Os Thread Condition

In this tutorial, you'll learn about How to Fix Condition Variable Errors. We cover key concepts, practical examples, and best practices.

Fix condition variable errors when spurious wakeup not handled or notify without state change.

Quick Fix

Wrong

import threading
cv=threading.Condition(); ready=False
def waiter():
    with cv:
        cv.wait()  # spurious wakeup may occur!
        print('Proceed')  # assumes ready is True but may not be!

wait() may return without notify() being called (spurious wakeup). Proceed without ready being True.

Right

import threading
cv=threading.Condition(); ready=False
def waiter():
    with cv:
        while not ready:  # always loop, never if!
            cv.wait()
        print('Proceed')
def signaler():
    global ready
    with cv:
        ready=True
        cv.notify()
threading.Thread(target=waiter).start()
threading.Thread(target=signaler).start()

'Proceed' printed only when ready=True and notified. Spurious wakeup re-checks condition.

Prevention

Always use while loop for condition check, not if. Spurious wakeups can occur.

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