If you want to use the PTHREAD_XXX_INITIALIZER
macros you should use them in the variable declaration. Also use PTHREAD_COND_INITIALIZER
for condition variables:
// Locks & Condition Variables
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; // Lock shared resources among theads
pthread_cond_t full = PTHREAD_COND_INITIALIZER; // Condition indicating queue is full
pthread_cond_t empty = PTHREAD_COND_INITIALIZER; // Condition indicating queue is empty
Don't use those macros to initialize the mutex or condition variable later. If you need to do it later (for example if the object is dynamically allocated), use the appropriate init function:
pthread_mutex_init( &lock, NULL);
pthread_cond_init( &full, NULL);
pthread_cond_init( &empty, NULL);
To check a condition variable you must use a loop in order to avoid spurious unblocks and you must lock the mutex when:
- checking the condition
- changing the state that indicates the current condition
- calling
pthread_cond_wait()
So whatever is waiting for an is-empty condition might look like:
pthread_mutex_lock(&lock);
while (!isEmpty) {
pthread_cond_wait(&empty, &lock);
}
// isEmpty is non-zero and the lock is held
Whatever is signalling that something is-empty might look like:
pthread_mutex_lock(&lock);
// ...
// we have emptied the queue while holding the lock
isEmpty = 1;
pthread_mutex_unlock(&lock);
pthread_cond_signal(&empty);