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Posted by Jim Michaels on 04/23/06 11:08
"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.
>
>>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().
>
> 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()
> $_SERVER['UNIQUE_ID'] (Apache only, and may need a module turned on)
> $_SERVER['REMOTE_ADDR']
> $_SERVER['REMOTE_PORT']
>
> Actually, using just the first two should be sufficient, as the
> pair (microtime(), getmypid()) should be unique. However, to get
> more entropy, throw in some of the others.
>
> If you are using pseudo-random numbers to select one of 100 images,
> expect the same image twice in a row about 1% of the time, unless
> you have code to deal with this. My suggestion is to not attempt
> to avoid this.
>
>>I suppose I could use the current rancom number as the seed for the next
>>random number. but would that really work?
>
> It's a very BAD idea to seed a pseudo-random number generator
> multiple times in the same run. Using the output of the random
> number generator as a new seed is also not a good idea. It's very
> easy to cripple a good pseudo-random number generator with a poor
> method of choosing a seed. The problem you're having with microtime()
> demonstrates this. Seed once. Use a good source of (pseudo-)randomness.
> microtime() alone doesn't qualify if you require rapid-fire results.
>
> Gordon L. Burditt
well, I have one last thing I can do. I asked about doing XML inline images
and got this really cook info. hope it would work with HTML.
echo "<img src=\"data:$mimetype;base64," .
base64_encode(file_get_contents($filepath)) . "\" alt=\"$alt\" width=\"$w\"
height=\"$h\" style=\"$style\"$end>";
just insert this into a function. from http://www.ietf.org/rfc/rfc2397.txt .
calling a function to generate an imline image should allow me to use the
random number generator as it's supposed to be used. then I don't have to
use <img src="img.php">
just tried it. output looks correct, but doesn't show an image. :-(
and base64_encode() sure is slow!
Alas, I found out this only works in NS, not IE. IE requires MIME encoding,
which I've never done, and since it looks like it requires headers, I may be
back to square one problem with the seed.
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