pthread

 

https://computing.llnl.gov/tutorials/pthreads/

Routines:

      pthread_mutex_init (mutex,attr)

      pthread_mutex_destroy (mutex)

      pthread_mutexattr_init (attr)

      pthread_mutexattr_destroy (attr)

Usage :

Mutex variables must be declared with type pthread_mutex_t, and must be initialized before they can be used. There are two ways to initialize a mutex variable:

   1. Statically, when it is declared. For example:
      pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

   2. Dynamically, with the pthread_mutex_init() routine. This method permits setting mutex object attributes, attr.

The mutex is initially unlocked.

# The attr object is used to establish properties for the mutex object, and must be of type pthread_mutexattr_t if used (may be specified as NULL to accept defaults). The Pthreads standard defines three optional mutex
attributes:

    * Protocol: Specifies the protocol used to prevent priority inversions for a mutex.
    * Prioceiling: Specifies the priority ceiling of a mutex.
    * Process-shared: Specifies the process sharing of a mutex.

 

Note that not all implementations may provide the three optional mutex attributes.

# The pthread_mutexattr_init() and pthread_mutexattr_destroy() routines are used to create and destroy mutex attribute objects respectively.

# pthread_mutex_destroy() should be used to free a mutex object which is no longer needed.

 

Locking and Unlocking Mutexes

Routines:

      pthread_mutex_lock (mutex)

      pthread_mutex_trylock (mutex)

      pthread_mutex_unlock (mutex)

 

Usage:

    * The pthread_mutex_lock() routine is used by a thread to acquire a lock on the specified mutex variable. If the mutex is already locked by another thread, this call will block the calling thread until the mutex is unlocked.

    * pthread_mutex_trylock() will attempt to lock a mutex. However, if the mutex is already locked, the routine will return immediately with a "busy" error code. This routine may be useful in preventing deadlock conditions, as
in a priority-inversion situation.

    * pthread_mutex_unlock() will unlock a mutex if called by the owning thread. Calling this routine is required after a thread has completed its use of protected data if other threads are to acquire the mutex for their work
with the protected data. An error will be returned if:
          o If the mutex was already unlocked
          o If the mutex is owned by another thread

Question: When more than one thread is waiting for a locked mutex, which thread will be granted the lock first after it is released?

 

Answer : Unless thread priority scheduling(not covered) is used, the assignment will be left to the native system scheduler and appear to be more or less random.