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Bug #77081 ftruncate() changes seek pointer in c mode
Submitted: 2018-10-29 15:21 UTC Modified: 2018-10-30 22:53 UTC
From: Assigned: ab (profile)
Status: Closed Package: Filesystem function related
PHP Version: 7.3Git-2018-10-29 (Git) OS: Windows
Private report: No CVE-ID: None
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 [2018-10-29 15:21 UTC]
The documentation of fopen() mode c[1] states:

| The file pointer is positioned on the beginning of the file.

However, this does not happen with PHP 7.3 on Windows; instead the
file pointer is positioned at the end of the file, even though
ftell() claims otherwise.  It works as expected on Linux and with
PHP 7.2 on Windows.

[1] <>

Test script:
$filename = __DIR__ . '/fopen-c.txt';
file_put_contents($filename, 'foo');
$stream = fopen($filename, 'c');
ftruncate($stream, 0);
// fseek($stream, 0);
fwrite($stream, 'bar');

Expected result:
string(3) "bar"

Actual result:
string(6) "   bar"


restore-old-position (last revision 2018-10-29 22:31 UTC by

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 [2018-10-29 22:31 UTC]
The following patch has been added/updated:

Patch Name: restore-old-position
Revision:   1540852290
 [2018-10-29 22:31 UTC]
-Assigned To: +Assigned To: ab
 [2018-10-29 22:31 UTC]
This regression has been introduced with commit 7728160[1].
Anatol, what do you think about something like the attached
restore-old-position patch?

[1] <;a=commit;h=7728160784785771d1933f5f0a8c4d7a735470b0>
 [2018-10-30 18:05 UTC]
Thanks for the ping, Christoph. Perhaps, if there's something wrong with fopen, it should be fixed in first place. I'm going to check it first.

 [2018-10-30 18:16 UTC]
-Summary: fopen() c mode positions file pointer at the end +Summary: ftruncate() changes seek pointer in c mode
 [2018-10-30 18:16 UTC]
> […] if there's something wrong with fopen […]

Sorry, my mistake.  Actually, the problem is not with fopen() per
se, but rather with ftruncate() on a file freshly opened in c
mode.  If I comment out the ftruncate() call in the given reproduce
script, I get the expected:

    string(3) "bar"

Also, writing only the single character 'b' instead of 'bar'
gives the expected:

    string(3) "boo"

The problem is that the ftruncate() call changes the seek pointer,
which it is not supposed to.
 [2018-10-30 19:18 UTC]
I see. As fopen() with 'c' doesn't truncate, the file pointer suddenly jumps to the end of file after ftruncate(). This is obviously wrong leftover from the linked patch for bug #76803. I'm going to fix it using your solution and revamp some more.

 [2018-10-30 19:51 UTC]
Automatic comment on behalf of ab
Log: Fixed bug #77081 ftruncate() changes seek pointer in c mode
 [2018-10-30 19:51 UTC]
-Status: Assigned +Status: Closed
 [2018-10-30 22:53 UTC]
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