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standards

C and UNIX Standards

The STANDARDS section that appears in many manual pages identifies various standards to which the documented interface conforms. The following list briefly describes these standards.

Version 7 (also known as Seventh Edition) UNIX, released by AT&T/Bell Labs in 1979. After this point, UNIX systems diverged into two main dialects: BSD and System V. Sources are available online.

4.2BSD
This is an implementation standard defined by the 4.2 release of the Berkeley Software Distribution, released by the University of California at Berkeley. This was the first Berkeley release that contained a TCP/IP stack and the sockets API. 4.2BSD was released in 1983. Sources are available online.
Earlier major BSD releases included 3BSD (1980), 4BSD (1980), and 4.1BSD (1981).
4.3BSD
The successor to 4.2BSD, released in 1986. Sources are available online.
4.4BSD
The successor to 4.3BSD, released in 1993. This was the last major Berkeley release. Sources are available online.

Unix/TS 4
This was an internal release of Unix that was never released to the public (in which case it would have been called System IV). The previous major AT&T release was System III, released in 1981.
This is an implementation standard defined by AT&T's milestone 1983 release of its commercial System V (five) release.
This was the next System V release, made in 1985. The SVr2 was formally described in the System V Interface Definition Issue 1 (SVID 1) published in 1985.
This was the successor to SVr2, released in 1986. This release was formally described in the System V Interface Definition Issue 2 (SVID 2).
This was the successor to SVr3, released in 1989. This version of System V is described in the "Programmer's Reference Manual: Operating System API (Intel processors)" (Prentice-Hall 1992, ISBN 0-13-951294-2) This release was formally described in the System V Interface Definition Third Edition (SVID 3), and is considered the definitive System V release.
System V Interface Definition version 4, issued in 1995. Available online at http://www.sco.com/developers/devspecs/.

This refers to the C language as specified in “The C Programming Language” [Brian W. Kernighan and Dennis M. Ritchie; Prentice Hall; 1st Ed., 1978].
This was the first C language standard, ratified by ANSI (American National Standards Institute) in 1989 (X3.159-1989). Sometimes this is known as ANSI C, but since C99 is also an ANSI standard, this term is ambiguous. It is available online.
The rationale for the C89 standard is also available online.
This standard was also ratified by ISO (International Standards Organization) and IEC (International Electrotechnical Commision) in 1990 (ISO/IEC 9899:1990), and is thus occasionally referred to as ISO C90.
Technical Corrigendum 1 of C90 was ratified by ISO/IEC in 1994 (ISO/IEC 9899:1990/Cor1:1994).
This amendment of the C language standard was ratified by ISO/IEC in 1995 (ISO/IEC 9899:1990/Amd1:1995). A draft is available online.
Technical Corrigendum 2 of C90 was ratified by ISO/IEC in 1996 (ISO/IEC 9899:1990/Cor2:1996). A draft is available online.
This revision of the C language standard was ratified by ISO/IEC in 1999 (ISO/IEC 9899:1999).
Technical Corrigendum 1 of C99 was ratified by ISO/IEC in 2001 (ISO/IEC 9899:1999/Cor1:2001).
Technical Corrigendum 2 of C99 was ratified by ISO/IEC in 2004 (ISO/IEC 9899:1999/Cor2:2004).
Technical Corrigendum 3 of C99 was ratified by ISO/IEC in 2007 (ISO/IEC 9899:1999/Cor3:2007). A draft is available online.
The rationale for the C99 standard is also available online.
This revision of the C language standard was ratified by ISO/IEC in 2011 (ISO/IEC 9899:2011). A draft is available online.
Technical Corrigendum 1 of C11 was ratified by ISO/IEC in 2012 (ISO/IEC 9899:2011/Cor1:2012).
This revision of the C language standard was ratified by ISO/IEC in 2018 (ISO/IEC 9899:2018). A draft is available online.
This revision of the C language standard was ratified by ISO/IEC in 2024 (ISO/IEC 9899:2024). A draft is available online.

