import logging
import multiprocessing
import multiprocessing.sharedctypes

"""Test showing that multiprocessing.Value instances are not synchronized.

Following tests performed on 2013-01-19.

Note that on a single CPU machine the error is not reported for every
execution but only for about every third run.
$ python --version
Python 2.6.5
$ uname -a
Linux ubuntu-desktop 2.6.32.11+drm33.2 #2 Fri Jun 18 20:30:49 CEST 2010 i686 GNU/Linux
$ python test_multiprocessing.py
ERROR:root:Unexpected value: 99 (expected: 100)


On a quadcore, the error occurs almost every time.
$ uname -a
Linux PC 2.6.35.13 #4 SMP Tue Dec 20 15:22:02 CET 2011 x86_64 GNU/Linux
~/local/Python-2.7.3/python test_multiprocessing.py
ERROR:root:Unexpected value: 95 (expected: 100)
~/local/Python-3.3.0/python test_multiprocessing.py
ERROR:root:Unexpected value: 86 (expected: 100)
"""


def do_inc(integer):
    """Increment integer.value for multiprocessing.Value integer."""
    integer.value += 1

def do_test(notasks):
    """Create notasks processes, each incrementing the same Value.

    As the Value is initialized to 0, its final value is expected to be
    notasks.
    """
    tasks = list()
    integer = multiprocessing.sharedctypes.Value("i", 0)
    for run in range(notasks):
        proc = multiprocessing.Process(target=do_inc, args=(integer,))
        proc.start()
        tasks.append(proc)
    for proc in tasks:
        proc.join()
    if integer.value != notasks:
        logging.error(
            "Unexpected value: %d (expected: %d)", integer.value, notasks)

if __name__ == "__main__":
    do_test(100)