Bug in rand() and srand()

[Tim Smith]

In article <320 at cubsvax.UUCP> peters at cubsvax.UUCP (Peter S. Shenkin) writes:
>In attempting to use these functions under 4.1bsd on a VAX/780, I found that
>rand() returns strictly alternating even and odd numbers, regardless of
>seed!!!  So I went and tried the same thing on a 4.2bsd system, and found
>the same behavior.  
>

If BSD rand() is at all like System V rand(), this is not a bug.  Rand()
is a multiplicitive congruential (sp?) random number generator, which means
it works like this:

        R    = A * R  + C  mod M
         i+1        i

So, depending on the parity of A and C, you have one of the following cases:

	Ri+1 is always odd
	Ri+1 is always even
	Ri+1 is always same parity as Ri
or	Ri+1 is always opposite parity of Ri

In this sort of number generator, the high bits are "more random" than the
low bits.  Thus, for example, to get a stream or random bits, one would
look at the top bit ( assuming M is a power of two... ), not the bottom bit.

Look at Knuth, Vol. II for almost all that is known on this topic.
-- 
Duty Now for the Future
					Tim Smith
			ihnp4!wlbr!callan!tim or ihnp4!cithep!tim