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Bug #44069 Huge memory usage with concatenation using . instead of .=
Submitted: 2008-02-07 13:39 UTC Modified: 2008-02-14 15:20 UTC
Votes:3
Avg. Score:5.0 ± 0.0
Reproduced:3 of 3 (100.0%)
Same Version:3 (100.0%)
Same OS:3 (100.0%)
From: frode dot guldberg at exense dot com Assigned: dmitry
Status: Closed Package: Performance problem
PHP Version: 5.2.5 OS: *
Private report: No CVE-ID:
 [2008-02-07 13:39 UTC] frode dot guldberg at exense dot com
Description:
------------
In a for-loop I concatenate two strings. If I at the same time fill a relatively small array, I get a huge memory consumption. This only happens if I concatenate the two strings using '.'. If I use '.=' the memory usage is as expected.

Reproduce code:
---------------
<?php

$string = str_repeat('This is a teststring.', 50);

echo 'Length: '.strlen($string).'<br>';
echo 'Memory Before:'.memory_get_usage(true).'<br>';

for($i = 1; $i <= 2000; $i++)
{
//	$newstring .= $string; //This uses an expected amount of mem.
	$newstring = $newstring . $string; //This uses very much mem.

	for($j = 1; $j <= 10; $j++)
	{
		$array[] = 'test';
	}
}
echo 'Memory After:'.memory_get_usage(true).'<br>';
echo 'Total Length of String: '.strlen($newstring).'<br>';

?>

Expected result:
----------------
Length: 1050
Before:262144
After:4456448
Length: 2100000


Actual result:
--------------
Length: 1050
Before:262144
After:161742848
Length: 2100000

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 [2008-02-13 18:25 UTC] jani@php.net
Dmitry, can you check this out please? The example script kills my machine totally if I don't use the .= :)
 [2008-02-14 14:49 UTC] dmitry@php.net
This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
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