The fcntl module

(Unix only). This module provides an interface to the ioctl and fcntl functions on Unix. They are used for “out of band” operations on file handles and I/O device handles. This includes things like reading extended attributes, controlling blocking, modifying terminal behavior, etc.

Exactly how to use these functions are highly platform dependent. For more information on what you can do on your platform, check the corresponding Unix manual pages.

This module also provides an interface to Unix’ file locking mechanisms. The following example uses the flock function to place an advisory lock on the file, while it is being updated.

The output shown below was obtained by running three instances of the program in parallel, like this: python python python (all on one command line). If you comment out the call to flock, the counter will not be updated properly.

Example: Using the fcntl module for locking
# File:

import fcntl, FCNTL
import os, time

FILE = "counter.txt"

if not os.path.exists(FILE):
    # create the counter file if it doesn't exist
    file = open(FILE, "w")

for i in range(20):
    # increment the counter
    file = open(FILE, "r+")
    fcntl.flock(file.fileno(), FCNTL.LOCK_EX)
    counter = int(file.readline()) + 1
    file.close() # unlocks the file
    print os.getpid(), "=>", counter
$ python
30940 => 1
30942 => 2
30941 => 3
30940 => 4
30941 => 5
30942 => 6

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