libc

Libc, short for the C standard library, is a collection of standard C functions that provide the basic runtime support for C programs. It supplies facilities for input and output, memory management, string handling, mathematics, time, environment and process control, error reporting, and interfacing with the operating system. A libc implementation sits between application code and the kernel, offering a portable API that translates C calls into system calls and library services.

Among the most widely used implementations are the GNU C Library (glibc), musl libc, and uClibc. Other examples include Bionic (used on Android), Newlib, and dietlibc. Linux distributions typically ship with glibc or musl; BSD systems ship their own libc variants.

Libc consists of the standard library functions, a dynamic linker/loader when using dynamic linking, and supporting components like locale data, thread support, and memory allocation facilities. It may provide wrappers around system calls to offer consistent behavior across kernels and architectures. Some implementations offer extensions beyond the ISO C and POSIX standards.

Programs can link to libc statically or dynamically; dynamic linking via a shared library reduces memory usage and enables updates, at the cost of startup latency. The dynamic linker, often invoked automatically by the program loader, resolves dependencies at startup.

Libc implementations aim to comply with the C standard and POSIX, while often including extensions. Licensing varies: glibc is released under the LGPL with a static linking exception; musl is permissively licensed; uClibc uses LGPL; Bionic has its own licensing. This affects distribution and redistribution in some environments.

Because most Unix-like systems rely on libc for basic functionality, it is a foundational component of software compatibility, influencing performance, internationalization, threading, and security patches. Ongoing development focuses on correctness, portability, size reduction for embedded systems, and improved support for modern C features and standards.