#22988 [Opn]: I'm feeling lucky - rand() bug

ID: 22988 User updated by: vakmajom at freemail dot hu Reported By: vakmajom at freemail dot hu Status: Open Bug Type: Math related Operating System: Windows 2000 PHP Version: 4.3.2RC1 New Comment: About the original bug... Nothing has happened since April 1st, so i browsed the source code a little bit: I think it is not just a carryover bug, it is more than that. It is because RAND_MAX on Windows, and RAND_RANGE. The problem is the follownig: On Windows systems RAND_MAX is 32767, so only 32768 values can be returned if I call rand(1, 2147483647) from a PHP script. That's why rand(1, 2147483647) or rand(1, 65535) returns always an even number. Maybe a RAND_MAX specific hack to the code: e.g. calling rand() three times and creating a 45 bit random value and then reducing it to a 32bit one, or somthing like that would help, but this would dirty the code... Alright, let's use mt_rand(), it is a 31 bit random number on every system, and it is better than a 15 bit random number... So if I call mt_rand(-2147483648, 2147483647) then I should get 2147483648 results. This is less than it should (4294967296), but more than 32768. And now, the suprise: the result is always the same: -2147483648. This IS a carryover bug, and unless I am mistaken it can be solved by changing line 43 in file ext/standard/php_rand.h from (__n) = (__min) + (long) ((double) ((__max) - (__min) + 1.0) * ((__n) / ((__tmax) + 1.0))) to (__n) = (__min) + (long) ((double) ((double)(__max) - (__min) + 1.0) * ((__n) / ((__tmax) + 1.0))) Previous Comments: ------------------------------------------------------------------------ [2003-04-01 08:01:28] vakmajom at freemail dot hu Sorry: srand((double)microtime()*100000); ------------------------------------------------------------------------ [2003-04-01 07:59:49] vakmajom at freemail dot hu The same case. Only even numbers. And a new rand() bug has also appeared: Without srand(microtime()*100000), always the same 10-20 numbers are returned. You should try this bug after the restart of Apache, to be sure that none of the php scripts called srand() before. ------------------------------------------------------------------------ [2003-03-31 17:49:50] sniper@php.net Please try using this CVS snapshot: http://snaps.php.net/php4-STABLE-latest.tar.gz For Windows: http://snaps.php.net/win32/php4-win32-STABLE-latest.zip ------------------------------------------------------------------------ [2003-03-31 15:22:49] pollita@php.net This is being caused by a carryover (2147483647 + 1 == -2147483648). The short answer is: "If it hurts, don't do it!" Of course that's not a proper answer. The API Macro could probably use some tweaking... ------------------------------------------------------------------------ [2003-03-31 12:36:52] vakmajom at freemail dot hu The function rand() always returns an even number! This bug appears when the parameters of rand() are near to the limits of int (i.e. 2147483647) Example: <?php echo(rand(1, 2147483647)); ?> Result: always an even number. I'm using Apache 2.0.44 with PHP4.3.2RC1 (original Win32 binary) as an apache module. Tests with different systems: Maybe this is a Windows bug, it still exists in Windows .Net server build 3718 with PHP 4.3.0 RC3 But everything is fine on a unix system with PHP 4.1.0 Strange... :)) ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=22988&edit=1