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

# 2. Standards

This chapter describes each of the standards which RTEMS tracks API alignment with. As a general rules, these standards are related to the POSIX or C programming language standards. Many are the result of domain specific efforts to define subsets or profiles or the full POSIX standard which are suitable for a specific domain. Each API set is considered a “profile” against which the full capability set of RTEMS is evaluated.

The RTEMS Complete Profile is the complete set of POSIX, BSD, and C programming language methods supported by RTEMS. This profile is independent of any standard and represents a union of multiple standards. For example, RTEMS supports BSD derived methods that are not in POSIX.

The IEEE Standard 1003.1 is the POSIX standard. Specifically, IEEE Standard 1003.1-2008 is the 2003 edition of the POSIX standard and IEEE Standard 1003.1-2008 is the 2008 edition. The 2008 is is an update from the 2003 edition. Each edition of the POSIX standard tends to add some methods, deprecate some methods, and obsolete (e.g. remove) other methods.

PSE51 through PSE54 are Open Group defined profiles of the 2003 edition of the POSIX standard. These profiles are:

- Profile 54 - Multipurpose

  - 1003.1-2003 Base Multi-process, Threads and File System

- Profile 53 - Dedicated

  - Multi-process, Threads and File System

- Profile 52 - Controller

  - Single Process, Threads, and File System

- Profile 51 - Minimal

  - Single Process, Threads, with No File System

The C99 Programming Language standard defines the Standard C Library. This library is largely included by reference in the POSIX standard.

The C11 Programming Language standard defines also defines an updated version of the Standard C Library. It deletes a few methods from the C99 version but adds many methods. A large portion of these methods are optional and not commonly implemented.

The Open Group FACE Consortium (https://www.opengroup.org/face) has defined four POSIX profiles targetting the avionics application domain. The FACE Technical Standard has been through multiple revisions and the POSIX API profiles are identical in Editions 1.0, 2.0, 2.1, and 2.1.1. In these editions, the profiles are as follows:

- Security - 163 APIs, single process, no FILE *

- Safety Basic - 246 APIs, single process, some FILE *

- Safety Extended - 335 APIs, multi-process, more FILE *

- General Purpose - 812 APIs, multi-process, much more

FACE Technical Standard Edition 3.0 adds the requirement for an operating system to support `clock_nanosleep()` in all profiles and defines one additional subcommand for the `posix_devctl()` methods.

RTEMS provides all of the methods required by the FACE Safety BASE profile and all of the methods in the Safety Extended profile which do not require multiple processes. Similarly, RTEMS provides most of the methods in the General Purpose profile which do not require multiple processes.

The Software Communications Architecture (SCA) specification targets the requirements for software-defined radios. This specification was originally developed in support of the Joint Tactical Radio System (JTRS) program in conjunction with the Object Management Group (OMG). This standard is now maintained by the Wireless Innovation Forum with support from the U.S. Navy Joint Tactical Network Center (JTNC). Some URLs of interest:

- SCA at Wireless Innovation Forum - http://www.wirelessinnovation.org/sca-based-standards-library

- JTRS - https://en.wikipedia.org/wiki/Joint_Tactical_Radio_System

- JTNC - http://www.public.navy.mil/jtnc/Pages/home.aspx

The SCA standard is hosted at the Wireless Innovation Forum with JTNC hosting supplemental information.

RTEMS includes all methods required by the SCA POSIX profiles.