Description

   The setuid function can be used with POSIX style identifiers
   enabled or disabled.

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

   With POSIX IDs disabled (the default), the setuid function is
   implemented for program portability and serves no function. It
   returns 0 (to indicate success).

   With POSIX style IDs enabled:

   o  If the process has the IMPERSONATE privilege, the setuid
      function sets the real user ID, effective user ID, and the
      saved set-user-ID to uid.

   o  If the process does not have appropriate privileges but uid
      is equal to the real user ID or to the saved set-user-ID, then
      the setuid function sets the effective user ID to uid. The
      real user ID and saved set-user-ID remain unchanged.

   See also getuid to know how UIC is represented.