pthread_cleanup_pop

[fguy817817]

gcc is telling me that "data" is undefined on the line "return data;". Does anyone know why? I think it has something to do with pthread_cleanup_pop because the code compiles when I comment that line. Thoughts?

void* queue_remove(queue_t* queue) 
{		
	/* Lock the queue */
	pthread_mutex_lock(&queue->queue_mutex);
	
	/* Push this thread's cleanup handler onto the stack */
	pthread_cleanup_push(cleanup_handler,(void*)queue);
	
	if(queue->num_workers > 0)
	{
		/* This loop is to prevent more than one thread from accessing the queue at a time in the event of a spurious wakeup. */
		while(queue->length == 0)
		{	
			/* Wait for queue_add to signal that it has added a new node */
			pthread_cond_wait(&queue->ready, &queue->queue_mutex);
		}
	}
	
	/* Temporary node pointer that will allow us to free the node being removed */				
	node_t *tmp_node;
	
	/* Point the first node of the queue to the new temporary node */
	tmp_node = queue->front;
    
	/* The second node in the queue is going to become the front so point the front to the second node */
	queue->front = tmp_node->next;
	
	/* Point the item we will be removing to the data portion of the node being removed */
	void* data = tmp_node->data;
	
	/* Decrease the amount of memory being used by the queue by the size of the data being removed. */
	queue->memory_allocated -= queue->node_size + sizeof(node_t);

	/* Free the node we are removing */
	free(tmp_node);

	/* Decrease the number of nodes in the queue */
	queue->length--;
	
	/* If the queue becomes empty as a result of removing this element ... */
	if (queue->front == NULL) 
	{
		/* Assign the back of the queue to null */
		queue->back = NULL;
	}

	pthread_cleanup_pop(1);
	
	return data;
}

[Rubberman]

I think you are onto the problem in that the cause is pthread_cleanup_pop(). According to the man page

       The pthread_cleanup_pop() function shall remove the  routine  at  the
       top of the calling thread’s cancellation cleanup stack and optionally
       invoke it (if execute is non-zero).

Just a guess, but it is this function, and as a result, all automatic variables are fubar. Try a local static variable and return that instead. IE

static void* data = 0;
void* queue_remove(queue_t* queue) 
{		
	/* Lock the queue */
	pthread_mutex_lock(&queue->queue_mutex);
	
	/* Push this thread's cleanup handler onto the stack */
	pthread_cleanup_push(cleanup_handler,(void*)queue);
	
	if(queue->num_workers > 0)
	{
		/* This loop is to prevent more than one thread from accessing the queue at a time in the event of a spurious wakeup. */
		while(queue->length == 0)
		{	
			/* Wait for queue_add to signal that it has added a new node */
			pthread_cond_wait(&queue->ready, &queue->queue_mutex);
		}
	}
	
	/* Temporary node pointer that will allow us to free the node being removed */				
	node_t *tmp_node;
	
	/* Point the first node of the queue to the new temporary node */
	tmp_node = queue->front;
    
	/* The second node in the queue is going to become the front so point the front to the second node */
	queue->front = tmp_node->next;
	
	/* Point the item we will be removing to the data portion of the node being removed */
	data = tmp_node->data;
	
	/* Decrease the amount of memory being used by the queue by the size of the data being removed. */
	queue->memory_allocated -= queue->node_size + sizeof(node_t);

	/* Free the node we are removing */
	free(tmp_node);

	/* Decrease the number of nodes in the queue */
	queue->length--;
	
	/* If the queue becomes empty as a result of removing this element ... */
	if (queue->front == NULL) 
	{
		/* Assign the back of the queue to null */
		queue->back = NULL;
	}

	pthread_cleanup_pop(1);
	
	return data;
}

[fguy817817]

If multiple threads are accessing this routine won't there be a race condition on data? The queue is unlocked before data is returned.