flock
(PHP 3 >= 3.0.7, PHP 4, PHP 5)
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). The lock is released also by fclose()
(which is also called automatically when script finished).
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);
?>
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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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