flock

Portable advisory file locking

&reftitle.description; `bool` `flock` `resource` `stream` `int` `operation` `int` `would_block`&null;

`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 lock is released also by `fclose`, or when `stream` is garbage collected.

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). By default, this function will block until the requested lock is acquired; this may be controlled with the `LOCK_NB` option documented below.

&reftitle.parameters;

`stream`

- &fs.file.pointer;

`operation`

- `operation` is one of the following:

  - `LOCK_SH` to acquire a shared lock (reader).

  - `LOCK_EX` to acquire an exclusive lock (writer).

  - `LOCK_UN` to release a lock (shared or exclusive).

  It is also possible to add `LOCK_NB` as a bitmask to one of the above operations, if `flock` should not block during the locking attempt.

`would_block`

- The optional third argument is set to 1 if the lock would block (EWOULDBLOCK errno condition).

&reftitle.returnvalues;

&return.success;

&reftitle.examples;

### `flock` example

    <?php

    $fp = fopen("/tmp/lock.txt", "r+");

    if (flock($fp, LOCK_EX)) {  // acquire an exclusive lock
        ftruncate($fp, 0);      // truncate file
        fwrite($fp, "Write something here\n");
        fflush($fp);            // flush output before releasing the lock
        flock($fp, LOCK_UN);    // release the lock
    } else {
        echo "Couldn't get the lock!";
    }

    fclose($fp);

    ?>

### `flock` using the `LOCK_NB` option

    <?php
    $fp = fopen('/tmp/lock.txt', 'r+');

    /* Activate the LOCK_NB option on an LOCK_EX operation */
    if(!flock($fp, LOCK_EX | LOCK_NB)) {
        echo 'Unable to obtain lock';
        exit(-1);
    }

    /* ... */

    fclose($fp);
    ?>

&reftitle.notes;

`flock` uses mandatory locking instead of advisory locking on Windows. Mandatory locking is also supported on Linux and System V based operating systems via the usual mechanism supported by the fcntl() system call: that is, if the file in question has the setgid permission bit set and the group execution bit cleared. On Linux, the file system will also need to be mounted with the mand option for this to work.

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`).

May only be used on file pointers returned by `fopen` for local files, or file pointers pointing to userspace streams that implement the `streamWrapper::stream_lock` method.

Assigning another value to `stream` argument in subsequent code will release the lock.

On some operating systems `flock` is implemented at the process level. When using a multithreaded server API 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 these environments.

On Windows, if the locking process opens the file a second time, it cannot access the file through this second handle until it unlocks the file.