libc - overview of standard C libraries on Linux
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 http://www.gnu.org/software/libc/">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 libc.so.6.
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
uClibc
dietlibc
and
musl libc
Details of these libraries are covered by the man-pages
project, where they are known.