The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++ (and, indirectly, other programming languages). It was started in the 1980s by the Free Software Foundation (FSF) for the GNU operating system. Released under the GNU Lesser General Public License, glibc is free software. The GNU C Library project provides the core libraries for the GNU system, as well as many systems that use Linux as the kernel. These libraries provide critical APIs including ISO C11, POSIX.1-2008, BSD, OS-specific APIs and more. These APIs include such foundational facilities as open, read, write, malloc, printf, getaddrinfo, dlopen, pthread create, crypt, login, exit and more.
The glibc project was initially written mostly by Roland McGrath, working for the Free Software Foundation (FSF) in the 1980s as a teenager. 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. In September 1995 Ulrich Drepper made his first contribution to the glibc and by 1997 most commits were made by him. Drepper held the maintainership position for many years and until 2012 accumulated 63% of all commits to the project.
In May 2009 glibc was migrated to a Git repository.
In 1994, the developers of the Linux kernel forked glibc. Their fork, "Linux libc", was maintained separately until around 1998. Because the copyright attribution was insufficient, changes could not be merged back to the GNU Libc.
When the FSF released glibc 2.0 in January 1997, the kernel developers discontinued Linux libc due to glibc 2.0’s superior compliance with POSIX standards. glibc 2.0 also had better internationalisation and more in-depth translation, IPv6 capability, 64-bit data access, facilities for multithreaded applications, future version compatibility, and the code was more portable.
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
Starting in 2001 the library's development had been overseen by a committee, with Ulrich Drepper kept as the lead contributor and maintainer. The steering committee installation was surrounded by a public controversy as it was openly described by Ulrich Drepper as a failed hostile takeover maneuver by Richard Stallman.
In March 2012, the steering committee voted to disband itself and remove Drepper in favor of a community-driven development process, with Ryan Arnold, Maxim Kuvyrkov, Joseph Myers, Carlos O'Donell, and Alexandre Oliva holding the responsibility of GNU maintainership (but no extra decision-making power).
In July 2017, 30 years after he started glibc, Roland McGrath announced his departure, "declaring myself maintainer emeritus and withdrawing from direct involvement in the project. These past several months, if not the last few years, have proven that you don't need me any more".
After longstanding controversies around Drepper's leadership style and external contribution acceptance, Debian switched publicly to the glibc fork EGLIBC in 2009  and back to glibc with the Debian 8.0 (Jessie) release in April 2015.
For most systems, the version of glibc can be obtained by executing the lib file (for example, /lib/libc.so.6).
|0.1 – 0.6||October 1991 – February 1992|
|1.01 – 1.09.3||March 1992 – December 1994|
|1.90 – 1.102||May 1996 – January 1997|
|2.3.2||February 2003||Debian 3.1 (Sarge)|
|2.3.4||December 2004||Standard for Linux Standard Base (LSB) 3.0||RHEL 4 (Update 5)|
|2.3.5||April 2005||SLES 9|
|2.3.6||November 2005||Debian 4.0 (Etch)|
|2.4||March 2006||Standard for LSB 4.0, initial inotify support||SLES 10|
|2.5||September 2006||Full inotify support||RHEL 5|
|2.7||October 2007||Debian 5 (Lenny), Ubuntu 8.04|
|2.11||October 2009||SLES 11, Ubuntu 10.04, eglibc used in Debian 6 (Squeeze)|
|2.12||May 2010||RHEL 6|
|2.13||January 2011||eglibc 2.13 used in Debian 7 (Wheezy)|
|2.15||March 2012||Ubuntu 12.04 and 12.10|
|2.16||June 2012||x32 ABI support, ISO C11 compliance, SystemTap|
|2.17||December 2012||64-bit ARM support||Ubuntu 13.04, RHEL 7|
|2.18||August 2013||Improved C++11 support. Support for Intel TSX lock elision. Support for the Xilinx MicroBlaze and IBM POWER8 microarchitectures.||Fedora 20|
|2.19||February 2014||SystemTap probes for malloc. GNU Indirect Function (IFUNC) support for ppc32 and ppc64. New feature test macro _DEFAULT_SOURCE to replace _SVID_SOURCE and _BSD_SOURCE. Preliminary safety documentation for all functions in the manual. ABI change in ucontext and jmp_buf for s390/s390x.||Ubuntu 14.04, eglibc 2.19 used in Debian 8 (Jessie), openSUSE 13, SLES 12|
|2.20||September 2014||Support for file description locks||Fedora 21|
|2.21||February 2015||New semaphore implementation||Ubuntu 15.04, Fedora 22|
|2.22||August 2015||Support to enable Google Native Client (NaCl), that originally ran on x86, running on ARMv7-A, Unicode 7.0||Fedora 23|
|2.23||February 2016||Unicode 8.0||Fedora 24, Ubuntu 16.04|
|2.24||August 2016||Some deprecated features have been removed||Fedora 25, Ubuntu 16.10 and 17.04, Debian 9 (Stretch)|
|2.26||August 2017||Improved performance (per-thread cache for malloc), Unicode 10 support||Fedora 27, Ubuntu 17.10|
|2.27||February 2018||Performance optimizations. RISC-V support.||Fedora 28, Ubuntu 18.04|
||Ubuntu 18.10, RHEL 8.0.0, Debian 10 (Buster), Fedora 29|
||Ubuntu 19.04, Fedora 30|
|2.30||August 2019||Unicode 12.1.0, the dynamic linker accepts the
||Ubuntu 19.10, Fedora 31|
|2.31||February 2020||Initial C2x standard support||Ubuntu 20.04, Fedora 32|
|2.32||August 2020||Unicode 13.0, 'access' attribute for better warnings in GCC 10, i.e. to "help detect buffer overflows and other out-of-bounds accesses"||Ubuntu 20.10, Fedora 33|
|2.33||February 2021||HWCAPS||Ubuntu 21.04|
|2.34||August 2021||libpthread, libdl, libutil, libanl has been integrated into libc.||Ubuntu 21.10|
glibc provides the functionality required by the Single UNIX Specification, POSIX (1c, 1d, and 1j) and some of the functionality required by ISO C11, ISO C99, Berkeley Unix (BSD) interfaces, the System V Interface Definition (SVID) and the X/Open Portability Guide (XPG), Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems along with all X/Open UNIX extensions.
In addition, glibc also provides extensions that have been deemed useful or necessary while developing GNU.
glibc is used in systems that run many different kernels and different hardware architectures. Its most common use is in systems using the Linux kernel on x86 hardware, however, officially supported hardware includes: 32-bit ARM and its newer 64-bit ISA (AArch64), C-SKY, DEC Alpha, IA-64, Motorola m68k, MicroBlaze, MIPS, Nios II, PA-RISC, PowerPC, RISC-V, s390, SPARC, and x86 (old versions support TILE). It officially supports the Hurd and Linux kernels. Additionally, there are heavily patched versions that run on the kernels of FreeBSD and NetBSD (from which Debian GNU/kFreeBSD and Debian GNU/NetBSD systems are built, respectively), as well as a forked-version of OpenSolaris. It is also used (in an edited form) and named libroot.so in BeOS and Haiku.
glibc has been criticized as being "bloated" and slower than other libraries in the past, e.g. by Linus Torvalds and embedded Linux programmers. For this reason, several alternative C standard libraries have been created which emphasize a smaller footprint. 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).
There are compatibility layers ("shims") to allow programs written for other ecosystems to run on glibc interface offering systems. These include libhybris, a compatibility layer for Android's Bionic, and Wine, which can be seen as compatibility layer from Windows APIs to glibc and other native APIs available on Unix-like systems.