Description

   The setregid function is used to set the real and effective group
   IDs of the calling process. If rgid is -1, the real group ID
   is not changed; if egid is -1, the effective group ID is not
   changed. The real and effective group IDs can be set to different
   values in the same call.

   Only a process with the IMPERSONATE privilege can set the real
   group ID and the effective group ID to any valid value.

   A nonprivileged process can set either the real group ID to the
   saved set-group-ID from an exec function, or the effective group
   ID to the saved set-group-ID or the real group ID.

   Any supplementary group IDs of the calling process remain
   unchanged.

   If a set-group-ID process sets its effective group ID to its real
   group ID, it can still set its effective group ID back to the
   saved set-group-ID.

   This function requires that long (32-bit) UID/GID support be
   enabled. See 32-Bit UID and GID Macro (Integrity servers, Alpha)
   for more information.