How to stop all processes if one of them changes a global "stop" variable to True-CodePudding

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.

CodePudding user response：

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.

CodePudding user response：

Whenever you want to synchronise threads you need some shared context and make sure it is safe. as @Tim Roberts mentioned These can be taken from (https://docs.python.org/3/library/multiprocessing.html#synchronization-primitives)

Try something like this:

import multiprocessing
from multiprocessing import Event
from time import sleep


def makeprocesses():
    processes = []
    e = Event()

    for i in range(50):
        p = multiprocessing.Process(target=runprocess,args= (e,i))
        p.start()
        processes.append(p)

    for p in processes:
        p.join()


def runprocess(e: Event() = None,name = 0):
   while not e.is_set():
        sleep(1)
        if name == 1:
            e.set()  # here we make all other processes to stop
   print("end")


if __name__ == '__main__':
    makeprocesses()

My favorite way is using cancelation token which is a object wrapping what we did here