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, the ancestral UNIX from Bell Labs.

       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

              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 porta-
              ble  operating  systems,  ratified  by  ISO  in  1996   (ISO/IEC

              IEEE Std 1003.1c-1995 describing the POSIX threads interfaces.

              IEEE  Std  1003.1c-1999  describing  additional real-time exten-

              IEEE Std  1003.1g-2000  describing  networking  APIs  (including

              IEEE Std 1003.1j-2000 describing advanced real-time extensions.

              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

       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 <
              /austin/>.    The    standard    is    available    online    at
              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 Corrigenda 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,

       feature_test_macros(7), libc(7), posixoptions(7)

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