Question

I’ve a situation like this:

The main process generate some sub-process that they should write the result in a shared object in string and numeric types, for the numeric types there’s no problem but with the string the value will be lost.

import multiprocessing as mp
from ctypes import Structure, c_double, c_wchar_p, c_int

# shared obj class
class SharedObj(Structure):
    _fields_ = [('name', c_wchar_p), ('val', c_double) ]

def run_mp( values , lock , s ) :
    for i in range( s , len( values ) , 2 ):
        lock.acquire()
        values[i].name = str( i ) # write the string value in the shared obj
        values[i].val = float( i )
        print( "tmp: %d" % i )
        lock.release()

def main():
    # creating the shared obj and mutex
    values = mp.Array(  SharedObj , [SharedObj() for i in range( 10 )] )
    lock_j = mp.Lock()

    # creating two sub-process form the function run_mp
    p1 = mp.Process( target=run_mp , args=( values , lock_j , 0 ))
    p2 = mp.Process( target=run_mp , args=( values , lock_j , 1 ))

    p1.start()
    p2.start()

    p1.join()
    p2.join()

    for v in  values:
        print()
        print( "res name: %s" % v.name )
        print( "res val: %f " % v.val )


if __name__ == '__main__':
    main()

As a result the field in the shared object containing the c_double is written in the field, but the string generated in the sub-processes rum-mp ( string values[i].name = str( i ) ) will be lost in the main process.

There is a method for saving the strings generated in sub-process?

The output of this code looks like:

Where the resulting string in the main process are completely random.

tmp: 0
tmp: 2
tmp: 3
tmp: 4

res name: ����羍����羍
res val: 0.000000
res name: ����羍����羍
res val: 1.000000
res name:
res val: 2.000000   ....

Answer 1

Here is a slightly modified version of your code:

#!/usr/bin/env python

import multiprocessing as mp


def run_mp( values ):
    for c_arr, c_double in values:
        c_arr.value = 'hello foo'
        c_double.value = 3.14

def main():
    lock = mp.Lock()
    child_feed = []
    for i in range(10):
        child_feed.append((
            mp.Array('c', 15, lock = lock),
            mp.Value('d', 1.0/3.0, lock = lock)
        ))

    p1 = mp.Process( target=run_mp , args=(child_feed,))
    p2 = mp.Process( target=run_mp , args=(child_feed,))

    p1.start()
    p2.start()

    p1.join()
    p2.join()

    for c_arr, c_double in child_feed:
        print()
        print( "res name: %s" % c_arr.value )
        print( "res val: %f " % c_double.value )


if __name__ == '__main__':
    main()

Take a look at http://docs.python.org/library/multiprocessing.html There is an example of using Array of chars.

There is also mmap module alowing to share memory http://docs.python.org/library/mmap.html but with this you have to Sync access yourself possibly by semaphores. If you like more simple approach just use pipes.