PHP flock file lock_PHP tutorial

//Introduction to flock function usage on w3school:
PHP flock() function

Definition and Usage

The flock() function locks or releases a file.

If successful, returns true. On failure, returns false.

Grammar
flock(file,lock,block)
Parameter Description
file required. Specifies an open file to be locked or released.
lock required. Specifies which lock type to use.
block is optional. If set to 1 or true, blocks other processes while locking.
Description
The file used by flock() must be an open file pointer.

The lock parameter can be one of the following values:

To obtain a shared lock (reading program), set lock to LOCK_SH (set to 1 in versions prior to PHP 4.0.1).
To obtain an exclusive lock (writing program), set lock to LOCK_EX (set to 2 in versions prior to PHP 4.0.1).
To release a lock (whether shared or exclusive), set lock to LOCK_UN (set to 3 in versions prior to PHP 4.0.1).
If you do not want flock() to block on lock, add LOCK_NB to lock (set to 4 in versions prior to PHP 4.0.1).
Tips and Notes

Tip: The lock operation can be released through fclose(), which will be automatically called when the code is completed.

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 (adding "w" or "w+" to the fopen() function).

Example

==================================================
From: http://hxsdit.com/1110
PHP comes with a file lock function:
bool flock ( int $handle , int $operation [, int &$wouldblock ] )
$handle is the open file pointer;
$operation can be
"LOCK_SH", shared lock; "LOCK_EX", exclusive lock; "LOCK_UN", releases the lock; "LOCK_NB", prevents blocking when flock is locked.
Here we mainly talk about "LOCK_EX" and "LOCK_NB".
For example, we have two files, as follows.
flocka.php

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$file = 'temp.txt'; $fp = fopen($file,'a'); for($i = 0;$i < 5;$i++) { fwrite($fp, "11111111n"); sleep (1); } fclose($fp);

flockb.php

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$file = 'temp.txt'; $fp = fopen($file,'a'); for($i = 0;$i < 5;$i++) { fwrite($fp, "22222222n"); } fclose($fp);
Run flocka.php first, then flockb.php immediately.
Result:
11111111
22222222
22222222
22222222
22222222
22222222
11111111
11111111
11111111
11111111
Note that when no file lock is added, the two files will write to the txt file at the same time.
Let’s modify the code of the two php files.
flocka.php

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$file = 'temp.txt'; $fp = fopen($file,'a'); if(flock($fp,LOCK_EX)) { for($i = 0;$i < 5;$i++) { fwrite($fp, "11111111n");                   sleep(1);                          flock($fp,LOCK_UN); flockb.php

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$file = 'temp.txt'; $fp = fopen($file,'a'); if(flock($fp,LOCK_EX)) { for($i = 0;$i < 5;$i++) { fwrite($fp, "22222222n"); } } flock($fp,LOCK_UN); } fclose($fp);
Also run flocka.php first, and then flockb.php immediately.
You will find that flockb.php is in a waiting state before flocka.php finishes running. Only when flocka.php finishes running, flockb.php will continue to execute.
Output result:
11111111
11111111
11111111
11111111
11111111
22222222
22222222
22222222
22222222
22222222
In addition, when flock is executed, the file lock is automatically released.