INTRO(3)

NAME
intro - introduction to the C libraries

SYNOPSIS
cc [flags] file ... [-llibrary]

DESCRIPTION
The manual pages in section 3 provide an overview of the C library functions, their error returns, and other common definitions and concepts. Most of these functions are available from the C library, libc. Other libraries, such as the math library, libm, must be indicated at compile time with the -l option of the compiler.

The various libraries (followed by the loader flag):

libasn1 (-lasn1)
    The abstract syntax notation (ASN) library provides routines for the specification of abstract data types.

libc (-lc)
    Standard C library functions. When using the C compiler cc(1), it is not necessary to supply the loader flag -lc for these functions. There are several “libraries” or groups of functions included inside of libc: the standard I/O routines, database routines, bit operators, string operators, character tests and character operators, DES encryption routines, storage allocation, time functions, signal handling, and more.

libcom_err (-lcom_err)
    The common error description library.

libcompat (-lcompat)
    Functions which are obsolete but are available for compatibility with 4.3BSD. In particular, a number of system call interfaces provided in previous releases of BSD have been included for source code compatibility. Use of these routines should, for the most part, be avoided. The manual page entry for each compatibility routine indicates the proper interface to use.

libcrypto (-lcrypto)
    The OpenSSL crypto library. Implements a range of cryptographic algorithms, providing such functionality as symmetric encryption, public key cryptography, and certificate handling. See crypto(3).

libcurses (-lcurses)
libncurses (-lncurses)
libncursesw (-lncursesw)
libtermcap (-ltermcap)
libtermlib (-ltermlib)
    Terminal-independent screen management routines for two-dimensional non-bitmap display terminals. This implementation is “new curses” and is a replacement for 4.2BSD classic curses. The libraries libncurses, libncursesw, libtermcap, and libtermlib are all hard links to libcurses. This is for compatibility purposes only; new programs should link with -lcurses. See curses(3) and termcap(3).

libedit (-ledit)
    Generic line editing and history functions, similar to those found in sh(1). Functions using the libedit library must be linked with the libcurses library, i.e. -ledit -lcurses. See editline(3).

libevent (-levent)
    Provides a mechanism to execute a function when a specific event on a file descriptor occurs or after a given time has passed. See event(3).

libexpat (-lexpat)
    Library routines for parsing XML documents.

libform (-lform)
libformw (-lformw)
    Terminal-independent facilities for composing form screens on character-cell terminals. Functions using the libform library must be linked with the libcurses library, i.e. -lform -lcurses. libformw is a hard link to libform intended for use with libncursesw wide-character functions. See form(3).

libgcc (-lgcc)
    GCC runtime support, including long arithmetic, propolice, and language independent exception support. Note: users do not normally have to explicitly link with this library.

libgssapi (-lgssapi)
    The Generic Security Service Application Program Interface (GSS-API) provides security services to callers in a generic fashion. See gssapi(3).

libhdb (-lhdb)
    The Heimdal Kerberos 5 authentication/authorisation database access library.

libiberty (-liberty)
    Collection of subroutines missing in other operating systems, as well as the C++ demangler and other functions used by the GNU toolchain.

libkadm5clnt (-lkadm5clnt)
    Kerberos administration client library, for talking to a Kerberos database. Clients communicate via the network.

libkadm5srv (-lkadm5srv)
    Kerberos administration server library, for talking to a Kerberos database. Servers talk directly to the database.

libkafs5 (-lkafs5)
    The Kerberos 5 AFS library. See kafs5(3).

libkeynote (-lkeynote)
    System library for the keynote trust-management system. Trust-management systems provide standard, general-purpose mechanisms for specifying application security policies and credentials. Functions using the libkeynote library must be linked with the libm and libcrypto libraries, i.e. -lkeynote -lm -lcrypto. See keynote(3) and keynote(4).

libkrb5 (-lkrb5)
    Kerberos 5 library. See krb5(3).

libkvm (-lkvm)
    Kernel memory interface library. Provides a uniform interface for accessing kernel virtual memory images, including live systems and crash dumps. See kvm(3).

libl (-ll)
libfl (-lfl)
    The library for lex(1), a lexical analyzer generator. The libfl library is a hard link to libl.

libm (-lm)
    Mathematical functions which comprise the C math library, libm.

libmenu (-lmenu)
libmenuw (-lmenuw)
    Terminal-independent facilities for composing menu systems on character-cell terminals. Functions using the libmenu library must be linked with the libcurses library, i.e. -lmenu -lcurses. libmenuw is a hard link to libmenu intended for use with libncursesw wide-character functions. See menu(3).

libmilter (-lmilter)
    The sendmail(8) mail filter API. See the documentation in /usr/share/doc/html/milter/.

libobjc (-lobjc)
    Library for Objective C, an object-oriented superset of ANSI C. Use this to compile Objective C programs.

libocurses (-locurses)
libotermcap (-lotermcap)
    Routines to provide the user with a method of updating screens with reasonable optimisation. The ocurses(3) library is compatible with the curses library provided in 4.3. libotermcap is the 4.3-compatible termcap library, and is a hard link to libocurses. See otermcap(3).

libossaudio (-lossaudio)
    Provides an emulation of the OSS (Linux) audio interface. This is used only for porting programs. See ossaudio(3).

libpanel (-lpanel)
libpanelw (-lpanelw)
    Terminal-independent facilities for stacked windows on character-cell terminals. Functions using the libpanel library must be linked with the libcurses library, i.e. -lpanel -lcurses. libpanelw is a hard link to libpanel intended for use with libncursesw wide-charcter functions. See panel(3).

libpcap (-lpcap)
    Packet capture library. All packets on the network, even those destined for other hosts, are accessible through this library. See pcap(3).

