The GNU C Library, commonly known as glibc, is the C standard library released by the GNU Project. Originally written by the Free Software Foundation (FSF) for the GNU operating system, the library's development has been overseen by a committee since 2001, with Ulrich Drepper from Red Hat as the lead contributor and maintainer.
In February 1988, FSF described glibc as having nearly completed the functionality required by ANSI C. By 1992, it had the ANSI C-1989 and POSIX.1-1990 functions implemented and work was under way on POSIX.2.
When FSF released glibc 2.0 in 1996, it had much more complete POSIX standards support, better internationalisation/multilingual support, support for IPv6, 64-bit data access, support for multithreaded applications, future version compatibility support, and the code was more portable. At this point, the Linux kernel developers discontinued their fork and returned to using FSF's glibc.
The last used version of Linux libc used the internal name (soname) libc.so.5. Following on from this, glibc 2.x on Linux uses the soname libc.so.6 (Alpha and IA64 architectures now use libc.so.6.1, instead). The soname is often abbreviated as libc6 (for example in the package name in debian) following the normal conventions for libraries.
According to Richard Stallman, the changes that had been made in Linux libc could not be merged back into glibc because the authorship status of that code was unclear and the GNU project is quite strict about recording copyright and authors.
In addition, glibc also provides extensions which have been deemed useful or necessary while developing GNU.
However, many small-device projects use GNU libc over the smaller alternatives because of its application support, standards compliance, and completeness. Examples include OpenMoko and Familiar Linux for iPaq handhelds (when using the GPE display software).