Process::GID.switch

static VALUE
p_gid_switch(VALUE obj)
{
    rb_gid_t gid, egid;

    check_gid_switch();

    gid = getgid();
    egid = getegid();

    if (gid != egid) {
        proc_setegid(obj, GIDT2NUM(gid));
        if (rb_block_given_p()) {
            under_gid_switch = 1;
            return rb_ensure(rb_yield, Qnil, p_gid_sw_ensure, SAVED_GROUP_ID);
        }
        else {
            return GIDT2NUM(egid);
        }
    }
    else if (egid != SAVED_GROUP_ID) {
        proc_setegid(obj, GIDT2NUM(SAVED_GROUP_ID));
        if (rb_block_given_p()) {
            under_gid_switch = 1;
            return rb_ensure(rb_yield, Qnil, p_gid_sw_ensure, egid);
        }
        else {
            return GIDT2NUM(gid);
        }
    }
    else {
        rb_syserr_fail(EPERM, 0);
    }

    UNREACHABLE_RETURN(Qnil);
}