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# flock

(PHP 3>= 3.0.7, PHP 4 )

flock -- Portable advisory file locking

## Description

bool flock ( int fp, int operation [, int &wouldblock])

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).

flock() operates on `fp` which must be an open file pointer. `operation` is one of the following values:

- To acquire a shared lock (reader), set `operation` to LOCK_SH (set to 1 prior to PHP 4.0.1).

- To acquire an exclusive lock (writer), set `operation` to LOCK_EX (set to 2 prior to PHP 4.0.1).

- To release a lock (shared or exclusive), set `operation` to LOCK_UN (set to 3 prior to PHP 4.0.1).

- If you don't want flock() to block while locking, add LOCK_NB (4 prior to PHP 4.0.1) to `operation`.

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 optional third argument is set to `TRUE` if the lock would block (EWOULDBLOCK errno condition)

flock() returns `TRUE` on success and `FALSE` on error (e.g. when a lock could not be acquired).

> Note: 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()).

Warning
flock() will not work on NFS and many other networked file systems. Check your operating system documentation for more details. On some operating systems flock() is implemented at the process level. When using a multithreaded server API like ISAPI 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 this environments (this is especially true for Windows 98 users).

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