# File: msvcrt-example-3.py

import msvcrt
import os, time

LK_UNLCK = 0 # unlock the file region
LK_LOCK = 1 # lock the file region
LK_NBLCK = 2 # non-blocking lock
LK_RLCK = 3 # lock for writing
LK_NBRLCK = 4 # non-blocking lock for writing

FILE = "counter.txt"

if not os.path.exists(FILE):
    file = open(FILE, "w")
    file.write("0")
    file.close()

for i in range(20):
    file = open(FILE, "r+")
    # lock from current position (0) to end of file
    msvcrt.locking(file.fileno(), LK_LOCK, os.path.getsize(FILE))
    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)