1. Preface — RTEMS POSIX 1003.1 Compliance Guide 5.4-rc2 (21st August 2026) documentation

# 1. Preface

RTEMS supports a variety of POSIX and BSD features including some POSIX methods that are now deemed obsolete and some methods for compatibility with GNU/Linux and FreeBSD. There are multiple POSIX standard versions as well as multiple efforts to tailor (e.g. profile) POSIX for embedded environments. They range in size from less than 200 required capabilities to the full POSIX standard which has over 1200 required capabilities. This document reports on the alignment of RTEMS with various standard versions and defined profiles.

RTEMS supports a number of POSIX process, user, and group oriented routines in what is referred to as a “SUSP” (Single-User, Single Process) manner. RTEMS supports a single process, multithreaded POSIX environment. In a pure world, there would be no reason to even include routines like `getpid()` when there can only be one process. But providing routines like `getpid()` and making them work in a sensible fashion for an embedded environment while not returning `ENOSYS` (for not implemented) makes it significantly easier to port code from a UNIX environment without modifying it.

In general, adding missing methods is always an open project for a volunteer. If considering addressing missing methods, please discuss this on mailing list. Some are properly implemented in the Newlib C Standard Library used by RTEMS. Others may require target architecture specific implementations. Still others may be impossible to implement without multiple processes or can only be implemented in a restricted fashion.

Missing methods required by the C99 standard or FACE Technical Standard Edition 3.0 General Purpose Profile are good candidates to add. Proposals to add missing methods from the C11 standard should be reviewed by RTEMS core developers to ensure the effort is well spent. There are rumors that some optional methods that are not being widely implemented will be removed in a future versino of the C Programming Language standard.

The next chapter in this document describes each of the standards with which the RTEMS alignment is tracked. Each subsequent chapter in this document presents the alignment of RTEMS with a specific standard version or defined profile. Each section with a chapter details the alignment of a specific header file relative to the chapter’s standard or profile. The implementation status of the items required by the standard are listed.