共享数组在 python 多处理中未正确共享

2022-01-12 00:00:00 python multiprocessing

问题描述

我正在 Python 中尝试多处理，并尝试在两个进程之间共享一个字符串数组.这是我的python代码:

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

当我运行脚本时，我看到 arr 已正确填充并在 f1() 中可用，但在 f2() 中不可用.结果如下:

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

我是否忽略了什么?

提前感谢您的反馈.:)

Thanks in advance for your feedback. :)

解决方案

我的猜测是:

arr 存储 3 个指针.f1() 将它们分配给没有意思是在当前流程之外.f2() 尝试访问此时包含垃圾的无意义地址.

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.

分配给在所有过程中都有意义的值似乎有帮助:

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()

输出

Before : [None, None, None] 0
f1 :  ['aaaa', 'bbbb', 'cccc'] 1
f2 :  ['aaaa', 'bbbb', 'cccc'] 2
After :  ['aaaa', 'bbbb', 'cccc'] 2

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