flock() allows you to perform a simple reader/writer
model which can be used on virtually every platform (including most Unix
derivatives and even Windows).
The lock is released also by fclose() (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).
Parameters
handle
An open file pointer.
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).
LOCK_NB if you don't want
flock() to block while locking.
(not supported on Windows)
wouldblock
The optional third argument is set to TRUE if the lock would block
(EWOULDBLOCK errno condition).
Return Values
Returns TRUE on success or FALSE on failure.
ChangeLog
Version
Description
4.0.1
The LOCK_XXX constants were added. Prior to that
you must use 1 for LOCK_SH, 2 for
LOCK_EX, 3 for LOCK_UN and
4 for LOCK_NB
Examples
Example #1 flock() example
<?php
$fp = fopen("/tmp/lock.txt", "w+");
if (flock($fp, LOCK_EX)) { // do an exclusive lock fwrite($fp, "Write something here\n"); flock($fp, LOCK_UN); // release the lock } else { echo "Couldn't lock the file !"; }
fclose($fp);
?>
Notes
Note:
flock() locks mandatory under Windows.
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()).
Warning
flock() will not work on NFS and many other networked
file systems. Check your operating system documentation for more
details.
On some operating systems flock() is implemented at
the process level. When using a multithreaded server API like ISAPI 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 this environments (this is especially true for
Windows 98 users).
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