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Bug #42465 rand() and mt_rand() return same values from forked processes
Submitted: 2007-08-28 19:31 UTC Modified: 2007-08-29 08:59 UTC
Votes:2
Avg. Score:5.0 ± 0.0
Reproduced:2 of 2 (100.0%)
Same Version:2 (100.0%)
Same OS:2 (100.0%)
From: bugs at nazarenko dot net Assigned:
Status: Not a bug Package: Math related
PHP Version: 5CVS-2007-08-28 (snap) OS: Linux, Solaris
Private report: No CVE-ID: None
 [2007-08-28 19:31 UTC] bugs at nazarenko dot net
Description:
------------
Using CLI, if more than 1 process is forked with pcntl_fork() both functions mt_rand() and rand() return same values in all child processes.

This only happens when there was a call to mt_rand()/rand() in the parent process before forking. 

Reproduce code:
---------------
Compiled latest available snapshot under Solaris 10 and SuSE Linux 10.1 with the following configure options:

--disable-all --disable-cgi --enable-pcntl --disable-ipv6


The test script is the following:

<?php

echo mt_rand(0,255)."\n";   # comment this line out to see difference

for ($i=0; $i<3; $i++)
{
	if (pcntl_fork() == 0)
	{
		die(mt_rand(0,255)."\n");
	}
}

?>


Expected result:
----------------
Expected result is 4 random numbers generated by 4 different Unix processes (1 parent and 3 children).

Actual result:
--------------
The actual result (both on Solaris and Linux) is that all 3 numbers generated by the children are the same.

Now comment out the first echo command belonging to the parent process and the result is 3 random numbers generated by children.

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 [2007-08-28 22:09 UTC] bugs at nazarenko dot net
I am sorry but I have been through these pages before submitting the report.

I know you are very busy people, but may I kindly ask you to provide tiny bit more information as I could not find anything related to pcntl_fork() problem that I experience on those manual pages.

Thank you...
 [2007-08-28 22:22 UTC] bugs at nazarenko dot net
... and if I understand right I am being suggested to use srand() or mt_srand(), which according to the documentation is not required since PHP 4.2.

So what I am missing?
 [2007-08-29 08:59 UTC] jani@php.net
It doesn't count for run-once SAPIs like CLI.

 
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