This was the first POSIX standard, ratified by IEEE in 1998 (IEEE Std 1003.1-1988). The term "POSIX" was coined by Richard Stallman. It is available online.
Part 1, "Portable Operating System Interface for Computing Environments". Ratified by IEEE in 1990, (IEEE Std 1003.1-1990), and ratified by ISO/IEC in 1990 (ISO/IEC 9945-1:1990). It is available online.
Part 2, which describes commands and utilities. Ratified by IEEE in 1992 (IEEE Std 1003.2-1992), and ratified by ISO/IEC in 1993 (ISO/IEC 9945-2:1993).
Part 1b, which describes real-time facilities for portable operating systems. Ratified by IEEE in 1993 (IEEE Std 1003.1b-1993).
Part 1c, which describes the POSIX threads interfaces. Ratified by IEEE in 1995 (IEEE Std 1003.1c-1995).
Part 1d, which describes additional real-time extensions. Ratified by IEEE in 1999 (IEEE Std 1003.1d-1999).
Part 1g, which describes networking APIs (including sockets). Ratified by IEEE in 2000 (IEEE Std 1003.1g-2000).
Part 1j, which describes advanced real-time extensions. Ratified by IEEE in 2000 (IEEE Std 1003.1j-2000).
A revision of POSIX.1 which incorporated POSIX.1b and POSIX.1c. Ratified by ISO/IEC in 1996 (ISO/IEC 9945-1:1996).
Released in 1989, this was the first release of the X/Open Portability Guide to be based on a POSIX standard (POSIX.1-1988). It is also known as Issue 3. This multivolume guide was developed by the X/Open Group, a multivendor consortium.
A revision of the X/Open Portability Guide, released in 1992. It is also known as Issue 4. This revision incorporated POSIX.2.
A 1994 revision of XPG4. This is also referred to as Spec 1170, where 1170 referred to the number of interfaces defined by this standard.
Single UNIX Specification. This was a repackaging of XPG4v2 and other X/Open standards (X/Open Curses Issue 4 version 2, X/Open Networking Service (XNS) Issue 4). Systems conforming to this standard can be branded UNIX 95.
Single UNIX Specification version 2. Sometimes also referred to as Issue 5. This standard appeared in 1997. Systems conforming to this standard can be branded UNIX 98. See also http://www.unix.org/version2/.)
This was a 2001 revision and consolidation of the POSIX.1, POSIX.2, and SUS standards into a single document, conducted under the auspices of the Austin Group http://www.opengroup.org/austin/. The standard is available online at http://www.unix.org/version3/.
The standard defines two levels of conformance: POSIX conformance, which is a baseline set of interfaces required of a conforming system; and XSI Conformance, which additionally mandates a set of interfaces (the "XSI extension") which are only optional for POSIX conformance. XSI-conformant systems can be branded UNIX 03.
The POSIX.1-2001 document is broken into four parts:
XBD: Definitions, terms, and concepts, header file specifications.
XSH: Specifications of functions (i.e., system calls and library functions in actual implementations).
XCU: Specifications of commands and utilities (i.e., the area formerly described by POSIX.2).
XRAT: Informative text on the other parts of the standard.
POSIX.1-2001 is aligned with C99, so that all of the library functions standardized in C99 are also standardized in POSIX.1-2001.
The Single UNIX Specification version 3 (SUSv3) comprises the Base Specifications containing XBD, XSH, XCU, and XRAT as above, plus X/Open Curses Issue 4 version 2 as an extra volume that is not in POSIX.1-2001.
POSIX.1-2001 2002 edition
Technical Corrigendum 1 of POSIX.1-2001 was ratified by IEEE in 2002 (IEEE Std 1003.1-2001/Cor1-2002).
This is informally known as POSIX.1-2002.
POSIX.1-2001 2004 edition
Technical Corrigendum 2 of POSIX.1-2001 was ratified by IEEE in 2004 (IEEE Std 1003.1-2001/Cor2-2004). It is available online.
This is informally known as POSIX.1-2004.
Work on the next revision of POSIX.1/SUS was completed and ratified in 2008. The standard is available online at http://www.unix.org/version4/, and also here.
The changes in this revision are not as large as those that occurred for POSIX.1-2001/SUSv3, but a number of new interfaces are added and various details of existing specifications are modified. Many of the interfaces that were optional in POSIX.1-2001 become mandatory in the 2008 revision of the standard. A few interfaces that are present in POSIX.1-2001 are marked as obsolete in POSIX.1-2008, or removed from the standard altogether.
The revised standard is structured in the same way as its predecessor. The Single UNIX Specification version 4 (SUSv4) comprises the Base Specifications containing XBD, XSH, XCU, and XRAT, plus X/Open Curses Issue 7 as an extra volume that is not in POSIX.1-2008.
Again there are two levels of conformance: the baseline POSIX Conformance, and XSI Conformance, which mandates an additional set of interfaces beyond those in the base specification.
In general, where the STANDARDS section of a manual page lists POSIX.1-2001, it can be assumed that the interface also conforms to POSIX.1-2008, unless otherwise noted.
Further information can be found on the Austin Group web site, http://www.opengroup.org/austin/.
POSIX.1-2008 2013 edition
Technical Corrigendum 1 of POSIX.1-2008 was ratified by IEEE in 2013 (IEEE Std 1003.1-2008/Cor1-2013). It is available online.
This is informally known as POSIX.1-2013.
POSIX.1-2008 2016 edition
Technical Corrigendum 2 of POSIX.1-2008 was ratified by IEEE in 2016 (IEEE Std 1003.1-2008/Cor2-2016). It is available online.
SUSv4 2016 edition
This is equivalent to POSIX.1-2008 2016 edition, with the addition of the XCurses specification.
This revision of POSIX is technically identical to POSIX.1-2008 2016 edition. Ratified by IEEE in 2017 (IEEE Std 1003.1-2017). It is available online.
SUSv4 2018 edition
This is equivalent to POSIX.1-2017, with the addition of the XCurses specification.
Work on the next revision of POSIX.1/SUS was completed and ratified by IEEE in 2024 (IEEE Std 1003.1-2024). It is available online.
POSIX.1-2024 is aligned with C17, so that all of the library functions standardized in C17 are also standardized in POSIX.1-2024.
Further information can be found on the Austin Group web site.

The interfaces documented in POSIX.1/SUS are available as manual pages under sections 0p (header files), 1p (commands), and 3p (functions); thus one can write man 3p open.

The Large File Summit specification, completed in 1996. This specification defined mechanisms that allowed 32-bit systems to support the use of large files (i.e., 64-bit file offsets). See https://www.opengroup.org/platform/lfs.html.

getconf(1), confstr(3), pathconf(3), sysconf(3), attributes(7), feature_test_macros(7), libc(7), posixoptions(7), system_data_types(7)