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Pthread II

Pthread II. Outline. Join Mutex Variables Condition Variables. Join Mutex Variables Condition Variables. "Joining" is one way to accomplish synchronization between threads. Declare a pthread attribute variable of the pthread_attr_t data type

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Pthread II

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  1. Pthread II

  2. Outline • Join • Mutex Variables • Condition Variables

  3. Join • Mutex Variables • Condition Variables

  4. "Joining" is one way to accomplish synchronization between threads.

  5. Declare a pthread attribute variable of the pthread_attr_t data type • Initialize the attribute variable with pthread_attr_init() • Set the attribute detached status with pthread_attr_setdetachstate() • When done, free library resources used by the attribute with pthread_attr_destroy()

  6. int pthread_join( pthread_t tid, void **status ); • If successful, the pthread_join() function shall return zero; • otherwise, an error number shall be returned to indicate the error (the number>0)

  7. Join • Mutex Variables • Condition Variables

  8. Typical Sequence • Create and initialize a mutex variable • Several threads attempt to lock the mutex • Only one succeeds and that thread owns the mutex • The owner thread performs some set of actions • The owner unlocks the mutex • Another thread acquires the mutex and repeats the process • Finally the mutex is destroyed

  9. Creating and Destroying Mutexes • pthread_mutex_init (mutex,attr) • pthread_mutex_destroy (mutex) • pthread_mutexattr_init (attr) • pthread_mutexattr_destroy (attr)

  10. Initialize Mutex variables must be declared with type pthread_mutex_t , and must be initialized before they can be used. 1. pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER; 2. pthread_mutex_init() routine. This method permits setting mutex object attributes, attr The mutex is initially unlocked

  11. Locking and Unlocking Mutexes • pthread_mutex_lock (mutex) • pthread_mutex_trylock (mutex) • pthread_mutex_unlock (mutex)

  12. pthread_mutex_unlock() will unlock a mutex if called by the owning thread An error will be returned if: • If the mutex was already unlocked • If the mutex is owned by another thread

  13. The programmer have to to insure that the necessary threads all make the the mutex lock and unlock calls correctly • E.g. lock ….. lock Deadlock

  14. Join • Mutex Variables • Condition Variables

  15. pthread_cond_init (condition,attr) • pthread_cond_destroy (condition) • pthread_condattr_init (attr) • pthread_condattr_destroy (attr)

  16. Condition variable initialization • pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER; • pthread_cond_init() The ID of the created condition variable is returned to the calling thread through the condition parameter

  17. Waiting and Signaling on Condition Variables • pthread_cond_wait (condition,mutex) • pthread_cond_signal (condition) • pthread_cond_broadcast (condition)

  18. more than one thread is in a blocking wait state =>pthread_cond_broadcast() • It is a logical error to call pthread_cond_signal() before calling pthread_cond_wait()

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