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Bug #51969 fwrite returns 1 when trying to write to a locked file
Submitted: 2010-06-01 23:45 UTC Modified: 2015-04-12 21:19 UTC
Votes:7
Avg. Score:4.4 ± 0.9
Reproduced:4 of 4 (100.0%)
Same Version:2 (50.0%)
Same OS:1 (25.0%)
From: scarduzio at gmail dot com Assigned: cmb (profile)
Status: Not a bug Package: Filesystem function related
PHP Version: 5.3.2 OS: Windows 7 x86
Private report: No CVE-ID: None
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 [2010-06-01 23:45 UTC] scarduzio at gmail dot com 
Description:
------------
When I try to write any length string with fwrite using a file handle I previously 
locked with flock, fwrite is of course unable to write, but will return 
unexplicable value 1 instead of false.

Test script:
---------------
/*
   Run two instances of this script in a rapid sequence
*/

$fp1 = fopen("/cache/lockable.txt","w");
if(flock($fp1, LOCK_EX | LOCK_NB)){
    echo "locked fp1\n";
    sleep(20);
}else{
        $res = fwrite($fp1, "ABCDE");
        if(false === $res){
           echo "failed the write\n";   
        }
        echo "I managed! fwrite returned: "; print_r($res);
}

Expected result:
----------------
I expect to read "failed to write" when a second instance of the script is run, 
since fwrite should return false as documentation says when fwrite is not able to 
write.

Actual result:
--------------
fwrite returns integer 1.

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 [2010-06-10 12:04 UTC] fa@php.net
-Status: Open +Status: Feedback
 [2010-06-10 12:04 UTC] fa@php.net 
From the manual:
"It is also possible to add LOCK_NB as a bitmask to one of the above operations if you don't want flock() to block while locking. (not supported on Windows)"

Then again path "/cache/lockable.txt" doesn't look like windows - so what OS are you using?

And while I know that's possibly not directly related to your problem - please clarify.
 [2010-06-10 12:26 UTC] scarduzio at gmail dot com
-Status: Feedback +Status: Open
 [2010-06-10 12:26 UTC] scarduzio at gmail dot com 
Hi, thanks for replying.

The OS in use is Windows 7 x86 genuine with all windows updates. 
The path with forward slash that I use in my code is unix style because I 
noticed 
PHP always translates it correctly into "C:\cache\lockable.txt", and it's always 
been fine. Also this time, the file is always found.

About the manual line stating LOCK_NB bitmask is not supported: as evinced from 
my tests, including this one, the use of LOCK_NB is behaving perfectly since it 
detects file is locked, and executes my "else".

I also do not think LOCK_NB is the core of this problem since I could split this 
script in two (one with the flock+sleep and the other with the "else" content) 
and reproduce the same bug.

I later tested this in linux environment, fwrite will return 5 (not 1). Which is 
still funny, since writing in a locked file should in my opinion return false.

I look forward for your further analysis.
BR,

Simone
 [2010-06-10 14:50 UTC] fa@php.net 
Ah, ok, thank you for the clarification.

Tried to reproduce now, indeed fwrite in the else clause ignores the flock() - but then again you already checked if there is a lock, that is exactly why you are in the else code path, so I'm not exactly sure how flock *should* behave ...

Trying to open the locked file gives a windows warning that the file is indeed locked, though.

Same result with PHP 5.3.2 (cli) on Debian lenny, btw - minus the warning on accessing the file directly.
 [2015-04-12 21:19 UTC] cmb@php.net
-Status: Open +Status: Not a bug -Assigned To: +Assigned To: cmb
 [2015-04-12 21:19 UTC] cmb@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

You can't lock a file that you're opening in "w" mode reliably. See
also the respective note in the flock man page[1] and the
documentation of the $mode parameter of fopen[2].

On Windows locking a file opened in "w" mode might work reliably.
However, testing on PHP 5.4.19 revealed that fwrite() returned 0 in
the test script.

[1] <http://php.net/manual/en/function.flock.php#refsect1-function.flock-notes>
[2] <http://php.net/manual/en/function.fopen.php>
 
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