The getgid function can be used with POSIX style identifiers or
    with UIC-based identifiers.

    POSIX style IDs are supported on OpenVMS Version 7.3-2 and
    higher.

    With POSIX style IDs disabled (the default), the getegid and
    getgid functions are equivalent and return the group number from
    the current UIC. For example, if the UIC is [313,031], 313 is the
    group number.

    With POSIX style IDs enabled, getegid returns the effective group
    ID of the calling process, and getgid returns the real group ID
    of the calling process. The real group ID is specified at login
    time. The effective group ID is more transient, and determines
    additional access permission during execution of a set-group-ID
    process. It is for such processes that the getgid function is
    most useful.

    See also geteuid and getuid.