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Detailed explanation of flock file lock in php

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Jul 25, 2016 am 09:03 AM

$fp = fopen("/tmp/lock.txt", "w+");
if (flock($fp, LOCK_EX)) { // Perform exclusive lock
fwrite ($fp, "Write something here ");
flock($fp, LOCK_UN); // Release the lock
} else {
echo "Couldn't lock the file !";
}
fclose($fp);
?>

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Remarks: Since flock() requires a file pointer, you may have to use a special lock file to protect access to files intended to be opened in write mode (adding "w" or "w+" to the fopen() function).

Note: flock() cannot be used with NFS and some other network file systems. Check your operating system's documentation for details. On 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 the 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 prior to PHP 4.0.1) 3. To release the lock (whether shared or exclusive), set operation to LOCK_UN (set to 3 in versions prior to PHP 4.0.1) 4. If you don’t want flock() to block when locked, add LOCK_NB to the operation (set to 4 in versions prior to PHP 4.0.1)

Look at the code below. File: 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);
?>

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file: b.php

$file = "temp.txt";
$fp = fopen($file, 'r');
echo fread($fp, 100);
fclose($fp) ;
?>

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After running a.php, run b.php immediately and you can see the output: abc After a.php has finished running, run b.php and you can see the output: abc 123 Obviously, when a.php writes the file, the data is too large and takes a long time. At this time, b.php reads the data incompletely and makes modifications to b.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);
?>

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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) before displaying: 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);
?>

Copy code

After running a.php, run b.php immediately and you can see the output: Lock file failed… It is proved that the lock file failure status can be returned instead of waiting for a long time as above. Script House editor’s conclusion: It is recommended to select relevant locks when caching files, otherwise the read data may be incomplete or the data may be written repeatedly. It seems that file_get_contents cannot 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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