2.3.1.2 Recursive Mutex

A recursive mutex can be locked more than once by a given thread without causing a deadlock. The thread must call the pthread_mutex_unlock routine the same number of times that it called the pthread_mutex_lock routine before another thread can lock the mutex. Recursive mutexes have the notion of a mutex owner. When a thread successfully locks a recursive mutex, it owns that mutex and the lock count is set to 1. Any other thread attempting to lock the mutex blocks until the mutex becomes unlocked. If the owner of the mutex attempts to lock the mutex again, the lock count is incremented, and the thread continues running. When an owner unlocks a recursive mutex, the lock count is decremented. The mutex remains locked and owned until the count reaches zero. It is an error for any thread other than the owner to attempt to unlock the mutex.

A recursive mutex is useful if a thread needs exclusive access to a piece of data, and it needs to call another routine (or itself) that needs exclusive access to the data. A recursive mutex allows nested attempts to lock the mutex to succeed rather than deadlock.

This type of mutex is called recursive because it allows you a capability not permitted by a normal (default) mutex. However, its use requires more careful programming. A recursive mutex should never be used with condition variables, because the unlock performed for a pthread_cond_wait or pthread_cond_timedwait might not actually release the mutex. In that case, no other thread can satisfy the condition of the predicate, and the thread waits indefinitely. See Section 2.3.2 for information on the condition variable wait and timed wait routines.