The  CONFORMING TO section that appears in many manual pages identifies
       various standards to which the documented interface conforms.  The fol-
       lowing 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 ver-
              sion 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 consid-
              ered the definitive System V release.

       SVID 4 System V Interface Definition version 4, issued in 1995.  Avail-
              able online at <>.

       C89    This was the first C language standard, ratified by ANSI (Ameri-
              can National Standards Institute) in 1989 (X3.159-1989).   Some-
              times  this  is  known  as ANSI C, but since C99 is also an ANSI
              standard, this term is ambiguous.  This standard was also  rati-
              fied  by  ISO  (International  Standards  Organization)  in 1990
              (ISO/IEC 9899:1990), and is thus occasionally referred to as ISO
              (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  por-
              table  operating  systems,  ratified  by  ISO  in  1996 (ISO/IEC

              IEEE Std 1003.1c-1995, which describes the POSIX threads  inter-

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

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

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

              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 mul-
              tivendor  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

       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 conforming to
              this    standard   can   be   branded   UNIX   98.    See   also

              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 (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  (referred  to
              as POSIX.1-2003), and TC2 in 2004 (referred to as POSIX.1-2004).

       POSIX.1-2008, SUSv4
              Work on the next revision of POSIX.1/SUS was completed and rati-
              fied 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 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  stan-
              dard.   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  speci-

              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 (referred to as POSIX.1-2013).

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