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.