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Bug #49874 ftell() and fseek() inconsistency when using stream filters
Submitted: 2009-10-14 11:39 UTC Modified: 2013-10-24 08:19 UTC
Votes:8
Avg. Score:4.2 ± 0.8
Reproduced:5 of 5 (100.0%)
Same Version:1 (20.0%)
Same OS:2 (40.0%)
From: jketterl at chipxonio dot de Assigned:
Status: Open Package: Filesystem function related
PHP Version: 5.5.4 OS: linux (ubuntu)
Private report: No CVE-ID: None
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 [2009-10-14 11:39 UTC] jketterl at chipxonio dot de 
Description:
------------
exact php version: PHP 5.2.11-0.dotdeb.1 with Suhosin-Patch 0.9.7 (cli) (built: Sep 20 2009 09:41:43)
this bug is also be filter-/stream-related. i just believe it might be easier to fix on the filesystem side, that's why i chose that category.

when using a php stream filter to convert input from utf-16 into iso8859 (or most probably from any 2byte-encoded charset into any single-byte-encode charset) the ftell() and fseek() functions start to behave inconsistently.

more precisely: fseek() jumps to exact offsets ignoring the 2byte-encoding, whereas ftell() seems to return the number of bytes read *after* the filter has been applied. thus it is not possible to fseek() back to a certain offset that has been stored with ftell() before.

the content of the testfile used in the code examples is as follows:
Line 01
Line 02
Line 03
Line 04

Reproduce code:
---------------
$file = 'test.csv';

$fp = fopen($file, 'r');
stream_filter_append($fp, 'convert.iconv.utf16/iso8859-15');
$line = fgets($fp);
var_dump($line);
$line = fgets($fp);
var_dump($line);
fclose($fp);

$fp = fopen($file, 'r');
stream_filter_append($fp, 'convert.iconv.utf16/iso8859-15');
$line = fgets($fp);
var_dump($line);
fseek($fp, ftell($fp)); // this shouldn't move anything - but it does...
$line = fgets($fp);
var_dump($line);
fclose($fp);

Expected result:
----------------
string(8) "Line 01
"
string(8) "Line 02
"
string(8) "Line 01
"
string(8) "Line 02
"

Actual result:
--------------
string(8) "Line 01
"
string(8) "Line 02
"
string(8) "Line 01
"
string(4) " 01
"

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 [2009-10-14 16:40 UTC] Sjoerd@php.net 
Thank you for your bug report. Does your test.csv file start with a BOM? You can determine this by viewing the file in a hex editor. If it starts with fffe or feff, it has a BOM (byte order mark).
 [2009-10-15 06:54 UTC] jketterl at chipxonio dot de 
thanks for having a look

i tried with and without. the challenge is to get it working without, because that's the worst case my app has to deal with, but the BOM doesn't seem to solve this.

$ hexdump test-with-bom.csv
0000000 feff 004c 0069 006e 0065 0020 0030 0031
0000010 000a 004c 0069 006e 0065 0020 0030 0032
0000020 000a 004c 0069 006e 0065 0020 0030 0033
0000030 000a 004c 0069 006e 0065 0020 0030 0034
0000040 000a
0000042

$ php test.php
string(8) "Line 01
"
string(8) "Line 02
"
string(8) "Line 01
"
string(5) "e 01
"

i also tried opening the file including the BOM without a stream filter, but that just resulted in php reading in two extra chars (the BOM converted in some way i guess) on the beginning of the first line.

i thought i'd attach the sample files to this bug, but it seems like i can't. i've uploaded them here instead: http://www.djmacgyver.net/tmp/php-ftell/
 [2013-10-24 08:19 UTC] yohgaki@php.net
-PHP Version: 5.2.11 +PHP Version: 5.5.4
 [2013-10-24 08:19 UTC] yohgaki@php.net 
It seems 5.5 has this problem still

string(8) "Line 01
"
string(8) "Line 02
"
string(8) "Line 01
"
string(5) "e 01
"
[yohgaki@dev php-5.4]$ php -v
PHP 5.5.4 (cli) (built: Sep 19 2013 13:06:40)
 [2017-08-01 12:06 UTC] paul dot smith at deepseaplc dot com 
I'm also having this problem with the zlib.deflate filter in PHP 5.6.20.

I attempted to append a user filter after zlib.deflate with the intention of totalling the datalen of the buckets. The filter() function is called but stream_bucket_make_writeable($in) returns null. I suspect this is related to the ftell problem.
 [2018-07-08 10:14 UTC] php at bohwaz dot net 
Same issue with zlib:

<?php

$fh = fopen('php://temp', 'w+');

fwrite($fh, '123');
var_dump(ftell($fh));

stream_filter_append($fh, 'zlib.deflate', STREAM_FILTER_WRITE, 9);

fwrite($fh, 'abc');

var_dump(ftell($fh));

fseek($fh, 0);
var_dump(fread($fh, 1024));

fclose($fh);

?>

Will return:

int(3)
int(3)
string(3) "123"

See: https://3v4l.org/VAfmR
 
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