FLOCK(3) 1 FLOCK(3)
flock - Portable advisory file locking
SYNOPSIS
bool flock (resource $handle, int $operation, [int &$wouldblock])
DESCRIPTION
flock(3) allows you to perform a simple reader/writer model which can be used on virtually every platform (including most Unix derivatives
and even Windows).
On versions of PHP before 5.3.2, the lock is released also by fclose(3) (which is also called automatically when script finished).
PHP supports a portable way of locking complete files in an advisory way (which means all accessing programs have to use the same way of
locking or it will not work). By default, this function will block until the requested lock is acquired; this may be controlled with the
LOCK_NB option documented below.
PARAMETERS
o $handle
-A file system pointer resource that is typically created using fopen(3).
o $operation
-$operation is one of the following:
o LOCK_SH to acquire a shared lock (reader).
o LOCK_EX to acquire an exclusive lock (writer).
o LOCK_UN to release a lock (shared or exclusive).
It is also possible to add LOCK_NB as a bitmask to one of the above operations if you don't want flock(3) to block while locking.
o $wouldblock
- The optional third argument is set to 1 if the lock would block (EWOULDBLOCK errno condition).
RETURN VALUES
Returns TRUE on success or FALSE on failure.
CHANGELOG
+--------------+---------------------------------------------------+
| Version | |
| | |
| | Description |
| | |
+--------------+---------------------------------------------------+
|5.5.22, 5.6.6 | |
| | |
| | Added support for the $wouldblock parameter on |
| | Windows. |
| | |
| 5.3.2 | |
| | |
| | The automatic unlocking when the file's resource |
| | handle is closed was removed. Unlocking now |
| | always has to be done manually. |
| | |
+--------------+---------------------------------------------------+
EXAMPLES
Example #1
flock(3) example
<?php
$fp = fopen("/tmp/lock.txt", "r+");
if (flock($fp, LOCK_EX)) { // acquire an exclusive lock
ftruncate($fp, 0); // truncate file
fwrite($fp, "Write something here
");
fflush($fp); // flush output before releasing the lock
flock($fp, LOCK_UN); // release the lock
} else {
echo "Couldn't get the lock!";
}
fclose($fp);
?>
Example #2
flock(3) using the LOCK_NB option
<?php
$fp = fopen('/tmp/lock.txt', 'r+');
/* Activate the LOCK_NB option on an LOCK_EX operation */
if(!flock($fp, LOCK_EX | LOCK_NB)) {
echo 'Unable to obtain lock';
exit(-1);
}
/* ... */
fclose($fp);
?>
NOTES
Note
flock(3) uses mandatory locking instead of advisory locking on Windows. Mandatory locking is also supported on Linux and System V
based operating systems via the usual mechanism supported by the fcntl() system call: that is, if the file in question has the set-
gid permission bit set and the group execution bit cleared. On Linux, the file system will also need to be mounted with the mand
option for this to work.
Note
Because flock(3) requires a file pointer, you may have to use a special lock file to protect access to a file that you intend to
truncate by opening it in write mode (with a "w" or "w+" argument to fopen(3)).
Note
May only be used on file pointers returned by fopen(3) for local files, or file pointers pointing to userspace streams that imple-
ment the streamWrapper.stream_lock(3) method.
Warning
Assigning another value to $handle argument in subsequent code will release the lock.
Warning
On some operating systems flock(3) is implemented at the process level. When using a multithreaded server API like ISAPI you may
not be able to rely on flock(3) to protect files against other PHP scripts running in parallel threads of the same server instance!
flock(3) is not supported on antiquated filesystems like FAT and its derivates and will therefore always return FALSE under this
environments (this is especially true for Windows 98 users).
PHP Documentation Group FLOCK(3)