here is a sample; alter the names:


         Yoyodyne, Inc., hereby disclaims all copyright interest in the program
         `Gnomovision' (which makes passes at compilers) written by James Hacker.

         <signature of Ty Coon>, 1 April 1989
         Ty Coon, President of Vice



  This General Public License does not permit incorporating your program
  into proprietary programs.  If your program is a subroutine library,
  you may consider it more useful to permit linking proprietary applica-
  tions with the library.  If this is what you want to do, use the GNU
  Library General Public License instead of this License.


  A.2.  The Artistic License



                                The "Artistic License"

                                       Preamble



  The intent of this document is to state the conditions under which a
  Package may be copied, such that the Copyright Holder maintains some
  semblance of artistic control over the development of the package,
  while giving the users of the package the right to use and distribute
  the Package in a more-or-less customary fashion, plus the right to
  make reasonable modifications.

  Definitions:


               "Package" refers to the collection of files distributed by the
               Copyright Holder, and derivatives of that collection of files
               created through textual modification.

               "Standard Version" refers to such a Package if it has not been
               modified, or has been modified in accordance with the wishes
               of the Copyright Holder as specified below.

               "Copyright Holder" is whoever is named in the copyright or
               copyrights for the package.

               "You" is you, if you're thinking about copying or distributing
               this Package.

               "Reasonable copying fee" is whatever you can justify on the
               basis of media cost, duplication charges, time of people involved,
               and so on.  (You will not be required to justify it to the
               Copyright Holder, but only to the computing community at large
               as a market that must bear the fee.)

               "Freely Available" means that no fee is charged for the item
               itself, though there may be fees involved in handling the item.
               It also means that recipients of the item may redistribute it
               under the same conditions they received it.



  1. You may make and give away verbatim copies of the source form of
  the Standard Version of this Package without restriction, provided
  that you duplicate all of the original copyright notices and associ-
  ated disclaimers.

  2. You may apply bug fixes, portability fixes and other modifications
  derived from the Public Domain or from the Copyright Holder.  A
  Package modified in such a way shall still be considered the Standard
  Version.

  3. You may otherwise modify your copy of this Package in any way,
  provided that you insert a prominent notice in each changed file
  stating how and when you changed that file, and provided that you do
  at least ONE of the following:


           a) place your modifications in the Public Domain or otherwise make them
           Freely Available, such as by posting said modifications to Usenet or
           an equivalent medium, or placing the modifications on a major archive
           site such as uunet.uu.net, or by allowing the Copyright Holder to include
           your modifications in the Standard Version of the Package.

           b) use the modified Package only within your corporation or organization.

           c) rename any non-standard executables so the names do not conflict
           with standard executables, which must also be provided, and provide
           a separate manual page for each non-standard executable that clearly
           documents how it differs from the Standard Version.

           d) make other distribution arrangements with the Copyright Holder.



  4. You may distribute the programs of this Package in object code or
  executable form, provided that you do at least ONE of the following:


           a) distribute a Standard Version of the executables and library files,
           together with instructions (in the manual page or equivalent) on where
           to get the Standard Version.

           b) accompany the distribution with the machine-readable source of
           the Package with your modifications.

           c) give non-standard executables non-standard names, and clearly
           document the differences in manual pages (or equivalent), together
           with instructions on where to get the Standard Version.

           d) make other distribution arrangements with the Copyright Holder.



  5. You may charge a reasonable copying fee for any distribution of
  this Package.  You may charge any fee you choose for support of this
  Package.  You may not charge a fee for this Package itself.  However,
  you may distribute this Package in aggregate with other (possibly com-
  mercial) programs as part of a larger (possibly commercial) software
  distribution provided that you do not advertise this Package as a
  product of your own.  You may embed this Package's interpreter within
  an executable of yours (by linking); this shall be construed as a mere
  form of aggregation, provided that the complete Standard Version of
  the interpreter is so embedded.

  6. The scripts and library files supplied as input to or produced as
  output from the programs of this Package do not automatically fall
  under the copyright of this Package, but belong to whomever generated
  them, and may be sold commercially, and may be aggregated with this
  Package.  If such scripts or library files are aggregated with this
  Package via the so-called "undump" or "unexec" methods of producing a
  binary executable image, then distribution of such an image shall
  neither be construed as a distribution of this Package nor shall it
  fall under the restrictions of Paragraphs 3 and 4, provided that you
  do not represent such an executable image as a Standard Version of
  this Package.
  7. C subroutines (or comparably compiled subroutines in other
  languages) supplied by you and linked into this Package in order to
  emulate subroutines and variables of the language defined by this
  Package shall not be considered part of this Package, but are the
  equivalent of input as in Paragraph 6, provided these subroutines do
  not change the language in any way that would cause it to fail the
  regression tests for the language.

