flock
Portable advisory file locking
Description
bool flock(resource $stream, int $operation, int &$would_block = null)
The lock is released also by fclose,
or when stream is garbage collected.
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
-
stream
-
A file system pointer resource
that is typically created using fopen.
-
operation
-
operation is one of the following:
-
LOCK_SH to acquire a shared lock (reader).
-
LOCK_EX to acquire an exclusive lock (writer).
-
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 flock should not
block during the locking attempt.
-
would_block
-
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.
Examples
Example #1 flock 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\n");
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 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 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 setgid
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 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).
Note:
May only be used on file pointers returned by fopen
for local files, or file pointers pointing to userspace streams that
implement the streamWrapper::stream_lock method.
Warning
Assigning another value to stream argument in
subsequent code will release the lock.
Warning
On some operating systems flock is implemented at
the process level. When using a multithreaded server API you
may not be able to rely on flock to protect files
against other PHP scripts running in parallel threads of the same server
instance!
flock is not supported on antiquated filesystems like
FAT and its derivates and will therefore always
return false under these environments.
Note:
On Windows, if the locking process opens the file a second time, it cannot
access the file through this second handle until it unlocks the file.