From owner-freebsd-questions  Mon Nov 20 08:18:16 1995
Return-Path: owner-questions
Received: (from root@localhost)
          by freefall.freebsd.org (8.6.12/8.6.6) id IAA16340
          for questions-outgoing; Mon, 20 Nov 1995 08:18:16 -0800
Received: from abraxas.wustl.edu (abraxas.wustl.edu [128.252.227.108])
          by freefall.freebsd.org (8.6.12/8.6.6) with ESMTP id IAA16335
          for <freebsd-questions@freebsd.org>; Mon, 20 Nov 1995 08:18:10 -0800
Received: (from matt@localhost) by abraxas.wustl.edu (8.6.11/8.6.9) id KAA17976; Mon, 20 Nov 1995 10:20:36 GMT
Date: Mon, 20 Nov 1995 10:20:36 +0000 ()
From: Matt Massie <matt@abraxas.wustl.edu>
To: Frank Volf <volf@oasis.IAEhv.nl>
cc: freebsd-questions@freebsd.org
Subject: Re: What's wrong with this code???
In-Reply-To: <199511191351.OAA04737@oasis.IAEhv.nl>
Message-ID: <Pine.BSF.3.91.951120101824.17617T-100000@abraxas.wustl.edu>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: owner-questions@freebsd.org
Precedence: bulk

I really do not like srand() or rand().  If you want to really see what 
is going on.. make your own pseudo-random number generator like the one 
that follows:

/* If you pass it zero, if will seed itself or you can send it time(NULL)*/
float rnd_unit(long int seed)
{
        static long int num = 85784;
        long int div = 53159, mult = 18327;
        float result;
        if (seed) num = seed;

        num *= mult;    
        num = num % div;
        result = (float)num / div;
        return result;
}


---
Matt Massie		 
matt@abraxas.wustl.edu

On Sun, 19 Nov 1995, Frank Volf wrote:

> 
> Hi,
> 
> I have an application that uses random() to generate random numbers. For
> generating the random numbers I use the following piece of code:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/time.h>
> #include <unistd.h>
> 
> main()
> {
>   int i;
>   long t;
> 
>   t = time(NULL);
>   srandom((u_int)t);
>   printf("%ld %d %d %d\n",t, random() % 8, random() % 8, random() % 8); 
> }
> 
> To my surprise the range of numbers generated is not random at all between
> multiple invocations of the program. In fact the random generator seems
> to start again after 16 invocations (with 1 second delay between them
> so t has incremented):
> 
> 816788556 6 1 1
> 816788557 5 6 3
> 816788558 5 3 6
> 816788559 4 0 0
> 816788560 4 5 3
> 816788561 3 2 5
> 816788563 2 4 2
> 816788564 2 1 5
> 816788565 1 6 7
> 816788566 1 3 2
> 816788567 0 0 4
> 816788568 0 5 7
> 816788569 7 2 1
> 816788570 7 7 4
> 816788571 6 4 6
> 
> 816788572 6 1 1
> 816788573 5 6 3
> 816788574 5 3 6
> 816788575 4 0 0
> 
> etc.
> 
> What is wrong? Is it a bug in my program (maybe the cast?) or is there a
> problem with the random number generator used in FreeBSD?
> 
> Regards,
> 
>             Frank
> 
>  ----------------------------------------------------------------------------
>        Frank Volf - Internet Access Eindhoven - Digitale Stad Eindhoven 
>  ----------------------------------------------------------------------------
>   ||  volf@oasis.IAEhv.nl - use for personal mail                         ||
>   ||  volf@IAEhv.nl - use for Internet Access Eindhoven related mail      ||
>   ||  volf@dse.dse.nl - use for Digital City of Eindhoven related mail    ||
>  ----------------------------------------------------------------------------
>     IAE Public Access Unix System - Dial +31.40.2439436 and login as new.  
>  ----------------------------------------------------------------------------
>