Posted by "Richard Lynch" on 12/30/05 20:34

On Thu, December 29, 2005 5:37 pm, Michael Gross wrote:
> Hello
> I have to migrate a PHP-application to a new Linux-Box. Both the old
> and
> the new system are Linux and PHP 5.1.1. (the old one has a Pentium 4,
> the new one two Xeon CPUs). I have a problem using the
> Crypt_Xtea-Extension. I narrowed it down to the following right-shift
> operation:
>
> (-3281063054 >> 11) produces different results:
> Old System: 495070
> New System: -1048576
>
> I understand that both results are "wrong", but everything worked with
> the old behavior and I need that behavior back very urgent.
>
> Maybe someone can explain me in which way the bits are shifted so that
> the result is 495070? If I understand it, I implement my "own" shift
> function.

Assuming the previous hypothesis that it's 32-bit versus 64-bit
machines at work...

If you can determine the number of bits on your system, you could use
a different number from 11 on the two systems to get the answer you
want.

if (is_32_bit_machine()){
$y = $x >> 11;
}
else{
$y = $x >> 43; //11 + 32 (guess)
}

One hack for detecting 32-bit machine might be this:

function is_32_bit_machine(){
return mt_getrandmax() == 0xffffffff;
}

I suppose if you're going to do this right, what you REALLY should do
is code it to determine the number of bits, no matter how large, and
then bit-shift the correct amount based on that.

So when the 128-bit machines come out in a few years, you aren't
re-coding this same damn problem AGAIN.

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