The CONFORMING TO 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.
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.
Earlier major BSD releases included 3BSD (1980), 4BSD
(1980), and 4.1BSD (1981).
The successor to 4.2BSD, released in 1986.
The successor to 4.3BSD, released in 1993. This was the last major
This is an implementation standard defined by AT&T's milestone 1983
release of its commercial System V (five) release. The previous major
AT&T release was System III, released in 1981.
System V release 2 (SVr2)
This was the next System V release, made in 1985. The SVr2 was formally
described in the System V Interface Definition version 1 (SVID
1) published in 1985.
System V release 3 (SVr3)
This was the successor to SVr2, released in 1986. This release was
formally described in the System V Interface Definition version 2
System V release 4 (SVr4)
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
version 3 (SVID 3), and is considered the definitive System V
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. This standard was also ratified by ISO (International
Standards Organization) in 1990 (ISO/IEC 9899:1990), and is thus
occasionally referred to as ISO C90.
This revision of the C language standard was ratified by ISO in 2011
"Portable Operating System Interface for Computing
Environments". IEEE 1003.1-1990 part 1, ratified by ISO in 1990
(ISO/IEC 9945-1:1990). The term "POSIX" was coined by
IEEE Std 1003.2-1992, describing commands and utilities, ratified by ISO
in 1993 (ISO/IEC 9945-2:1993).
POSIX.1b (formerly known as POSIX.4)
IEEE Std 1003.1b-1993, describing real-time facilities for portable
operating systems, ratified by ISO in 1996 (ISO/IEC
IEEE Std 1003.1c-1995, which describes the POSIX threads interfaces.
IEEE Std 1003.1c-1999, which describes additional real-time
IEEE Std 1003.1g-2000, which describes networking APIs (including
IEEE Std 1003.1j-2000, which describes advanced real-time extensions.
A 1996 revision of POSIX.1 which incorporated POSIX.1b and POSIX.1c.
Released in 1989, this was the first significant release of the X/Open
Portability Guide, produced by the X/Open Company, a multivendor
consortium. This multivolume guide was based on the POSIX standards.
A revision of the X/Open Portability Guide, released in 1992.
A 1994 revision of XPG4. This is also referred to as Spec 1170,
where 1170 referred to the number of interfaces defined by this
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
Single UNIX Specification version 2. Sometimes also referred to as
XPG5. This standard appeared in 1997. Systems conforming to this
standard can be branded UNIX 98. See also
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
The standard is available online at
and the interfaces that it describes are also available in the Linux
manual pages package under sections 1p and 3p (e.g., "man 3p
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. (XSI
conformance constitutes the Single UNIX Specification version 3
The POSIX.1-2001 document is broken into four parts:
XBD: Definitions, terms and concepts, header file
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.
Two Technical Corrigenda (minor fixes and improvements) of the original
2001 standard have occurred: TC1 in 2003 (also known as
POSIX.1-2003), and TC2 in 2004 (also known as
Work on the next revision of POSIX.1/SUS was completed and ratified in
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
The revised standard is broken into the same four parts as POSIX.1-2001,
and 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 CONFORMING TO 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.
Technical Corrigendum 1 (minor fixes and improvements) of this standard
was released in 2013 (also known as POSIX.1-2013).
Technical Corrigendum 2 of this standard was released in 2016 (also known
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