multiproccesing and error The process has forked and you cannot use this CoreFoundation functionality safely

Question

I'm having this error when calling my function start and using the multiprocessing module on Python 2.7.8 . I'm using a mac OS 10.9.5.

The process has forked and you cannot use this CoreFoundation functionality safely. 
You MUST exec(). Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_
COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.

Here is the code, under the classe Lattice. My function sansfin is working well and is returning a boolean, it only take as an argument self and an integer to loop on.

    def start(self):
        if __name__ == '__main__':
            self.run = True
            p = Process(target=self.sansfin, args=(1000,))
            p.start()
            p.join()

    def stop(self):
        self.run = False

I am quite lost with this message. I don't have found anything helpful there and elsewhere. Some are suggesting a bug ...

the `if __name__ == '__main__':` should not be used inside a class. It allows you to define what is importable and executable in a script — CoMartel, Jun 05 '15 at 15:00
Thanks but if I put it in my mai I have the same problem `def start(L): if __name__ == '__main__': L.run = True p = Process(target=L.sansfin, args=(1000,)) p.start() p.join() def stop(L): L.run = False` — InfiniteLooper, Jun 05 '15 at 15:09

score 6 · Answer 1 · answered Jun 27 '20 at 20:45

To fix this error, you need to explicitly set the multiprocessing start method to spawn on MacOS. This can be achieved by adding the following immediately below if __name__ == '__main__'. For example:

import platform, multiprocessing
...
if __name__ == '__main__':
    if platform.system() == "Darwin":
        multiprocessing.set_start_method('spawn')

See Also:

score 0 · Answer 2 · edited May 23 '17 at 10:28

You should write your script in 2 parts : your function/classes first, and a second part where you create the instances you need and call the functions you want.

The if __name__ == '__main__': is here to split these 2 parts. As you put it in your example, I think every process will try to launch itself, and will fork forever, leading to this error.

Here is what your code should look like:

class Lattice:
    def __init__(self):
        # init what you need

    def start(self):
        self.run = True
        p = Process(target=self.sansfin, args=(1000,))
        p.start()
        p.join()

    def stop(self):
        self.run = False

if __name__ == '__main__':
    lattice_instance=Lattice()
    lattice_instance.start()
    # wait the time you need
    lattice_instance.stop()

Let me know if this help with your issue.

You can also find a very good explanation on the main here : What does if __name__ == "__main__": do?

multiproccesing and error The process has forked and you cannot use this CoreFoundation functionality safely

2 Answers2

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