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Bug #27683 rand() and mt_rand() require seeding
Submitted: 2004-03-24 18:08 UTC Modified: 2004-03-25 09:32 UTC
From: x13 at rpgsource dot net Assigned:
Status: Closed Package: Math related
PHP Version: 4.3.4 OS: Windows 2000 Professional (SP3)
Private report: No CVE-ID: None
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From: x13 at rpgsource dot net
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 [2004-03-24 18:08 UTC] x13 at rpgsource dot net
Description:
------------
Hello,

I run a script using the command-line interface which generates three random numbers and writes them, along with the date, to a logfile.  Using timers on the mIRC chat program, I run this script every 45 seconds:

dos SILENT C:\apache\php\php.exe D:\htdocs\private\sites\fake\fake.php

The "dos SILENT" command simply opens cmd.exe in the background and executes whatever parameters I pass.  It's equivalent to:

C:\>C:\apache\php\php.exe D:\htdocs\private\sites\fake\fake.php

I have many other applications (including other PHP scripts) running in this manner.  I am 99.9% sure the way I execute php.exe (with "dos SILENT") has nothing to do with this problem.

I run Apache/1.3.29 (Win32) with a basically-standard httpd.conf file and PHP 4.3.4 with an unedited php.ini file.  At first, using rand(), the random numbers were being generated just fine.  I checked the logfile a while later and noticed that identical random numbers were being chosen and logged:

03/23/04, 23:10:21 (8, 1, 358)
03/23/04, 23:11:02 (8, 1, 358)
03/23/04, 23:11:55 (8, 1, 358)
03/23/04, 23:12:36 (8, 1, 358)

I tried switching to mt_rand(); the problem persisted.  I then tried seeding the random number generator once, using the example provided in the manual, and it began working correctly:

03/24/04, 07:36:33 (8, 1, 634)
03/24/04, 07:37:18 (8, 0, 680)
03/24/04, 07:38:03 (4, 0, 677)
03/24/04, 07:38:48 (4, 0, 10)

I found nothing mentioned in the bug database or in the user comments for rand()'s manual section.  I'd be willing to experiment further, if necessary; just contact me.  Thank you.

Reproduce code:
---------------
<?php
function make_seed() {
   list($usec, $sec) = explode(' ', microtime());
   return (float) $sec + ((float) $usec * 100000);
}
mt_srand(make_seed());

$rand = array(mt_rand(1,9), mt_rand(0,1), mt_rand(1,700));

$log_p = fopen("log.txt", "a");
fwrite($log_p, date("m/d/y, H:i:s")." ($rand[0], $rand[1], $rand[2])\n");
fclose($log_p);
?>

Expected result:
----------------
I expected three random numbers to be generated and written to log.txt, such as:

03/24/04, 07:36:33 (8, 1, 634)
03/24/04, 07:37:18 (8, 0, 680)
03/24/04, 07:38:03 (4, 0, 677)
03/24/04, 07:38:48 (4, 0, 10)

Actual result:
--------------
Without the code:

function make_seed() {
   list($usec, $sec) = explode(' ', microtime());
   return (float) $sec + ((float) $usec * 100000);
}
mt_srand(make_seed());

...my results were:

03/23/04, 23:10:21 (8, 1, 358)
03/23/04, 23:11:02 (8, 1, 358)
03/23/04, 23:11:55 (8, 1, 358)
03/23/04, 23:12:36 (8, 1, 358)

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 [2004-03-25 09:32 UTC] derick@php.net
This bug has been fixed in CVS.

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