srand

/*
 *  call-seq:
 *     srand(number=0)    => old_seed
 *  
 *  Seeds the pseudorandom number generator to the value of
 *  <i>number</i>.<code>to_i.abs</code>. If <i>number</i> is omitted,
 *  seeds the generator using a combination of the time, the
 *  process id, and a sequence number. (This is also the behavior if
 *  <code>Kernel::rand</code> is called without previously calling
 *  <code>srand</code>, but without the sequence.) By setting the seed
 *  to a known value, scripts can be made deterministic during testing.
 *  The previous seed value is returned. Also see <code>Kernel::rand</code>.
 */

static VALUE
rb_f_srand(argc, argv, obj)
    int argc;
    VALUE *argv;
    VALUE obj;
{
    VALUE seed, old;

    rb_secure(4);
    if (rb_scan_args(argc, argv, "01", &seed) == 0) {
        seed = random_seed();
    }
    old = rand_init(seed);

    return old;
}