ID: 51073
Updated by: ras...@php.net
Reported By: jian at theorchard dot com
Status: Bogus
Bug Type: Feature/Change Request
Operating System: Linux RHEL 5
PHP Version: 5.3.1
New Comment:
Why can't you always use a signed checksum if that is what you want?
crc32 simply returns 32 bits. You always get the same 32 bits
regardless of the platform. If you want to always represent that as a
signed 32-bit integer, then do so. The fact that some platforms have
the ability to show those 32 bits without messing up the sign is
irrelevant. Just emulate it with something stupidly simple like:
$max32 = 2147483648;
$crc = crc32("884385799717_1_1");
if($crc>$max32-1) {
$crc = $crc-2*$max32;
}
echo $crc;
Previous Comments:
------------------------------------------------------------------------
[2010-02-19 04:20:00] jian at theorchard dot com
All I was saying is to point out that crc32 function is unable to
return the same result represented in the same way under any different
platforms.
The documentation does say to use sprintf function to format the result
to be an unsigned integer in order to the get same expected result from
all platforms.
In the real world, we actually do need to use signed checksum in some
cases. So with this shortcoming in mind, crc32 function cannot achieve
a truly signed checksum purpose.
------------------------------------------------------------------------
[2010-02-18 08:34:59] ras...@php.net
It does return the correct set of bits. Anything we do to change what
is currently being returned will seriously confuse everyone and the
documentation is quite clear on this.
------------------------------------------------------------------------
[2010-02-18 08:26:44] jian at theorchard dot com
On a second thought, however, this still poses an issue. As a
programmer, I shouldn't be worreid about all these reformatting and
work-arounds. If my function returns me a chicken under development
environment, it shouldn't return me an egg once it's released to the
production environment.
Even though it's the same value, they should also be represented in the
same way regardless which platform this function is used.
This issue should still be classified as a valid bug in my opinion.
------------------------------------------------------------------------
[2010-02-17 22:38:09] jian at theorchard dot com
Thanks Derick. That was much more helpful. Unfortunately, I cannot
use unsigned integer as our checksum. It has to be signed. I do in
fact have a work around function built to solve this issue. I was just
wondering whether this will get fixed in the future or not. Since it's
a platform dependent issue, it most likely won't get fixed I think.
Below is the function I have posted against the crc32 function as a
note. I hope this can help others out. Thanks again.
function get_signed_int($in) {
$int_max = pow(2, 31)-1;
if ($in > $int_max){
$out = $in - $int_max * 2 - 2;
}
else {
$out = $in;
}
return $out;
}
------------------------------------------------------------------------
[2010-02-17 22:32:04] der...@php.net
Instead of a plain echo, you can also do:
printf("%u", crc32("884385799717_1_1"));
that should output the same string on every platform (that is at least
32bit).
The reason why it is different on Windows vs. (any other) Unix, is
because on Windows even on 64 bit CPUs, "long" (the data type we use for
integers) is still only 32bit; on Unices, the "long" type is usually 64
bit on 64 bit CPUs.
------------------------------------------------------------------------
The remainder of the comments for this report are too long. To view
the rest of the comments, please view the bug report online at
http://bugs.php.net/51073
--
Edit this bug report at http://bugs.php.net/?id=51073&edit=1
