VMS Help  —  CRTL  Feature-Test Macros
    Feature-test macros provide a means for writing portable
    programs. They ensure that the C RTL symbolic names used
    by a program do not clash with the symbolic names supplied by the
    implementation.

    The C RTL header files are coded to support the use of
    a number of feature-test macros. When an application defines a
    feature-test macro, the C RTL header files supply the
    symbols and prototypes defined by that feature-test macro and
    nothing else. If a program does not define such a macro, the
    C RTL header files define symbols without restriction.

    The feature-test macros supported by the C RTL fall into
    the following broad categories for controlling the visibility of
    symbols in header files according to the following:

    o  Standards

    o  Multiple-version support

    o  Compatibility
Additional Information: explode extract
Standards Macros Selecting a Standard Multiple-Version-Support Macro Compatibility Modes Curses and Socket Compatibility Macros
2 GB File Size Macro
32-Bit UID and GID Macro (Integrity servers, Alpha)
Standard-Compliant stat Structure (Integrity servers, Alpha)
Using Legacy toupper and tolower Behavior (Integrity servers, Alpha) Using Faster, Inlined Put and Get Functions (Integrity servers, Alpha)
POSIX Style exit (Integrity servers, Alpha)
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