How to trigger a thread to start working using semaphore?

Question

How to trigger a thread to start working using semaphore?

I have find a lot of explanation about thread and semaphore but i still don't understand, any help please

Answer 1

When using semaphores or locks, you don't do anything else than acquiring the semaphore (resp. the lock), and then do what you want. For example, if you want to use a lock, then you typically have two threads

# thread 1
lock.acquire()
# here is all the work for thread 1
lock.release()

and the other thread is completely identical:

# thread 2
lock.acquire()
# here is all the work for thread 2
lock.release()

Now, the first to obtain the lock gets to run the code first, and the other thread needs to wait until the lock is released.

Here's an example of a Python program that prints a series of 0s and 1s depending on the order of execution:

import time
import threading

lock = threading.Lock()
lst = []


def f1():
    for i in range(10):
        lock.acquire()
        lst.append("0")
        lock.release()
        time.sleep(0.1)


def f2():
    for i in range(10):
        lock.acquire()
        lst.append("1")
        lock.release()
        time.sleep(0.1)


t1 = threading.Thread(target=f1)
t2 = threading.Thread(target=f2)
t1.start()
t2.start()
t1.join()
t2.join()

print("".join(lst))

Feel free to run it in your own terminal.

(Note that I used lock.acquire() and lock.release(), but the "correct" way is to use a context manager and use with lock: ....)

See also: Lock, mutex, semaphore… what's the difference?