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

       standards - C and UNIX Standards

       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 release.

       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).

              LFS 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

              This was the first POSIX standard, ratified by IEEE as IEEE Std
              1003.1-1988, and subsequently adopted (with minor revisions) as an
              ISO standard in 1990.  The term "POSIX" was coined by Richard

              "Portable Operating System Interface for Computing Environments".
              IEEE 1003.1-1990 part 1, ratified by ISO in 1990 (ISO/IEC

              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

       POSIX.1c  (formerly known as POSIX.4a)
              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

              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 release of the X/Open
              Portability Guide to be based on a POSIX standard (POSIX.1-1988).
              This multivolume guide was developed by the X/Open Group, a
              multivendor consortium.

       XPG4   A revision of the X/Open Portability Guide, released in 1992.
              This revision incorporated POSIX.2.

       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
              (incorrectly) 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 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

              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

              Two Technical Corrigenda (minor fixes and improvements) of the
              original 2001 standard have occurred: TC1 in 2003 and TC2 in 2004.

       POSIX.1-2008, SUSv4
              Work on the next revision of POSIX.1/SUS was completed and
              ratified in 2008.  The standard is available online at

              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 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

              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.

              Technical Corrigendum 2 of this standard was released in 2016.

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

       SUSv4 2016 edition
              This is equivalent to POSIX.1-2008, with the addition of Technical
              Corrigenda 1 and 2 and the XCurses specification.

              This revision of POSIX is technically identical to POSIX.1-2008
              with Technical Corrigenda 1 and 2 applied.

       SUSv4 2018 edition
              This is equivalent to POSIX.1-2017, with the addition of the
              XCurses specification.

       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".

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

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Linux                              2020-11-01                       STANDARDS(7)