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Bug #25701 Calling flush from within an output buffer prevents headers from being sent
Submitted: 2003-09-30 09:56 UTC Modified: 2004-04-21 20:28 UTC
Avg. Score:4.8 ± 0.4
Reproduced:5 of 5 (100.0%)
Same Version:5 (100.0%)
Same OS:4 (80.0%)
From: scottmacvicar at ntlworld dot com Assigned: iliaa (profile)
Status: Closed Package: Apache2 related
PHP Version: 4CVS OS: *
Private report: No CVE-ID: None
 [2003-09-30 09:56 UTC] scottmacvicar at ntlworld dot com
Calling flush in Apache and Apache 2 produce different results.

Within Apache after calling flush you can still send headers as the output buffer prevents it from actually being flushed.

Within Apache 2 after calling flush you can no longer send headers even though headers_sent still returns false.

Reproduce code:


echo 'test';

$newtext = ob_get_clean();

if (strpos(' ' . $_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') !== false AND !headers_sent())
	header('Content-Encoding: gzip');
	$newtext = gzencode($newtext, 1);

header('Content-Length: ' . strlen($newtext));
echo $newtext;


Expected result:

Actual result:
binary data


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 [2003-09-30 18:47 UTC]
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at and the instructions on how to report
a bug at

Unlike Apache 1, when Apache 2 recieves a directive to flush it does so right away sending any pending headers in the process. Since the headers & text were already sent it cannot send the header indicating the data that follows in gziped. Due to the missing header you get a whole bunch of binary data.
 [2003-09-30 19:22 UTC] scottmacvicar at ntlworld dot com
Shouldn't headers_sent() return true then?
 [2003-10-01 10:30 UTC] scottmacvicar at ntlworld dot com


echo 'test';

$newtext = ob_get_clean();

if (!headers_sent())
	echo 'in here<br />';

echo $newtext;


Based on what you've said above then you shouldn't see 'in here' within Apache 2.
 [2003-10-01 23:22 UTC]
This bug has been fixed in CVS.

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