线程如何获取互斥锁？

Question

My question here is about a thread acquiring a mutex lock.

We use pthread_mutex_t locks to ensure sequential access to variables, when being read/written in a multi-threaded program. But pthread_mutex_t lock ; is also a global variable shared between threads. Will accessing of this global variable (the lock) be the same as accessing other global variables? If it is the same, then won't we face the same problem that we will face for other global variables? And if not, then how accessing a lock data structure different from accessing other global variables?

In short: Why don't we need to lock access to locks themselves and so end up in a chicken-and-egg problem?

Answer 1

You don't really access pthread_mutex_t variables directly, you access them with some given functions ( pthread_mutex_lock for example).

These functions are built so that there is no concurrent access to these structures, either by the use of some test-and-set instruction ( http://en.wikipedia.org/wiki/Test-and-set ) or by using some atomic portion of code ( http://en.wikipedia.org/wiki/Linearizability ).