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 stan-
dard C library is somewhat ambiguous on Linux.
By far the most widely used C library on Linux is the GNU C Library
<http://www.gnu.org/software/libc/>, often referred to as glibc. This
is the C library that is nowadays used in all major Linux distribu-
tions. 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 sym-
bolic link that points to the location of the glibc library, and exe-
cuting 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). 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. Since this switch
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 some manual pages, in par-
ticular, references to libc4 and libc5.
Other C libraries
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 uClibc <http://www.uclibc.org/>, dietlibc
<http://www.fefe.de/dietlibc/>, and musl libc <http://www.musl-
libc.org/>. Details of these libraries are covered by the man-pages
project, where they are known.
syscalls(2), getauxval(3), proc(5), feature_test_macros(7), man-
pages(7), standards(7), vdso(7)
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