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")
    file.write("0")
    file.close()

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.seek(0)
    file.write(str(counter))
    file.close() # unlocks the file
    print os.getpid(), "=>", counter
    time.sleep(0.1)

## 30940 => 1
## 30942 => 2
## 30941 => 3
## 30940 => 4
## 30941 => 5
## 30942 => 6