Question

I’m wondering if it’s possible to lock multiple mutexes at the same time, like:

 Mutex1.Lock();
 {
     Mutex2.Lock();
     {
          // Code locked by mutex 1 and 2.
     }
     Mutex2.Unlock();

     // Code locked by mutex 1.
 }
 Mutex1.Unlock();

It would be very useful for some situations. Thanks.

Answer 1

It is possible but the order of locking must be consistent throughout the application otherwise deadlock is a likely result (if two threads acquire the locks in opposite order then each thread could be waiting on the other to release one of the locks).

Recommend using a scoped lock and unlock facility for exception safety, to ensure locks are always released (std::lock_guard with std::mutex for example):

std::mutex mtx1;
std::mutex mtx2;

std::lock_guard<std::mutex> mtx1_lock(mtx1);
{
    std::lock_guard<std::mutex> mtx2_lock(mtx2);
    {
    }
}

If your compiler does not support these C++11 features boost has similar in boost::mutex and boost::lock_guard.