amd64/121885: random() system call returning the same number on amd64

[Lewis]

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

From: Lewis <lewis at elasticmind.net>
To: Roland Smith <rsmith at xs4all.nl>
Cc: FreeBSD-gnats-submit at 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
 >