flock
(PHP 3>= 3.0.7, PHP 4 )
flock -- Portable advisory file locking
Description
bool flock ( resource handle, int operation [, int &wouldblock])
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).
Note: flock() is mandatory under Windows.
flock() operates on handle which must be an open file pointer.
operation is one of the following values:
To acquire a shared lock (reader), set operation to LOCK_SH
(set to 1 prior to PHP 4.0.1).
To acquire an exclusive lock (writer), set operation to
LOCK_EX (set to 2 prior to PHP 4.0.1).
To release a lock (shared or exclusive), set operation
to LOCK_UN (set to 3 prior to PHP 4.0.1).
If you don't want flock() to block while locking, add LOCK_NB
(4 prior to PHP 4.0.1) to operation.
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 optional third
argument is set to TRUE if the lock would block (EWOULDBLOCK
errno condition)
Returns TRUE on success or FALSE on failure.
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);
?>
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).
