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问题:
I am experimenting multiprocessing in Python and tried to share an Array of strings among two processes. Here is my python code :
from multiprocessing import Process, Array, Value import ctypes def f1(a, v): for i, l in enumerate(['a', 'b', 'c']): a[i] = l*3 v.value += 1 print "f1 : ", a[:], v.value def f2(a,v): v.value += 1 print "f2 : ", a[:], v.value if __name__ == '__main__': val = Value(ctypes.c_int, 0) arr = Array(ctypes.c_char_p, 3) print "Before :", arr[:], val.value p = Process(target=f1, args=(arr, val)) p2 = Process(target=f2, args=(arr, val)) p.start() p2.start() p.join() p2.join() print "After : ", arr[:], val.value
When I run the script I see that arr is correctly populated and available in f1() but not in f2(). Here is the result:
% python /tmp/tests.py Before : [None, None, None] 0 f1 : ['aaa', 'bbb', 'ccc'] 1 f2 : ['\x01', '\x11', '\x01'] 2 After : ['\x01', '\x01', '\x01'] 2
Did I overlooked something ?
Thanks in advance for your feedback. :)
回答1:
My guess is:
arr stores 3 pointers. f1() assigns them to memory addresses that have no meaning outside current process. f2() tries to access the meaningless addresses that contain junk at this point.
Assigning to values that have meaning in all processes seems to help:
from __future__ import print_function import ctypes import time from multiprocessing import Process, Array, Value values = [(s*4).encode('ascii') for s in 'abc'] def f1(a, v): for i, s in enumerate(values): a[i] = s v.value += 1 print("f1 : ", a[:], v.value) def f2(a,v): v.value += 1 print("f2 : ", a[:], v.value) def main(): val = Value(ctypes.c_int, 0) arr = Array(ctypes.c_char_p, 3) print("Before :", arr[:], val.value) p = Process(target=f1, args=(arr, val)) p2 = Process(target=f2, args=(arr, val)) p.start() p2.start() p.join() p2.join() print("After : ", arr[:], val.value) if __name__ == '__main__': main()
Output
Before : [None, None, None] 0 f1 : ['aaaa', 'bbbb', 'cccc'] 1 f2 : ['aaaa', 'bbbb', 'cccc'] 2 After : ['aaaa', 'bbbb', 'cccc'] 2
回答2:
I don't have a solution, but I can add more hints. I've stripped the script to pinpoint the problem. It looks to me that the problem is in the l * 3 operation. I don't know why, but moving the l * 3 operation in the enumerator works as expected:
from multiprocessing import Process, Array import ctypes def f1(a): # for i, l in enumerate(['a', 'b', 'c']): # a[i] = (l * 3) for i, l in enumerate(['a' * 3, 'b' * 3, 'c' * 3]): a[i] = l print "f1 : ", map(id, a), a[:] if __name__ == '__main__': arr = Array(ctypes.c_char_p, 3) print "Before :", map(id, arr), arr[:] p = Process(target=f1, args=(arr, )) p.start() p.join() print "After : ", map(id, arr), arr[:]
Result:
Before : [3077673516L, 3077673516L, 3077673516L] [None, None, None] f1 : [3073497784L, 3073497784L, 3073497784L] ['aaa', 'bbb', 'ccc'] After : [3073497784L, 3073497784L, 3073497784L] ['aaa', 'bbb', 'ccc']
