#42465 [Bgs->Opn]: rand() and mt_rand() return same values from forked processes

From: Date: Tue, 28 Aug 2007 22:09:22 +0000
Subject: #42465 [Bgs->Opn]: rand() and mt_rand() return same values from forked processes
References: 1  Groups: php.bugs 
Request: Send a blank email to php-bugs+get-116790@lists.php.net to get a copy of this message 
 ID:               42465
 User updated by:  bugs at nazarenko dot net
 Reported By:      bugs at nazarenko dot net
-Status:           Bogus
+Status:           Open
 Bug Type:         Math related
 Operating System: Linux, Solaris
 PHP Version:      5CVS-2007-08-28 (snap)
 New Comment:

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...


Previous Comments:
------------------------------------------------------------------------

[2007-08-28 21:37:34] jani@php.net

http://php.net/mt_srand
http://php.net/srand


------------------------------------------------------------------------

[2007-08-28 19:31:32] 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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