Creating and Destroying Mutexes

The mtx_init() function initializes the mutex specified by mtx. The following types of mutexes are valid and may be specified by the type argument:

mtx_plain

A simple, intra-process mutex.

mtx_timed

A simple, intra-process mutex, which allows timed locking operations.

mtx_plain | mtx_recursive

An intra-process mutex that may be acquired recursively by the same thread. It must be unlocked an equal number of times that it is locked.

mtx_timed | mtx_recursive

An intra-process mutex that supports timed locking operations and may be acquired recursively by the same thread. It must be unlocked an equal number of times that it is locked.

The mtx_destroy() function destroys the mutex pointed to by mtx. It is illegal for threads to be blocked waiting for mtx when mtx_destroy() is called .

Locking and Unlocking Mutexes

The mtx_lock() function attempts to lock the mutex mtx. When the function returns, it will be the sole owner of the mutex and must call mtx_unlock() when it is done, or risk inducing a deadlock in the process. Other threads that make calls to mtx_lock() after another thread has successfully completed its call to mtx_lock() will block. When they finally return, then they will have obtained the mutex mtx.

Unless a lock of type mtx_recursive was created, a thread calling mtx_lock() when it already holds mtx will cause the thread to deadlock. Otherwise, the lock will be successfully taken again. However, a thread must call mtx_unlock() for each time that it has acquired mtx.

The mtx_trylock() function will attempt to obtain the mutex pointed to by mtx. However, unlike mtx_lock(), if mtx is locked, then it will not block and wait for mtx and instead return thrd_busy to indicate that the lock is currently held.

The mtx_timedlock() function attempts to obtain the mutex pointed to by mtx. If it is unable to obtain it, then it will block for a set amount of time dictated by ts. The timeout in ts is treated as an absolute time in UTC to block until, measured based on the CLOCK_REALTIME clock.

The mtx_unlock() function unlocks the mutex pointed to by mtx, which allows another thread the opportunity to obtain it. If any threads are actively blocking on the mutex, one of them will obtain it and be woken up. It is an error to call mtx_unlock() on a mutex which the calling thread does not currently own.