|
|
Posted by Jim Michaels on 04/23/06 09:43
"Gordon Burditt" <gordonb.cg3n5@burditt.org> wrote in message
news:124luplqscmpm9f@corp.supernews.com...
> >I need to generate 2 random numbers in rapid sequence from either PHP or
>>mysql.
>
> Same page hit or different page hits? I cannot explain why you
> would get the same number in rapid sequence from several calls on
> the same hit.
same page hit on the webserver to a .html file, which has 2 <img
src=img.php> elements.
on a multiprocessor webserver, this may be occurring simultaneously.
>
>>I have not been able to do either. I get the same number back several
>>times
>>from PHP's mt_rand() and from mysql's RAND().
>>any ideas?
>
> When PHP starts up, the seed gets initialized to <SOMETHING>,
> possibly based on microtime(), but I really don't care. Apache/PHP
> don't get restarted very often. With a good host, it could easily
> be less than once a month. However, once PHP starts, it's fixed.
>
> It is likely that if Apache fork()s for a given hit, the seed stays
> initialized to <SOMETHING> unless you explicitly set it. Any changes
> to that by more calls to get pseudo-random numbers are discarded
> when that instance of Apache exits. To get different values, you
> need to explicitly set the seed to a function of something other
> than just microtime().
which makes sense. if all set to microtime, you would still get the same
images.
>
> Things to use for a seed (jumble them all together, as in concatenate,
> then take md5 of result, then convert some of md5 result to integer):
> microtime()
> getmypid()
believe it or not, on these 2 I get the same pictures. frustrating.
> $_SERVER['UNIQUE_ID'] (Apache only, and may need a module turned on)
Don't think I have that option. sure sounds good though.
> $_SERVER['REMOTE_ADDR']
> $_SERVER['REMOTE_PORT']
these two won't make it unique, because both images are going to be all on
the same page.
mt_srand(make_seed()+getmypid()+$_SERVER['UNIQUE_ID']);
$n=mt_rand(1,$row['a']);
tried this, but still doesn't do it. I don't even get an error on
$_SERVER['UNIQUE_ID']. I think it's NULL. is $_SERVER['UNIQUE_ID'] a
string I should hash, or an integer?
>
> Actually, using just the first two should be sufficient, as the
> pair (microtime(), getmypid()) should be unique. However, to get
> more entropy, throw in some of the others.
>
> If you are using pseudo-random numbers to select one of 100 images,
> expect the same image twice in a row about 1% of the time, unless
> you have code to deal with this. My suggestion is to not attempt
> to avoid this.
>
I get a different image on next page refresh, possibly due to the time
involved. but the same images all over the page, possibly due to the time
involved.
>>I suppose I could use the current rancom number as the seed for the next
>>random number. but would that really work?
>
> It's a very BAD idea to seed a pseudo-random number generator
> multiple times in the same run. Using the output of the random
> number generator as a new seed is also not a good idea. It's very
> easy to cripple a good pseudo-random number generator with a poor
> method of choosing a seed. The problem you're having with microtime()
I understand that. I was trying to avoid it. but nothing seems to work
short of having semaphore'd access to a file or database as the seed and
just incrementing it like a counter. And then I'm circumventing the web
server's whole concept of parallelism. :-/
> demonstrates this. Seed once. Use a good source of (pseudo-)randomness.
> microtime() alone doesn't qualify if you require rapid-fire results.
>
tell me about it. :-/
> Gordon L. Burditt
Navigation:
[Reply to this message]
|