VMS Help  —  CRTL  Feature-Test Macros, Selecting a Standard
    You can define a feature-test macro to select each standard. You
    can do this either with a #define preprocessor directive in your
    C source before the inclusion of any header file, or with the
    /DEFINE qualifier on the CC command line.

    Features not defined by one of the previously named standards are
    considered VSI C extensions and are selected by not defining
    any standards-related, feature-test macros.

    If you do not explicitly define feature test macros to control
    header file definitions, you implicitly include all defined
    symbols as well as VSI C extensions.
Additional Information: explode extract

XOPEN SOURCE EXTENDED XOPEN SOURCE
POSIX C SOURCE==199506 POSIX C SOURCE==2 POSIX C SOURCE==1
STDC VERSION ==199409
ANSI C SOURCE
Interactions with the /STANDARD Qualifier
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