Python multiprocessing exits without exception when accessing shared value. Is this normal?

Question

I am trying to share a string between processes with multiprocessing.Value(c_wchar_p, ""). Program exits unexpectedly without exception with code (0xC0000005). I am guessing that problem is somehow caused by main process trying to access memory of child process. Issue occurs only when ctypes c_char_p/c_wchar_p are used. Thread works fine. Is this expected beheavor?

Steps to reproduce:

import multiprocessing
import threading
import ctypes
import time


class ProcessExample:

    def __init__(self, proc: bool):
        self.shared_bool = multiprocessing.Value(ctypes.c_bool, False)
        self.shared_string = multiprocessing.Value(ctypes.c_wchar_p, "one")
        self.shared_byte = multiprocessing.Value(ctypes.c_char_p, b"one")
        if proc:
            self.th_proc = multiprocessing.Process(target=self.target, args=(self.shared_bool, self.shared_string, self.shared_byte))
        else:
            self.th_proc = threading.Thread(target=self.target, args=(self.shared_bool, self.shared_string, self.shared_byte))
        self.th_proc.start()

    @staticmethod
    def target(shared_bool, shared_string, shared_byte):
        shared_bool.value = True
        shared_string.value = "two"
        shared_byte.value = b"two"

    def print_values(self):
        self.th_proc.join()
        print(self.shared_bool.value)
        print(self.shared_string.value)  # Exits
        print(self.shared_byte.value)  # Exits


if __name__ == "__main__":
    # example = ProcessExample(False)  # Works
    example = ProcessExample(True)
    time.sleep(1)
    example.print_values()

Output

True

Process finished with exit code -1073741819 (0xC0000005)

Asked By: Jan H

||

Source

Answer 1

I think the problem is in multiprocessing.Value(ctypes.c_wchar_p, "one") and multiprocessing.Value(ctypes.c_char_p, b"one"). To have string/byte string initialized correctly, use multiprocessing.Array:

import multiprocessing
import threading
import ctypes
import time


class ProcessExample:
    def __init__(self, proc: bool):
        self.shared_bool = multiprocessing.Value(ctypes.c_bool, False)
        self.shared_string = multiprocessing.Array(ctypes.c_wchar, "one")
        self.shared_byte = multiprocessing.Array(ctypes.c_char, b"one")
        if proc:
            self.th_proc = multiprocessing.Process(
                target=self.target,
                args=(self.shared_bool, self.shared_string, self.shared_byte),
            )
        else:
            self.th_proc = threading.Thread(
                target=self.target,
                args=(self.shared_bool, self.shared_string, self.shared_byte),
            )
        self.th_proc.start()

    @staticmethod
    def target(shared_bool, shared_string, shared_byte):
        shared_bool.value = True
        shared_string[:] = "two"
        shared_byte[:] = b"two"

    def print_values(self):
        self.th_proc.join()
        print(self.shared_bool.value)
        print(self.shared_string[:])
        print(self.shared_byte[:])


if __name__ == "__main__":
    example = ProcessExample(True)
    time.sleep(1)
    example.print_values()

Prints:

True
two
b'two'

Answered By: Andrej Kesely

Python multiprocessing exits without exception when accessing shared value. Is this normal?

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