Semaphore in Python

Question

I'm trying to implement mutual exclusion using semaphore in Python. The two processes (proc1, proc2) are supposed to be two independent, concurrent processes. They do exactly the same thing: store n in array[n], then increment n.

The purpose of the program is to show that using semaphore we can ensure that the array is filled properly: [0,1,2,3,4,5,6,7,8,9], without skipping any index. However, my code seems to store [0,1,0,0,0,0,0,0,0,0]. I haven't used threads in python before, so I don't know what's going on.

import threading
import time

n = 0
array = [0]*10
sem = threading.Semaphore()

def proc1():
  global n, array
  while True:
    sem.acquire()
    array[n] = n
    n += 1
    sem.release()
    time.sleep(0.25)

def proc2():
  global n, array
  while True:
    sem.acquire()
    array[n] = n
    n += 1
    sem.release()
    time.sleep(0.25)


t = threading.Thread(target = proc1)
t.start()
t2 = threading.Thread(target = proc2)
t2.start()

print (array)

Zeh · Accepted Answer · 2018-03-20 12:51:46Z

3

the problem was that the OP tried to print the result before the threads were done. He should have waited for join.

import threading
import time

n = 0
array = [0]*10
sem = threading.Semaphore()

def proc(num):
  global n
  while True:
    sem.acquire()
    n = n+1
    sem.release()
    if n > 9:
        break
    array[n] = n
    print ("Thread {}: {}".format(num,array))
    time.sleep(0.25)

t1 = threading.Thread(target = proc, args=[1])
t2 = threading.Thread(target = proc, args=[2])
t1.start()
t2.start()
t1.join()
t2.join()

edited Mar 20, 2018 at 12:51

answered Mar 20, 2018 at 10:46

Zeh

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2 Comments

TTT Over a year ago

This doesn't exactly do what OP is expecting. He should be printing the list in the thread worker function since he wants to show that the threads are synchronized at every index

Zeh Over a year ago

rpg711 was right, I updated the answer to what I believe the OP intended.

Dan Leonard · Accepted Answer · 2022-04-09 20:40:43Z

Different take on a Semaphore pattern, handing the "tasks" within the Sempahore itself

class Sempahore:
    def __init__(self, max_threads):
        self.active_threads = 0
        self.max_threads = max_threads
        self.tasks = []

    def add_task(self, func, args):
        self.tasks.append(
            Task(
                func=func,
                args=args
            )
        )

    def run_task(self, task: Task):
        _func = task.func
        _args = task.args

        self.active_threads += 1
        _func(*_args)
        self.active_threads -= 1

    def run(self, blocking=False):
        if blocking:
            self._run()
        else:
            t = Thread(target=self._run)
            t.start()

    def _run(self):
        while True:
            if self.active_threads < self.max_threads:
                task = self.tasks.pop()
                logger.info(f'starting task: {task.task_id}')
                t = Thread(
                    target=self.run_task,
                    args=(task,))
                t.start()
            if len(self.tasks) == 0:
                break

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