Hi,
I'm currently experiencing a problem with a phpBB board that appears to be due to the way random number generation works in PHP. I have posted on serveral phpBB boards and have received no answers, so I think I need to understand how PHP is dealing with this in order to resolve it.
phpBB generates a unique index for search results using the following code (in search.php):
----- 8< ----- Code ----- 8< ----- mt_srand ((double) microtime() * 1000000); $search_id = mt_rand(); ----- 8< ----- Code ----- 8< -----
The problem is that the seed is generated from the system clock, and if two people search at the exact same moment (or more strictly, if the mt_srand statements get executed at the exact same moment), the random number generation system is initialised with the same seed in each case, so both searches receive the same $search_id, resulting in a MySQL error due to non-unique index fields.
Looking through the documentation for PHP, it seems that versions > 4.2.0 do not require the use of mt_srand as they will seed themselves when required. I've had a look at the PHP source code, and from my limited understanding, the seed appears to be generated using a function in php_rand.h:
----- 8< ----- Code ----- 8< -----
#define GENERATE_SEED() ((long) (time(0) * getpid() * 1000000 * php_combined_lcg(TSRMLS_C)))
----- 8< ----- Code ----- 8< -----
So this is still based to some extent on the current system clock, and the php_combined_lcg function in lcg.c also appears to rely on the current system clock along with the current process/thread ID.
There is also some code to ensure that seeding only occurs once. But is this once per thread? Once per process? Once per something-else?
What I'm trying to find out is whether this new seeding system will produce the same 'random' number if two mt_rand() statements are executing concurrently. My guess is that they won't, but I don't understand the internals of PHP or Apache well enough to know if their use of threads and processes will ensure uniqueness.
The problem is occuring with PHP 4.3.9 running under Apache 1.3.33 on Linux kernel 2.4.26 (on a fairly typical cPanel-based shared hosting box). Unfortunately due to the timing-based nature of the problem, it has proven very difficult to set up test cases.
I'd be very grateful for any assistance.
Gordon.
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