POSIX4

POSIX4 is the designation historically used for the Real-Time Extensions to the POSIX family of standards. It was defined by the IEEE as part of the 1003 standards and published under the IEEE Std 1003.4 in the 1990s. The purpose of POSIX4 was to provide portable interfaces for developing real-time and embedded software that requires predictable timing and bounded latency. The specification covers real-time process scheduling, high-resolution clocks and timers, real-time signals, and interprocess communication primitives such as message queues, semaphores, and shared memory, along with related facilities like memory locking and priority inheritance for priority-based scheduling.

In practice, POSIX4 was intended to complement the base POSIX specifications—POSIX.1 (base) and POSIX.2 (shell and utilities)—and its real-time features were gradually adopted in various forms across different operating systems. Over time, many real-time aspects were integrated into broader POSIX revisions or implemented through companion standards, such as POSIX.1b, rather than maintained as a separate, standalone standard.

Today the term POSIX4 is largely historical, with the real-time functionality it described commonly available in contemporary POSIX implementations. The influence of POSIX4 remains visible in how modern Unix-like systems expose real-time APIs, including timing, scheduling, and IPC mechanisms, through the continued evolution of POSIX.1 and related standards. Adopters typically reference the real-time extensions within POSIX rather than a separate POSIX4 document.