[FIXED] How to stop all processes if one of them changes a global "stop" variable to True

Issue

import multiprocessing


global stop
stop = False


def makeprocesses():
    processes = []
    for _ in range(50):
        p = multiprocessing.Process(target=runprocess)
        
        processes.append(p)
    
    for _ in range(50):
        processes[_].start()
    runprocess()


def runprocess():
    global stop
    while stop == False:
        
        x = 1 #do something here

        if x = 1:
            stop = True




makeprocesses()

while stop == True:
    x = 0
    makeprocesses()

How could I make all the other 49 processes stop if just one changes stop to True? I would think since stop is a global variable once one process changes stop all the others would stop.

Solution

No. Each process gets its own copy. It's global to the script, but not across processes. Remember that each process has a completely separate address space. It gets a COPY of the first process' data.

If you need to communicate across processes, you need to use one of the synchronization techniques in the multiprocessing documentation (https://docs.python.org/3/library/multiprocessing.html#synchronization-primitives), like an Event or a shared object.

Answered By - Tim Roberts