  8. Aggregation of this Package with a commercial distribution is
  always permitted provided that the use of this Package is embedded;
  that is, when no overt attempt is made to make this Package's
  interfaces visible to the end user of the commercial distribution.
  Such use shall not be construed as a distribution of this Package.

  9. The name of the Copyright Holder may not be used to endorse or
  promote products derived from this software without specific prior
  written permission.

  10. THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
  IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
  WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.



                            Table of Contents


  1. Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
  1.1. A Brief History of S-Lang . . . . . . . . . . . . . . . . . .   2
  1.2. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   2
  2. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .   4
  3. Interpreter Interface . . . . . . . . . . . . . . . . . . . . .   5
  3.1. Embedding the Interpreter . . . . . . . . . . . . . . . . . .   5
  3.2. Calling the Interpreter . . . . . . . . . . . . . . . . . . .   6
  3.2.1. Loading Files . . . . . . . . . . . . . . . . . . . . . . .   6
  3.2.2. Loading Strings . . . . . . . . . . . . . . . . . . . . . .   7
  3.3. Intrinsic Functions . . . . . . . . . . . . . . . . . . . . .   7
  3.3.1. Restrictions on Intrinsic Functions . . . . . . . . . . . .   8
  3.3.2. Adding a New Intrinsic  . . . . . . . . . . . . . . . . . .   9
  3.3.3. More Complicated Intrinsics . . . . . . . . . . . . . . . .  11
  3.4. Intrinsic Variables . . . . . . . . . . . . . . . . . . . . .  13
  3.5. Aggregate Data Objects  . . . . . . . . . . . . . . . . . . .  15
  3.5.1. Arrays  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
  3.5.2. Structures  . . . . . . . . . . . . . . . . . . . . . . . .  18
  3.5.2.1. Interpreter Structures  . . . . . . . . . . . . . . . . .  18
  3.5.2.2. Intrinsic Structures  . . . . . . . . . . . . . . . . . .  22
  4. Keyboard Interface  . . . . . . . . . . . . . . . . . . . . . .  24
  4.1. Initializing the Keyboard Interface . . . . . . . . . . . . .  24
  4.2. Resetting the Keyboard Interface  . . . . . . . . . . . . . .  25
  4.3. Initializing the SLkp  Routines . . . . . . . . . . . . . . .  25
  4.4. Setting the Interrupt Handler . . . . . . . . . . . . . . . .  26
  4.5. Reading Keyboard Input with SLang_getkey  . . . . . . . . . .  27
  4.6. Reading Keyboard Input with SLkp_getkey . . . . . . . . . . .  28
  4.7. Buffering Input . . . . . . . . . . . . . . . . . . . . . . .  29
  4.8. Global Variables  . . . . . . . . . . . . . . . . . . . . . .  30
  5. Screen Management . . . . . . . . . . . . . . . . . . . . . . .  31
  5.1. Initialization  . . . . . . . . . . . . . . . . . . . . . . .  31
  5.2. Resetting SLsmg . . . . . . . . . . . . . . . . . . . . . . .  31
  5.3. Handling Screen Resize Events . . . . . . . . . . . . . . . .  32
  5.4. SLsmg Functions . . . . . . . . . . . . . . . . . . . . . . .  33
  5.4.1. Positioning the cursor  . . . . . . . . . . . . . . . . . .  33
  5.4.2. Writing to the Display  . . . . . . . . . . . . . . . . . .  34
  5.4.3. Erasing the Display . . . . . . . . . . . . . . . . . . . .  35
  5.4.4. Setting Character Attributes  . . . . . . . . . . . . . . .  35
  5.4.5. Lines and Alternate Character Sets  . . . . . . . . . . . .  37
  5.4.6. Miscellaneous Functions . . . . . . . . . . . . . . . . . .  37
  5.5. Variables . . . . . . . . . . . . . . . . . . . . . . . . . .  38
  5.6. Hints for using SLsmg . . . . . . . . . . . . . . . . . . . .  38
  6. Signal Functions  . . . . . . . . . . . . . . . . . . . . . . .  39
  7. Searching Functions . . . . . . . . . . . . . . . . . . . . . .  41
  7.1. Regular Expressions . . . . . . . . . . . . . . . . . . . . .  41
  7.2. Simple Searches . . . . . . . . . . . . . . . . . . . . . . .  41
  7.3. Initialization  . . . . . . . . . . . . . . . . . . . . . . .  41
  7.4. SLsearch  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
  A. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . .  43
  A.1. The GNU Public License  . . . . . . . . . . . . . . . . . . .  43
  A.2. The Artistic License  . . . . . . . . . . . . . . . . . . . .  49



