STANDARDS(7)              Linux Programmer's Manual             STANDARDS(7)

NAME         top

       standards - C and UNIX Standards

DESCRIPTION         top

       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.

       V7     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.

       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.

              Earlier major BSD releases included 3BSD (1980), 4BSD (1980),
              and 4.1BSD (1981).

       4.3BSD The successor to 4.2BSD, released in 1986.

       4.4BSD The successor to 4.3BSD, released in 1993.  This was the last
              major Berkeley release.

       System V
              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 (SVID 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

       SVID 4 System V Interface Definition version 4, issued in 1995.
              Available online at ⟨⟩.

       C89    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.

       C99    This revision of the C language standard was ratified by ISO
              in 1999 (ISO/IEC 9899:1999).  Available online at 

       C11    This revision of the C language standard was ratified by ISO
              in 2011 (ISO/IEC 9899:2011).

              "Portable Operating System Interface for Computing Environ‐
              ments".  IEEE 1003.1-1990 part 1, ratified by ISO in 1990
              (ISO/IEC 9945-1:1990).  The term "POSIX" was coined by Richard

              IEEE Std 1003.2-1992, describing commands and utilities, rati‐
              fied 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

              IEEE Std 1003.1c-1999, which describes additional real-time

              IEEE Std 1003.1g-2000, which describes networking APIs
              (including sockets).

              IEEE Std 1003.1j-2000, which describes advanced real-time

              A 1996 revision of POSIX.1 which incorporated POSIX.1b and

       XPG3   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.

       XPG4   A revision of the X/Open Portability Guide, released in 1992.

       XPG4v2 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.

       SUS (SUSv1)
              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.

       SUSv2  Single UNIX Specification version 2.  Sometimes also referred
              to as XPG5.  This standard appeared in 1997.  Systems conform‐
              ing to this standard can be branded UNIX 98.  See also 

       POSIX.1-2001, SUSv3
              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 
              ⟨⟩.  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 confor‐
              mance, 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 Sin‐
              gle UNIX Specification version 3 (SUSv3).)

              The POSIX.1-2001 document is broken into four parts:

              XBD: Definitions, terms and concepts, header file specifica‐

              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

              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

       POSIX.1-2008, SUSv4
              Work on the next revision of POSIX.1/SUS was completed and
              ratified in 2008.

              The changes in this revision are not as large as those that
              occurred for POSIX.1-2001/SUSv3, but a number of new inter‐
              faces 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 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 as POSIX.1-2016).

              Further information can be found on the Austin Group web site,

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.02 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2017-11-26                     STANDARDS(7)

Pages that refer to this page: intro(1)intro(2)intro(3)matherr(3)intro(4)intro(5)feature_test_macros(7)intro(7)libc(7)man-pages(7)nptl(7)posixoptions(7)signal-safety(7)