Description

   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.