The GNU library is also compatible with the IEEE POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments. POSIX is derived mostly from various versions of the Unix operating system.
The library facilities specified by the POSIX standards are a superset of those required by ANSI C; POSIX specifies additional features for ANSI C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.
The GNU C library implements all of the functions specified in IEEE Std 1003.1-1990, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ANSI C facilities specified by this standard include file system interface primitives (see File System Interface), device-specific terminal control functions (see Low-Level Terminal Interface), and process control functions (see Processes).
Some facilities from IEEE Std 1003.2-1992, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (see Pattern Matching).