The term "libc" is commonly used as a shorthand for the "standard
C library", a library of standard functions that can be used by all C
programs (and sometimes by programs in other languages). Because of some
history (see below), use of the term "libc" to refer to the standard
C library is somewhat ambiguous on Linux.
By far the most widely used C library on Linux is the GNU C Library
often referred to as glibc. This is the C library that is nowadays used
in all major Linux distributions. It is also the C library whose details are
documented in the relevant pages of the man-pages project (primarily in
Section 3 of the manual). Documentation of glibc is also available in the
glibc manual, available via the command info libc. Release 1.0 of glibc
was made in September 1992. (There were earlier 0.x releases.) The next major
release of glibc was 2.0, at the beginning of 1997.
The pathname /lib/libc.so.6 (or something similar) is
normally a symbolic link that points to the location of the glibc library,
and executing this pathname will cause glibc to display various information
about the version installed on your system.
In the early to mid 1990s, there was for a while Linux libc, a fork of
glibc 1.x created by Linux developers who felt that glibc development at the
time was not sufficing for the needs of Linux. Often, this library was
referred to (ambiguously) as just "libc". Linux libc released major
versions 2, 3, 4, and 5, as well as many minor versions of those releases.
Linux libc4 was the last version to use the a.out binary format, and the first
version to provide (primitive) shared library support. Linux libc 5 was the
first version to support the ELF binary format; this version used the shared
library soname libc.so.5. For a while, Linux libc was the standard C
library in many Linux distributions.
However, notwithstanding the original motivations of the Linux
libc effort, by the time glibc 2.0 was released (in 1997), it was clearly
superior to Linux libc, and all major Linux distributions that had been
using Linux libc soon switched back to glibc. To avoid any confusion with
Linux libc versions, glibc 2.0 and later used the shared library soname
Since the switch from Linux libc to glibc 2.0 occurred long ago,
man-pages no longer takes care to document Linux libc details.
Nevertheless, the history is visible in vestiges of information about Linux
libc that remain in a few manual pages, in particular, references to
libc4 and libc5.
There are various other less widely used C libraries for Linux. These libraries
are generally smaller than glibc, both in terms of features and memory
footprint, and often intended for building small binaries, perhaps targeted at
development for embedded Linux systems. Among such libraries are
musl libc. Details
of these libraries are covered by the man-pages project, where they are
This page is part of release 5.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at https://www.kernel.org/doc/man-pages/.