ankithbti / readerwriterqueue

A fast single-producer, single-consumer lock-free queue for C++

A single-producer, single-consumer lock-free queue for C++

This mini-repository has my very own implementation of a lock-free queue (that I designed from scratch) for C++.

It only supports a two-thread use case (one consuming, and one producing). The threads can't switch roles, though you could use this queue completely from a single thread if you wish (but that would sort of defeat the purpose!).

Note: If you need a general-purpose multi-producer, multi-consumer lock free queue, I have one of those too.

Features

• Blazing fast
• Compatible with C++11 (supports moving objects instead of making copies)
• Fully generic (templated container of any type) -- just like std::queue, you never need to allocate memory for elements yourself (which saves you the hassle of writing a lock-free memory manager to hold the elements you're queueing)
• Allocates memory up front, in contiguous blocks
• Provides a try_enqueue method which is guaranteed never to allocate memory (the queue starts with an initial capacity)
• Also provides an enqueue method which can dynamically grow the size of the queue as needed
• Also provides a blocking version with wait_dequeue
• Completely "wait-free" (no compare-and-swap loop). Enqueue and dequeue are always O(1) (not counting memory allocation)
• On x86, the memory barriers compile down to no-ops, meaning enqueue and dequeue are just a simple series of loads and stores (and branches)

Use

Simply drop the readerwriterqueue.h and atomicops.h files into your source code and include them :-) A modern compiler is required (MSVC2010+, GCC 4.7+, ICC 13+, or any C++11 compliant compiler should work).

Note: If you're using GCC, you really do need GCC 4.7 or above -- 4.6 has a bug that prevents the atomic fence primitives from working correctly.

Example:

using namespace moodycamel;

ReaderWriterQueue<int> q(100);       // Reserve space for at least 100 elements up front

q.enqueue(17);                       // Will allocate memory if the queue is full
bool succeeded = q.try_enqueue(18);  // Will only succeed if the queue has an empty slot (never allocates)
assert(succeeded);

int number;
succeeded = q.try_dequeue(number);  // Returns false if the queue was empty

assert(succeeded && number == 17);

// You can also peek at the front item of the queue (consumer only)
int* front = q.peek();
assert(*front == 18);
succeeded = q.try_dequeue(number);
assert(succeeded && number == 18);
front = q.peek(); 
assert(front == nullptr);           // Returns nullptr if the queue was empty

The blocking version has the exact same API, with the addition of wait_dequeue and wait_dequeue_timed methods:

BlockingReaderWriterQueue<int> q;

std::thread reader([&]() {
    int item;
    for (int i = 0; i != 100; ++i) {
        // Fully-blocking:
        q.wait_dequeue(item);

        // Blocking with timeout
        if (q.wait_dequeue_timed(item, std::chrono::milliseconds(5)))
            ++i;
    }
});
std::thread writer([&]() {
    for (int i = 0; i != 100; ++i) {
        q.enqueue(i);
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
});
writer.join();
reader.join();

assert(q.size_approx() == 0);

Note that wait_dequeue will block indefinitely while the queue is empty; this means care must be taken to only call wait_dequeue if you're sure another element will come along eventually, or if the queue has a static lifetime. This is because destroying the queue while a thread is waiting on it will invoke undefined behaviour.

Disclaimers

The queue should only be used on platforms where aligned integer and pointer access is atomic; fortunately, that includes all modern processors (e.g. x86/x86-64, ARM, and PowerPC). Not for use with a DEC Alpha processor (which has very weak memory ordering) :-)

Note that it's only been tested on x86(-64); if someone has access to other processors I'd love to run some tests on anything that's not x86-based.

Finally, I am not an expert. This is my first foray into lock-free programming, and though I'm confident in the code, it's possible that there are bugs despite the effort I put into designing and testing this data structure.

Use this code at your own risk; in particular, lock-free programming is a patent minefield, and this code may very well violate a pending patent (I haven't looked). It's worth noting that I came up with this algorithm and implementation from scratch, independent of any existing lock-free queues.

More info

See the LICENSE.md file for the license (simplified BSD).

My blog post introduces the context that led to this code, and may be of interest if you're curious about lock-free programming.