Introduction to synchronization using mutexes

• if multiple threads are simply reading the same variable, there is no problem.

• however, if multiple threads update the same variable, the order of those updates - the order in which the threads execute - is important as it affects the values of the variable, over time, and the final value of that variable.

Thread libraries support an abstraction named a mutex, an abbreviation of mutual exclusion. Mutex variables are one of the primary means providing synchronization of threads - the order in which they execute - and for protecting shared data when multiple writes can occur.

A mutex variable acts like a lock protecting access to a shared data resource.

The basic concept of a mutex as used in pthreads is that only one thread can lock (or own) a mutex variable at any given time.

Thus, even if several threads try to lock a mutex only one thread will be successful. No other thread can own that mutex until the owning thread unlocks that mutex. Threads must take turns accessing protected data.