INTRO(3)

NAME
intro - introduction to the C libraries

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

DESCRIPTION
This section provides 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):

libbluetooth
    (-lbluetooth) The bluetooth library. See bluetooth(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:

    standard I/O routines
        see stdio(3)

    database routines
        see db(3)

    bit string operators
        see bitstring(3)

    bit and byte utilities
        see stdbit(3)

    string operators
        see string(3) and bstring(3)

    character tests and character operators
        see ctype(3)

    storage allocation
        see mpool(3)

    regular-expressions
        see regex(3)

    remote procedure calls (RPC)
        see rpc(3)

    time functions
        see time(3)

    signal handling
        see signal(3)

libcalendar
    (-lcalendar) The calendar arithmetic library. See calendar(3).

libcam
    (-lcam) The common access method user library. See cam(3).

libcrypt
    (-lcrypt) The crypt library. See crypt(3).

libcurses
    (-lcurses -ltermcap) Terminal independent screen management routines for two dimensional non-bitmap display terminals. See ncurses(3).

libcuse
    (-lcuse) The userland character device library. See cuse(3).

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.

libdevinfo
    (-ldevinfo) The Device and Resource Information Utility library. See devinfo(3).

libdevstat
    (-ldevstat) The Device Statistics library. See devstat(3).

libdwarf
    (-ldwarf) The DWARF access library. See dwarf(3).

libelf
    (-lelf) The ELF access library. See elf(3).

libfetch
    (-lfetch) The file transfer library. See fetch(3).

libfigpar
    (-lfigpar) The configuration file parsing library. See figpar(3).

libgpio
    (-lgpio) The general-purpose input output library (GPIO). See gpio(3).

libgssapi
    (-lgssapi) The generic security service application programming interface. See gssapi(3).

libjail
    (-ljail) The jail library. See jail(3).

libkvm
    (-lkvm) Functions used to access kernel memory are in this library. They can be used against both a running system and a crash dump. See kvm(3).

libl
    (-ll) The library for lex(1).

libm
    (-lm) The math library. See math(3).

libmd
    (-lmd) The message digest library. See md4(3), md5(3), sha(3), sha256(3), sha512(3), ripemd(3), skein(3).

libmp
    (-lmp)

libpam
    (-lpam) The pluggable authentication module library. See pam(3).

libpcap
    (-lpcap) The packet capture library. See pcap(3).

libpmc
    (-lpmc) The performance counters library. See pmc(3).

libpthread
    (-lpthread) The POSIX threads library. See pthread(3).

libstdthreads
    (-lstdthreads) The ISO C11 standard <threads.h> library. See thrd_create(3).

libsysdecode
    (-lsysdecode) The system argument decoding library. See sysdecode(3).

libtermcap
    (-ltermcap) The terminal independent operation library package. See termcap(3).

libusb
    (-lusb) The USB access library. See usb(3).

libvgl
    (-lvgl) The video graphics library. See vgl(3).

liby
    (-ly) The library for yacc(1).

libz
    (-lz) The general-purpose data compression library. See zlib(3).

FILES
/usr/lib/libc.a
    the C library
/usr/lib/libm.a
    the math library

LIBRARY TYPES
The system libraries are located in /lib and /usr/lib. A library has the following naming convention:

    libc.so.7

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

Libraries with a ‘.so.X’ suffix are dynamic libraries. When code is linked dynamically, the library code that the application needs is not included in 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 into the process virtual address space. rtld(1) loads the shared libraries when the program is executed.

‘X’ represents the library version number of the library. In the example above, a binary linked with libc.so.8 would not be usable on a system where only libc.so.7 is available.

The advantages of dynamic libraries are that multiple instances of the same library can share address space, and the physical size of the binary is smaller. A namespace per shared library is available via hidden visibility, allowing multiple compilation units in a library to share things without making them available to other libraries. It is possible to load libraries dynamically via dlopen(3). 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. Calls across shared libraries are also slightly slower and cannot be inlined, not even with link time optimization. The C compiler, cc(1), can be instructed to link dynamically by specifying the -shared flag.

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. The C compiler, cc(1), can be instructed to generate PIC code by specifying the -fPIC flag.

Static 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.

SEE ALSO
ar(1), cc(1), ld(1), nm(1), intro(2), math(3), stdio(3), make.conf(5), src.conf(5)

HISTORY
An intro manual appeared in AT&T UNIX.