process.c:1522
static VALUE
p_sys_setrgid(obj, id)
    VALUE obj, id;
{
#if defined HAVE_SETRGID
    rb_secure(2);
    if (under_gid_switch) {
	rb_raise(rb_eRuntimeError, "can't handle GID during evaluating the block given to the Process::GID.switch method");
    }
    if (setrgid(NUM2INT(id)) != 0) rb_sys_fail(0);
#else
    rb_notimplement();
#endif
    return Qnil;
}
