（摘抄） 源码分析multiprocessing的Value Array共享内存原理

原文地址：

http://xiaorui.cc/archives/3290

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摘抄内容：

接着粗略的聊聊multiprocessing共享内存的实现方法.

multiprocessing提前设定的ctypes映射表，这样对你来说只需要传递符号就可以了。

typecode_to_type = {
    'c': ctypes.c_char,  'u': ctypes.c_wchar,
    'b': ctypes.c_byte,  'B': ctypes.c_ubyte,
    'h': ctypes.c_short, 'H': ctypes.c_ushort,
    'i': ctypes.c_int,   'I': ctypes.c_uint,
    'l': ctypes.c_long,  'L': ctypes.c_ulong,
    'f': ctypes.c_float, 'd': ctypes.c_double
    }

下面的代码通过mmap ctypes实现了一个简单的数据共享, 这样对于我们来说，可以像操作python对象那样操作映射的内存地址

(经过修改后的Python3.7代码)

###  a.py

import ctypes
import mmap
import os
import struct
import time

def main():
    fd = os.open('/tmp/mmaptest', os.O_CREAT | os.O_TRUNC | os.O_RDWR)

    assert os.write(fd, b'\x00' * mmap.PAGESIZE) == mmap.PAGESIZE

    buf = mmap.mmap(fd, 0, mmap.MAP_SHARED, mmap.PROT_WRITE)

    i = ctypes.c_int.from_buffer(buf)

    # Before we create a new value, we need to find the offset of the next free
    # memory address within the mmap
    # print(offset, ctypes.sizeof(i))  # byte length
    offset = struct.calcsize(i._type_)

    # The offset should be uninitialized ('\x00')
    #assert buf[offset] == b'\x00'
    #print(type(buf[offset]))
    assert buf[offset] == 0x00

    # Now ceate a string containing 'foo' by first creating a c_char array
    s_type = ctypes.c_char * len('foot')

    # Now create the ctypes instance
    s = s_type.from_buffer(buf, offset)

    s.raw = b'foot'
    i.value = 10

    i.value *= i.value
    print( 'Changing i : {}'.format(i.value) )

    s.raw = b'bara'
    print( 'Changing s : {}'.format(s.raw) )

if __name__ == '__main__':
    main()

    time.sleep(60)

运行结果：

### b.py

import mmap
import os
import struct
import time

def main():
    # Open the file for reading
    fd = os.open('/tmp/mmaptest', os.O_RDONLY)

    # Memory map the file
    buf = mmap.mmap(fd, 0, mmap.MAP_SHARED, mmap.PROT_READ)

    i = None
    s = None

    new_i, = struct.unpack('i', buf[:4])
    new_s, = struct.unpack('4s', buf[4:8])

    if i != new_i or s != new_s:
        print( 'i: %s => %d' % (i, new_i) )
        print( 's: %s => %s' % (s, new_s) )
        i = new_i
        s = new_s

if __name__ == '__main__':
    main()

运行结果：