The /STANDARD qualifier selects the dialect of the C language
    supported.

    With the exception of /STANDARD=ANSI89 and /STANDARD=ISOC94, the
    selection of C dialect and the selection of Compaq C RTL APIs to
    use are independent choices. All other values for /STANDARD cause
    the entire set of APIs to be available, including extensions.

    Specifying /STANDARD=ANSI89 restricts the default API set to the
    ANSI C set. In this case, to select a broader set of APIs, you
    must also specify the appropriate feature-test macro. To select
    the ANSI C dialect and all APIs, including extensions, undefine
    __HIDE_FORBIDDEN_NAMES before including any header file.

    Compiling with /STANDARD=ISOC94 sets __STDC_VERSION__  to
    199409. Conflicts that arise when compiling with both XPG4 and
    ISO C Amendment 1 resolve in favor of ISO C Amendment 1. XPG4
    extensions to ISO C Amendment 1 are selected by defining _XOPEN_
    SOURCE.

    The following examples help clarify these rules:

    o  The fdopen function is an ISO POSIX-1 extension to <stdio.h>.
       Therefore, <stdio.h> defines fdopen only if one or more of the
       following is true:

       -  The program including it is not compiled in strict ANSI C
          mode (/STANDARD=ANSI89).

       -  _POSIX_C_SOURCE is defined as 1 or greater.

       -  _XOPEN_SOURCE is defined.

       -  _XOPEN_SOURCE_EXTENDED is defined.

    o  The popen function is an ISO POSIX-2 extension to <stdio.h>.
       Therefore, <stdio.h> defines popen only if one or more of the
       following is true:

       -  The program including it is not compiled in strict ANSI C
          mode (/STANDARD=ANSI89).

       -  _POSIX_C_SOURCE is defined as 2 or greater.

       -  _XOPEN_SOURCE is defined.

       -  _XOPEN_SOURCE_EXTENDED is defined.

    o  The getw function is an X/Open extension to <stdio.h>.
       Therefore, <stdio.h> defines getw only if one or more of the
       following is true:

       -  The program is not compiled in strict ANSI C mode
          (/STANDARD=ANSI89).

       -  _XOPEN_SOURCE is defined.

       -  _XOPEN_SOURCE_EXTENDED is defined.

    o  The X/Open Extended symbolic constants _SC_PAGESIZE,
       _SC_PAGE_SIZE, _SC_ATEXIT_MAX, and _SC_IOV_MAX were added
       to <unistd.h> to support the sysconf function. However, these
       constants are not defined by _POSIX_C_SOURCE.

       The <unistd.h> header file defines these constants only if
       a program does not define _POSIX_C_SOURCE and does define
       _XOPEN_SOURCE_EXTENDED.

       If _POSIX_C_SOURCE is defined, these constants are not visible
       in <unistd.h>. Note that _POSIX_C_SOURCE is defined only for
       programs compiled in strict ANSI C mode.

    o  The fgetname function is a Compaq C RTL extension to
       <stdio.h>. Therefore, <stdio.h> defines fgetname only
       if the program is not compiled in strict ANSI C mode
       (/STANDARD=ANSI89).

    o  The macro _PTHREAD_KEYS_MAX is defined by POSIX 1003.1c-1995.
       This macro is made visible in <limits.h> when compiling for
       this standard with _POSIX_C_SOURCE == 199506 defined, or
       by default when compiling without any standards-defining,
       feature-test macros.

    o  The macro WCHAR_MAX defined in <wchar.h> is required by ISO C
       Amendment 1 but not by XPG4. Therefore:

       -  Compiling for ISO C Amendment 1 makes this symbol visible,
          but compiling for XPG4 compliance does not.

       -  Compiling for both ISO C Amendment 1 and XPG4 makes this
          symbol visible.

       Similarly, the functions wcsftime and wcstok in <wchar.h>
       are defined slightly differently by the ISO C Amendment 1 and
       XPG4:

       -  Compiling for ISO C Amendment 1 makes the ISO C Amendment 1
          prototypes visible.

       -  Compiling for XPG4 compliance makes the XPG4 prototypes
          visible.

       -  Compiling for both ISO C Amendment 1 and XPG4 selects the
          ISO C prototypes because conflicts resulting from this mode
          of compilation resolve in favor of ISO C.

       -  Compiling without any standard selecting feature test
          macros makes ISO C Amendment 1 features visible.

       In this example, compiling with no standard-selecting
       feature-test macros makes WCHAR_MAX and the ISO C Amendment 1
       prototypes for wcsftime and wcstok visible.

    o  The wcswidth and wcwidth functions are XPG4 extensions to
       ISO C Amendment 1. Their prototypes are in <wchar.h>.

       These symbols are visible if:

       -  Compiling for XPG4 compliance by defining _XOPEN_SOURCE or
          _XOPEN_SOURCE_EXTENDED.

       -  Compiling for DEC C Version 4.0 compatibility or on pre-
          OpenVMS Version 7.0 systems.

       -  Compiling with no standard-selecting feature-test macros.

       -  Compiling for both ISO C Amendment 1 and XPG4 compilance
          because these symbols are XPG4 extensions to ISO C
          Amendment 1.

       Compiling for strict ISO C Amendment 1 does not make them
       visible.