Detailed introduction to PHP flock file lock_PHP tutorial
flock
(PHP 4, PHP 5)
flock — lightweight advisory file locking
Description
bool flock ( int $handle , int $operation [, int &$wouldblock ] )
PHP supports a lightweight method of locking all files in an advisory manner (that is, all accessing programs must lock in the same way, otherwise it will not work).
Note:
flock() will be enforced under Windows.
The handle of flock() operation must be an open file pointer. operation can be one of the following values:
To obtain a shared lock (reading program), set operation to LOCK_SH (set to 1 in versions prior to PHP 4.0.1).
To obtain an exclusive lock (writing program), set operation to LOCK_EX (set to 2 in versions prior to PHP 4.0.1).
To release a lock (whether shared or exclusive), set operation to LOCK_UN (set to 3 in versions prior to PHP 4.0.1).
If you do not want flock() to block when locked, add LOCK_NB to the operation (set to 4 in versions prior to PHP 4.0.1).
flock() allows the implementation of a simple read/write model that can be used on any platform (including most Unix derivatives and even Windows). The optional third parameter is set to TRUE if the lock would block (in case of EWOULDBLOCK error code). Lock operations can also be released by fclose() (also called automatically when the code completes execution).
Returns TRUE on success, or FALSE on failure.
Example #1 flock() Example
$fp = fopen("/tmp/lock.txt", "w+");
if (flock($ fp, LOCK_EX)) { // Lock exclusively
fwrite($fp, "Write something here ");
flock($fp, LOCK_UN); // Release lock
} else {
echo "Couldn't lock the file !";
}
fclose($fp);
?>
Note:
Due to flock() requires a file pointer, so you may have to use a special lock file to protect access to files intended to be opened in write mode (add "w" or "w+" to the fopen() function).
Warning
flock() cannot be used with NFS and some other network file systems. Check your operating system's documentation for details.
In some operating systems, flock() is implemented at the process level. When using a multi-threaded server API (such as ISAPI), it may not be possible to rely on flock() to protect the file, because the file can be processed by PHP scripts running in other parallel threads in the same server instance.
flock() does not support older file systems such as FAT and its derivatives. Therefore, FALSE is always returned in this environment (especially for Windows 98 users).
Introduction to the usage of the file lock function flock function in php:
Syntax:
bool flock (int $handle, int $operation [, int & $wouldblock ] )
The handle of flock() operation must be an open file pointer. operation can be one of the following values:
1. To obtain a shared lock (reader), set operation to LOCK_SH (set to 1 in versions prior to PHP 4.0.1)
2. To obtain an exclusive lock ( Writer), set operation to LOCK_EX (set to 2 in versions before PHP 4.0.1)
3. To release the lock (whether shared or exclusive), set operation to LOCK_UN (in versions before PHP 4.0.1) Set to 3 in the version)
4. If you don’t want flock() to block when locked, add LOCK_NB to the operation (set to 4 in versions before PHP 4.0.1)
See below Code:
a.php
$file = "temp.txt";
$fp = fopen($file, 'w');
if(flock($fp, LOCK_EX)) {
fwrite($fp, "abc" );
sleep(10);
fwrite($fp, "123");
flock($fp, LOCK_UN);
}
fclose($fp);
?>
b.php
$file = "temp.txt";
$fp = fopen($file, 'r');
echo fread($fp, 100);
fclose($fp);
?>
After running a.php, run b.php immediately. You can see the output:
abc
Wait for a.php to finish running and then run b.php , you can see the output:
abc
123
Obviously, when a.php writes the file, the data is too large, which takes a long time. At this time, b.php reads the data incompletely. Modify .php
Modify b.php to:
$file = "temp.txt";
$fp = fopen($file, 'r');
if(flock($fp, LOCK_EX) ){
echo fread($fp, 100);
flock($fp, LOCK_UN);
} else{
echo “Lock file failed…”;
}
fclose ($fp);
?>
After running a.php, run b.php immediately. You can find that b.php will wait until a.php is completed (that is, after 10 seconds ) to display:
abc
123
The read data is complete, but the time is too long. He has to wait for the write lock to be released before making modifications to b.php.
Modify b.php to:
$file = "temp.txt";
$fp = fopen($file, 'r');
if(flock($fp, LOCK_SH | LOCK_NB)) {
echo fread($fp, 100) ;
flock($fp, LOCK_UN);
} else{
echo “Lock file failed…”;
}
fclose($fp);
?>
After running a.php, run b.php immediately and you can see the output:
Lock file failed...
Proof that the lock file failure status can be returned instead of waiting for a long time like above .
Script House editor’s conclusion:
It is recommended to select relevant locks when caching files, otherwise it may result in incomplete reading of data or repeated writing of data.
file_get_contents seems to be unable to select the lock. I don’t know what lock it uses by default. Anyway, the output obtained by not locking is the same as incomplete data.
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