Will this make the object thread-safe?

Question

I have a native Visual C++ COM object and I need to make it completely thread-safe to be able to legally mark it as "free-threaded" in th system registry. Specifically I need to make sure that no more than one thread ever accesses any member variable of the object simultaneously.

The catch is I'm almost sure that no sane consumer of my COM object will ever try to simultaneously use the object from more than one thread. So I want the solution as simple as possible as long as it meets the requirement above.

Here's what I came up with. I add a mutex or critical section as a member variable of the object. Every COM-exposed method will acquire the mutex/section at the beginning and release before returning control.

I understand that this solution doesn't provide fine-grained access and this might slow execution down, but since I suppose simultaneous access will not really occur I don't care of this.

Will this solution suffice? Is there a simpler solution?

Answer 1

If you are certain that actual concurrent access is not going to happen in practice, then mutexing the entire execution is not an unreasonable approach.

Answer 2

This solution should work, but I'd recommend mutexes over critical sections as they handle time-outs, which provide some level of fall back in case of deadlock. You also want to be very careful that a function locking a mutex does not call another function that has already locked the same mutex in the same thread. This shouldn't be a problem for your COM interface, so long as you don't add extra functionality on top of your mutex to the interface. You could hit issues if the COM includes call backs.