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Bug #45953 Sign-Bit Manipulating is buggy
Submitted: 2008-08-30 10:23 UTC Modified: 2008-08-30 11:15 UTC
From: ben at umingo dot de Assigned:
Status: Not a bug Package: Output Control
PHP Version: 5.2.6 OS: Windows XP 32Bit
Private report: No CVE-ID: None
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 [2008-08-30 10:23 UTC] ben at umingo dot de 
Description:
------------
Problem is with sign bit. 
If i have 0x81234567 then first bit is set to 1 (sign bit).
I would expect that number to be negative, as all integers are signed 32 bit integers. 
If i echo that number, it is positive!
If i compute something with this number, it turns negative!

I have a standard PHP 5.2.6 Version and did not modify anything at it.



Reproduce code:
---------------
<?
//sign bit is not handled correctly

//a | a is expected to be a ...
//WORKS NOT
echo "with sign bit <br>";
echo (0x81234567 | 0x81234567); //result is negative
echo "<br>";
echo (0x81234567 );//result is positive

echo "<br>without sign bit <br>";
//WORKS
echo (0x71234567 | 0x71234567);
echo "<br>";
echo (0x71234567 );
?>

Expected result:
----------------
with sign bit 
-2128394905
-2128394905
without sign bit 
1898136935
1898136935


Actual result:
--------------
with sign bit 
-2128394905
2166572391
without sign bit 
1898136935
1898136935

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 [2008-08-30 11:15 UTC] mattwil@php.net 
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

What's happening is that when the script is parsed and compiled, 0x81234567 actually becomes a float/double type since, as you mention, it's too big for 32-bit signed range and the intended behavior is to keep it positive (as it would be on platforms with 64-bit longs) and not restrict it to integer type only. The bitwise operators like | only operate on integers, and it converts a float/double to int first, which results in "overflow" to a negative number that you're seeing.

Typecasting to (int) will also force the overflow (same internal conversion used by bitwise operators), and give the result you expect (on 32-bit platform anyway). Example:

<?php

var_dump(0x81234567);
var_dump(0x81234567 | 0x81234567);
var_dump((int) 0x81234567);


Output:

float(2166572391)
int(-2128394905)
int(-2128394905)
 
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