I am using a Python3 program to create two processes: the child born out of the parent process. Everything starts with the parent process. The motive is to start a process, give it the live sensor data, and end it when the sensor reading passes a certain threshold.
The sensor_data is obtained through a thread in the parent process as:
t = Thread(target=dump_sensor_data,args=())
t.start()
And I have tested that the live data can be used in any function as
while True:
global sensor_data
print(sensor_data)
The child process is defined as:
def fa():
global sensor_data
while sensor_data>300:
print(f'I am printing {sensor_data} from fa')
time.sleep(0.7)
I start it as:
processes_list=[]
pa=Process(target=fa, args=())
pa.start()
processes_list.append(pa)
Now, I create another watch-guard function that kills the child process fa() as soon as the sensor_data exceeds 300.
def watchguard():
global sensor_data
while True:
print(lidar1)
if sensor_data > 300:
for p in processes_list:
print(f'terminating process {p}')
p.terminate()
p.join()
return
But this is what happens
0
terminating process <Process(Process-1, started)>
The 0 is the wrong value of sensor_data which I have verified and should be above 1000. Also, this leads to the process getting killed.
Why is the data not getting passed to this child process ?
Ok, I got it.
As Python multiprocessing does not allow global variables to be passed between processes, the variable was never sent or updated. This is not the case with multithreading.