atom feed5 messages in org.freebsd.freebsd-amd64amd64/121885: random() system call re...
FromSent OnAttachments
LewisMar 20, 2008 12:39 am 
Roland SmithMar 20, 2008 7:19 am 
LewisMar 20, 2008 11:36 am 
Roland SmithMar 20, 2008 12:49 pm 
jh...@FreeBSD.orgMar 20, 2008 12:59 pm 
Subject:amd64/121885: random() system call returning the same number on amd64
From:Lewis (lew@elasticmind.net)
Date:Mar 20, 2008 11:36:08 am
List:org.freebsd.freebsd-amd64

The following reply was made to PR amd64/121885; it has been noted by GNATS.

From: Lewis <lew@elasticmind.net> To: Roland Smith <rsm@xs4all.nl> Cc: Free@freebsd.org Subject: Re: amd64/121885: random() system call returning the same number on amd64 Date: Thu, 20 Mar 2008 11:03:28 +0000

Hi,

Thanks for looking at this and spotting the problem, so much time was wasted trying to figure out what was wrong :( I guess much of the confusion was caused by the same code working on i386 but not amd64 and is what prompted people to suggest to me writing a PR. Would it have been because amd64 uses different sizes of 'double' than i386 or something like that?

Regards, Lewis.

Roland Smith wrote:

On Wed, Mar 19, 2008 at 10:02:31PM +0000, Lewis wrote:

--- random.c begins here --- #include <stdio.h> #include <strings.h> #include <stdlib.h> #include <time.h> #include <sys/types.h> #include <unistd.h>

int main(int argc, char ** argv) {

srandom(time(0) * getpid()); printf("time: %d\n", time(0)); printf("pid: %d\n", getpid());

double r_num;

This should be 'long random;', see random(3).

With this change it works. Or, see below.

int i, percent = 50; /* Constant 0x7fffffff is equal to (2**31)-1, which is the * maximum value returned by the random() number function. * http://web.mit.edu/answers/c/c_random_numbers.html 2008/03/19 */

for (i = 0; i < 100; i++) { /*r_num = ((float) random() / (float) 0x7fffffff);*/ r_num = random();

if (r_num < (percent / 100)) { printf("random: %u - Dropped packet.\n",r_num); } else { printf("random: %u - Sent packet.\n", r_num);

And if you want to use doubles, you should have used '%f' of '%g' here, instead of '%u'. In which case it works as well.

} } }

/* Sample program output:

(long r_num): time: 1205997021 pid: 59547 random: 1651842516 - Sent packet. random: 658819253 - Sent packet. random: 597332715 - Sent packet. random: 635700682 - Sent packet. random: 996321444 - Sent packet. random: 870878043 - Sent packet. random: 1977054922 - Sent packet. random: 955816479 - Sent packet. random: 367175873 - Sent packet. random: 16441391 - Sent packet. random: 1188837559 - Sent packet. random: 1906346020 - Sent packet. random: 151679052 - Sent packet.

(double r_num, with '%g' in printf) time: 1205997420 pid: 59748 random: 9.87536e+08 - Sent packet. random: 1.5497e+09 - Sent packet. random: 2.13094e+09 - Sent packet. random: 1.08434e+09 - Sent packet. random: 1.94301e+09 - Sent packet. random: 1.34453e+09 - Sent packet. random: 1.77993e+09 - Sent packet. random: 1.10691e+09 - Sent packet. random: 1.95755e+09 - Sent packet. random: 1.70928e+09 - Sent packet. random: 1.66913e+09 - Sent packet. random: 1.90308e+09 - Sent packet.

I think this PR can be closed.

Roland