libperl (-lperl)
    Support routines for perl(1).

libpthread (-lpthread)
    p1003.1-2001 threads API. See pthreads(3).

libreadline (-lreadline)
    Command line editing interface. See readline(3).

libroken (-lroken)
    A library containing compatibility functions used by Kerberos. It implements functionality required by the Kerberos implementation not implemented in the standard OpenBSD libraries.

librpcsvc (-lrpcsvc)
    Generated by rpcgen(1), containing stub functions for many common rpc(3) protocols.

libskey (-lskey)
    Support library for the S/Key one time password (OTP) authentication toolkit. See skey(3).

libsndio (-lsndio)
    Library for audio(4) hardware and the aucat(1) audio server. See sio_open(3).

libsqlite3 (-lsqlite3)
    SQL database library. See sqlite3(1).

libssl (-lssl)
    The OpenSSL ssl library implements the Secure Sockets Layer (SSL v3) and Transport Layer Security (TLS v1) protocols. See ssl(3).

libstdc++ (-lstdc++)
    GCC subroutine library for C++. See c++(1). Note: users do not normally have to explicitly link with this library.

libsupc++ (-lsupc++)
    (non GCC 2.95 systems only) C++ core language support (exceptions, new, typeinfo). Note: users do not normally have to explicitly link with this library.

libusbhid (-lusbhid)
    Routines to extract data from USB Human Interface Devices (HIDs). See usbhid(3).

libutil (-lutil)
    System utility functions. These are currently check_expire(3), fmt_scaled(3), fparseln(3), getmaxpartitions(3), getrawpartition(3), imsg_init(3), login(3), login_fbtab(3), opendev(3), opendisk(3), openpty(3), pidfile(3), pkcs5_pbkdf2(3), pw_init(3), pw_lock(3), readlabelfs(3) and uucplock(3).

libwind (-lwind)
    Heimdal Kerberos 5 implementation of stringprep and some of its profiles.

libwrap (-lwrap)
    TCP wrapper access control library. See hosts_access(3) and rfc1413(3).

liby (-ly)
    The library for yacc(1), an LALR parser generator.

libz (-lz)
    General purpose data compression library. The functions in this library are documented in compress(3). The data format is described in RFCs 1950 - 1952.

Platform-specific libraries:

libalpha (-lalpha)
    Alpha I/O and memory access functions. See inb(2).

libamd64 (-lamd64)
    AMD64 I/O and memory access functions. See amd64_iopl(2).

libarm (-larm)
    ARM I/O and memory access functions. See arm_drain_writebuf(2) and arm_sync_icache(2).

libi386 (-li386)
    i386 I/O and memory access functions. See i386_get_ioperm(2), i386_get_ldt(2), i386_iopl(2), and i386_vm86(2).

LIBRARY TYPES
The system libraries are located in /usr/lib. Typically, a library will have a number of variants:

    libc.a
    libc_p.a
    libc.so.30.1

Libraries with an ‘.a’ suffix are static. When a program is linked against a library, all the library code will be linked into the binary. This means the binary can be run even when the libraries are unavailable. However, it can be inefficient with memory usage. The C compiler, cc(1), can be instructed to link statically by specifying the -static flag.

Libraries with a ‘_p.a’ suffix are profiling libraries. They contain extra information suitable for analysing programs, such as execution speed and call counts. This in turn can be interpreted by utilities such as gprof(1). The C compiler, cc(1), can be instructed to generate profiling code, or to link with profiling libraries, by specifying the -pg flag.

Libraries with a ‘.so.X.Y’ suffix are dynamic libraries. When code is compiled dynamically, the library code that the application needs is not linked into the binary. Instead, data structures are added containing information about which dynamic libraries to link with. When the binary is executed, the run-time linker ld.so(1) reads these data structures, and loads them at a virtual address using the mmap(2) system call.

‘X’ represents the major number of the library, and ‘Y’ represents the minor number. In general, a binary will be able to use a dynamic library with a differing minor number, but the major numbers must match. In the example above, a binary linked with minor number ‘3’ would be linkable against libc.so.30.1, while a binary linked with major number ‘31’ would not.

The advantages of dynamic libraries are that multiple instances of the same program can share address space, and the physical size of the binary is smaller. The disadvantage is the added complexity that comes with loading the libraries dynamically, and the extra time taken to load the libraries. Of course, if the libraries are not available, the binary will be unable to execute. The C compiler, cc(1), can be instructed to link dynamically by specifying the -shared flag, although on systems that support it, this will be the default and need not be specified.

Shared libraries, as well as static libraries on architectures which produce position-independent executables (PIEs) by default, contain position-independent code (PIC). Normally, compilers produce relocatable code. Relocatable code needs to be modified at run-time, depending on where in memory it is to be run. PIC code does not need to be modified at run-time, but is less efficient than relocatable code. The C compiler, cc(1), can be instructed to generate PIC code by specifying the -fpic or -fPIC flags.

With the exception of dynamic libraries, libraries are generated using the ar(1) utility. The libraries contain an index to the contents of the library, stored within the library itself. The index lists each symbol defined by a member of a library that is a relocatable object file. This speeds up linking to the library, and allows routines in the library to call each other regardless of their placement within the library. The index is created by ranlib(1) and can be viewed using nm(1).

The building of dynamic libraries can be prevented by setting the variable NOPIC in /etc/mk.conf. The building of profiling versions of libraries can be prevented by setting the variable NOPROFILE in /etc/mk.conf. See mk.conf(5) for more details.

SEE ALSO
ar(1), cc(1), gcc-local(1), gprof(1), ld(1), ld.so(1), nm(1), ranlib(1), mk.conf(5)

HISTORY
An intro manual appeared in AT&T UNIX.