CODEWITHSUNDEEP
pythonpython-3.xmultithreadingmutex
Amine Harbaoui
Amine Harbaoui

Reputation: 1295

Is this the correct way to use variable between 2 Threads?

I have a script that read data from Serial Port, so I have an infinite loop that always fill my data to global variable, and also I schedule a function that run every X seconds to post in the database, and this function also use the same global variable.

Here's a small example I create it to show you my situation :

import serial
import schedule
import threading

shared_var = []

def save_to_db():
    print(threading.current_thread())
    global shared_var
    for l in shared_var:
        print(l)

    shared_var.clear()

def run_threaded(job_func):
    job_thread = threading.Thread(target=job_func)
    job_thread.start()

ser = serial.Serial()  # initialize the serial
ser.baudrate = 115200  # set the baud rate : default 115200
ser.port = "/dev/ttyUSB0"  # set the port to use
ser.timeout = 30
ser.write_timeout = None

if not ser.is_open:
    ser.open()  # Open port

ser.write(b'scan=01\r\n')  # Stop scan if already started
schedule.every(5).seconds.do(run_threaded, save_to_db)

while 1:
    schedule.run_pending()
    line = ser.readline()
    shared_var.append(line)
    print(threading.current_thread())

Is this code can cause a problem ? more specific what will happen if the MainThread (the one that read from Serail Port and write to shared_var) write to the shared variable between the 2 thread and in the same moment the other Thread read from the variable, is This will cause a problem because the 2 threads gonna access the same global variable in the same time ? and if yes It's a problem should I use mutex mechanism for that ?

Upvotes: 1

Views: 72

Answers (1)

Hcetipe
Hcetipe

Reputation: 466

Yes it is sure that you will have a problem if 2 processes affect the same variable at the same time.

To overcome this you must use threading.Lock() (this is Threading's mutex system).

lock = threading.Lock()
lock.acquire()
try:
    yourVariable += 1
finally:
    lock.release()

I hope I helped you.

Upvotes: 1

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