Detailed introduction to PHP flock file lock_php basics

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:

Under Windows flock() will be enforced.

The handle of the 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).

Return TRUE on success, or FALSE on failure.


Example #1 flock() Example

Copy code The code is as follows:

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

Note:

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 (add "w to the fopen() function " or "w ").

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 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 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 do not want flock() to block when locked, add LOCK_NB to the operation (set to 4 in previous versions of PHP 4.0.1)

See below Code:

a.php

Copy code The code is as follows:

$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

Copy code The code is as follows:

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

After running a.php, run b.php immediately and you can see the output:
abc
Wait for a.php to run and then run b.php and you can see the output:
abc
>123
Obviously, when a.php writes a file, the data is too large and takes a long time. At this time, b.php reads incomplete data. After modifying b.php,
modify b.php to ：

Copy code The code is as follows:

$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) before displaying:
abc
123
Read The data is complete, but the time is too long. He has to wait for the write lock to be released before making changes to b.php.
Modify b.php to:

Copy code The code is as follows:

php
$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 It takes a long time to wait 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.
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.