Qt example: no mutex lock when reading, why?

Question

I am looking at this example, and it raises a queston for me: when looking at this file a mutex is being used when accessing the global variable m_abort when its being changed. But when the run method reads from it, it does nothing with the mutex at all. Why?

Answer 1

Well, I cannot see the purpose of the mutex at all. Both reading and writing m_abort can be performed without mutex protection as it's a bool variable, and its value is changed atomically (at least on most platforms). And even if the write itself is not atomic and the variable can have an inconsistent value - it can be either false or true, and it will eventually be read as true (since the only value that is written there is true ).

Answer 2

Will work most of the time, but isn't the correct thing to do. It should be locked.

Also, check Is Mutex required for 1 byte shared memory for further information.