Interactions_with_the_/STANDARD_Qualifier

   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 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 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.