c++ - Thread safety for STL queue

Question

I am using a queue to communicate between threads. I have one reader and multiple writer threads. My question is do I need to lock the queue every time when I use push/front/pop from the queue for the reader? Can I do something like the following:

//reader threads
getLock();
get the number of elements from the queue
releaseLock();

int i = 0;
while( i < numOfElements){
    queue.front();
    queue.pop();
    i++
}

The idea is that I want to reduce the granularity of the locked code and since the writer thread would only write to the back of the queue and there is only a single reader thread. As long as I get the number of elements, then I could get the elements from the queue OR do I need to enclose the front() and pop() in the lock as well?

Answer 1

As others have already mentioned, standard containers are not required to guarantee thread safety so what you're asking for cannot be implemented portably. You can reduce the time your reader thread is locking the writers out by using 2 queues and a queue pointer that indicates the queue that is currently in use by the writers.

Each writer would:

• Acquire lock
• Push element(s) into the queue currently pointed to by the queue pointer
• Release lock

The reader can then do the following:

• Acquire lock
• Switch queue pointer to point to the second queue
• Release lock
• Process elements from the first queue