New functions are added to the VSI C Run-Time Library with each
    version of VSI C. These functions are implemented and shipped with
    the OpenVMS operating system, while the documentation and header
    files containing their prototypes are shipped with versions of
    the VSI C compiler.

    You might have a newer version of VSI C that has header files
    and documentation for C RTL functions that are not supported on
    your older OpenVMS system. For example, if your target operating
    system platform is OpenVMS Version 7.2, you cannot use C RTL
    functions introduced on OpenVMS Version 7.3, even though they are
    documented in this manual.

    The following tables list what C RTL functions are supported
    on recent OpenVMS versions. This is helpful for determining the
    functions to avoid using on your target OpenVMS platforms.