POSIXcompliant

POSIXcompliant is an adjective used to describe software or systems that adhere to the POSIX standards. POSIX, originally developed as part of the IEEE 1003 series and now maintained by The Open Group, defines a portable interface for applications, including the C library APIs, as well as the shell and utilities used on the command line. The core standard, POSIX.1, specifies system calls and library routines; POSIX.2 defines a standard set of utilities and command syntax. Additional components cover threading (POSIX threads), real-time extensions, and other optional features.

Conformance means that a system's headers, libraries, runtime behavior, and command-line tools conform to the specified interfaces and semantics. Vendors may declare POSIX conformance levels, and conformance is evaluated with standardized test suites such as the POSIX conformance test suite. Modern operating systems generally claim POSIX compliance for core features to varying degrees: Linux, macOS, FreeBSD, OpenBSD, NetBSD, and Solaris provide broad POSIX support, often with vendor-specific extensions. Windows has historically offered partial POSIX support through subsystems and compatibility layers; contemporary approaches emphasize running POSIX-using software via environments like Windows Subsystem for Linux rather than native POSIX certification.

POSIX compliance supports portability by providing a consistent interface across platforms, reducing the need for platform-specific code. However, true portability is limited by non-POSIX extensions (GNU, BSD, or vendor-specific features), differences in libraries and toolchains, and varying degrees of conformance. Some vendors also pursue certification under the Single UNIX Specification, which encompasses POSIX requirements as part of its criteria.