switch

/*
 *  call-seq:
 *     Process::UID.switch              => fixnum
 *     Process::UID.switch {|| block}   => object
 *
 *  Switch the effective and real user IDs of the current process. If
 *  a <em>block</em> is given, the user IDs will be switched back
 *  after the block is executed. Returns the new effective user ID if
 *  called without a block, and the return value of the block if one
 *  is given.
 *
 */

static VALUE
p_uid_switch(obj)
    VALUE obj;
{
    int uid, euid;

    check_uid_switch();

    uid = getuid();
    euid = geteuid();

    if (uid != euid) {
        proc_seteuid(obj, INT2FIX(uid));
        if (rb_block_given_p()) {
            under_uid_switch = 1;
            return rb_ensure(rb_yield, Qnil, p_uid_sw_ensure, SAVED_USER_ID);
        } else {
            return INT2FIX(euid);
        }
    } else if (euid != SAVED_USER_ID) {
        proc_seteuid(obj, INT2FIX(SAVED_USER_ID));
        if (rb_block_given_p()) {
            under_uid_switch = 1;
            return rb_ensure(rb_yield, Qnil, p_uid_sw_ensure, euid);
        } else {
            return INT2FIX(uid);
        }
    } else {
        errno = EPERM;
        rb_sys_fail(0);
    }

#else
static VALUE
p_uid_sw_ensure(obj)
    VALUE obj;
{
    under_uid_switch = 0;
    return p_uid_exchange(obj);
}