Should I use different mutexes for different objects?

Question

I am new to threading . Correct me if I am wrong that mutex locks the access to a shared data structure so that it cannot be used by other threads until it is unlocked . So, lets consider that there are 2 or more shared data structures . So , should I make different mutex objects for different data structures ? If no ,then how std::mutex will know which object it should lock ? What If I have to lock more than 1 objects at the same time ?

Answer 1

There are several points in your question that can be made more precise. Perhaps clearing this will solve things for you.

To begin with, a mutex, by itself, does not lock access to anything. It is basically something that your code can lock and unlock, and some "magic" ensures that only one thread can lock it at a time.

If, by convention, you decide that any code accessing some data structure foo will first begin by locking a mutex foo_mutex, then it will have the effect of protecting this data structure.

So, having said that, regarding your questions:

• It depends on whether the two data structures need to be accessed together or not (e.g., can updating one without the other leave the system in an inconsistent state). If so, you should lock them with a single mutex. If not, you can improve parallelism by using two.

• The mutex does not lock anything. It is you who decide by convention whether you can access 1, 2, or a million data structures, while holding it.

Answer 2

If you always needs to access both structures then it could be considered as a single resource so only a single lock is needed.

If you sometimes, even just once, need to access one of the structures independently then they can no longer be considered a single resource and you might need two locks. Of course, a single lock could still be sufficient, but then that lock would lock both resources at once, prohibiting other threads from accessing any of the structures.