Pthread Locks

This lesson gives a brief introduction to locks in the POSIX library.

The name that the POSIX library uses for a lock is a mutex, as it is used to provide mutual exclusion between threads, i.e., if one thread is in the critical section, it excludes the others from entering until it has completed the section. Thus, when you see the following POSIX threads code, you should understand that it is doing the same thing as above (we again use our wrappers that check for errors upon lock and unlock):

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pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
Pthread_mutex_lock(&lock); // wrapper; exits on failure
balance = balance + 1;
Pthread_mutex_unlock(&lock);

You might also notice here that the POSIX version passes a variable to lock and unlock, as one may be using different locks to protect different variables. Doing so can increase concurrency: instead of one big lock that is used any time any critical section is accessed (a coarse-grained locking strategy), one will often protect different data and data structures with different locks, thus allowing more threads to be in locked code at once (a more fine-grained approach).

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