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):

libLLVM (-lLLVM)
    LLVM components in a single library.

libagentx (-lagentx)
    AgentX client library. Used for applications to export metrics to AgentX capable snmp daemons. See agentx(3).

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, cryptographic routines, storage allocation, time functions, signal handling, and more.

libc++ (-lc++)
    LLVM standard C++ library. Note: users do not normally have to explicitly link with this library.

libc++abi (-lc++abi)
    LLVM C++ runtime library. Note: users do not normally have to explicitly link with this library.

libcbor (-lcbor)
    An implementation of the Concise Binary Object Representation (CBOR) encoding format defined in RFC 7049.

libcrypto (-lcrypto)
    Provides functionality such as symmetric encryption, public key cryptography, digests, message authentication codes, 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).

libelf (-lelf)
    Library routines for manipulating ELF objects. See elf(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).

libexecinfo (-lexecinfo)
    Library providing backtrace functions. See backtrace(3).

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

libfido2 (-lfido2)
    Library for communication with U2F/FIDO2 devices over USB.

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).

libfuse (-lfuse)
    File system in userland library. See fuse_main(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.

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

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).

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).

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-character 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_open_live(3).

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

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

libradius (-lradius)
    Support routines for the RADIUS library. See radius_new_request_packet(3).

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

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 sndiod(8) audio server. See sio_open(3).

libssl (-lssl)
    Implements the Transport Layer Security (TLS) protocol, the successor to the Secure Sockets Layer (SSL) protocol. See ssl(3).

libstdc++ (-lstdc++)
    GNU standard C++ library. Note: users do not normally have to explicitly link with this library.

libsupc++ (-lsupc++)
    GNU C++ runtime library. Note: users do not normally have to explicitly link with this library.

libtls (-ltls)
    A Transport Layer Security library with a clean and easy to use interface. See tls_init(3).

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

libutil (-lutil)
    System utility functions.

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).

libi386 (-li386)
    i386 I/O and memory access functions. See i386_iopl(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), gprof(1), ld(1), ld.so(1), nm(1), ranlib(1), mk.conf(5)

HISTORY
An intro manual for section 3 first appeared in AT&T UNIX.