A project I'm working on uses multiple threads to do work on a collection of files. Each thread can add files to the list of files to be processed, so I put together (what I thought was) a thread-safe queue. Relevant portions follow:
// qMutex is a std::mutex intended to guard the queue// populatedNotifier is a std::condition_variable intended to// notify waiting threads of a new item in the queuevoid FileQueue::enqueue(std::string&& filename){std::lock_guard<std::mutex> lock(qMutex);q.push(std::move(filename));// Notify anyone waiting for additional files that more have arrivedpopulatedNotifier.notify_one();}std::string FileQueue::dequeue(const std::chrono::milliseconds& timeout){std::unique_lock<std::mutex> lock(qMutex);if (q.empty()) {if (populatedNotifier.wait_for(lock, timeout) == std::cv_status::no_timeout) {std::string ret = q.front();q.pop();return ret;}else {return std::string();}}else {std::string ret = q.front();q.pop();return ret;}}However, I am occasionally segfaulting inside the if (...wait_for(lock, timeout) == std::cv_status::no_timeout) { } block, and inspection in gdb indicates that the segfaults are occurring because the queue is empty. How is this possible? It was my understanding that wait_for only returns cv_status::no_timeout when it has been notified, and this should only happen after FileQueue::enqueue has just pushed a new item to the queue.
Best Answer
It is best to make the condition (monitored by your condition variable) the inverse condition of a while-loop:while(!some_condition). Inside this loop, you go to sleep if your condition fails, triggering the body of the loop.
This way, if your thread is awoken--possibly spuriously--your loop will still check the condition before proceeding. Think of the condition as the state of interest, and think of the condition variable as more of a signal from the system that this state might be ready. The loop will do the heavy lifting of actually confirming that it's true, and going to sleep if it's not.
I just wrote a template for an async queue, hope this helps. Here, q.empty() is the inverse condition of what we want: for the queue to have something in it. So it serves as the check for the while loop.
#ifndef SAFE_QUEUE#define SAFE_QUEUE#include <queue>#include <mutex>#include <condition_variable>// A threadsafe-queue.template <class T>class SafeQueue{public:SafeQueue(void): q(), m(), c(){}~SafeQueue(void){}// Add an element to the queue.void enqueue(T t){std::lock_guard<std::mutex> lock(m);q.push(t);c.notify_one();}// Get the "front"-element.// If the queue is empty, wait till a element is avaiable.T dequeue(void){std::unique_lock<std::mutex> lock(m);while(q.empty()){// release lock as long as the wait and reaquire it afterwards.c.wait(lock);}T val = q.front();q.pop();return val;}private:std::queue<T> q;mutable std::mutex m;std::condition_variable c;};#endifAccording to the standard condition_variables are allowed to wakeup spuriously, even if the event hasn't occured. In case of a spurious wakeup it will return cv_status::no_timeout (since it woke up instead of timing out), even though it hasn't been notified. The correct solution for this is of course to check if the wakeup was actually legit before proceding.
The details are specified in the standard §30.5.1 [thread.condition.condvar]:
—The function will unblock when signaled by a call to notify_one(), a call to notify_all(), expiration of the absolute timeout (30.2.4) specified by abs_time, or spuriously.
...
Returns: cv_status::timeout if the absolute timeout (30.2.4) specifiedby abs_time expired, other-ise cv_status::no_timeout.
This is probably how you should do it:
void push(std::string&& filename){{std::lock_guard<std::mutex> lock(qMutex);q.push(std::move(filename));}populatedNotifier.notify_one();}bool try_pop(std::string& filename, std::chrono::milliseconds timeout){std::unique_lock<std::mutex> lock(qMutex);if(!populatedNotifier.wait_for(lock, timeout, [this] { return !q.empty(); }))return false;filename = std::move(q.front());q.pop();return true; }Adding to the accepted answer, I would say that implementing a correct multi producers / multi consumers queue is difficult (easier since C++11, though)
I would suggest you to try the (very good) lock free boost library, the "queue" structure will do what you want, with wait-free/lock-free guarantees and without the need for a C++11 compiler.
I am adding this answer now because the lock-free library is quite new to boost (since 1.53 I believe)
I would rewrite your dequeue function as:
std::string FileQueue::dequeue(const std::chrono::milliseconds& timeout){std::unique_lock<std::mutex> lock(qMutex);while(q.empty()) {if (populatedNotifier.wait_for(lock, timeout) == std::cv_status::timeout ) return std::string();}std::string ret = q.front();q.pop();return ret;}It is shorter and does not have duplicate code like your did. Only issue it may wait longer that timeout. To prevent that you would need to remember start time before loop, check for timeout and adjust wait time accordingly. Or specify absolute time on wait condition.
There is also GLib solution for this case, I did not try it yet, but I believe it is a good solution.https://developer.gnome.org/glib/2.36/glib-Asynchronous-Queues.html#g-async-queue-new
BlockingCollection is a C++11 thread safe collection class that provides support for queue, stack and priority containers. It handles the "empty" queue scenario you described. As well as a "full" queue.
This is my implementation of a thread-queue in C++20:
#pragma once#include <deque>#include <mutex>#include <condition_variable>#include <utility>#include <concepts>#include <list>template<typename QueueType>concept thread_queue_concept =std::same_as<QueueType, std::deque<typename QueueType::value_type, typename QueueType::allocator_type>>|| std::same_as<QueueType, std::list<typename QueueType::value_type, typename QueueType::allocator_type>>;template<typename QueueType>requires thread_queue_concept<QueueType>struct thread_queue{using value_type = typename QueueType::value_type;thread_queue();explicit thread_queue( typename QueueType::allocator_type const &alloc );thread_queue( thread_queue &&other );thread_queue &operator =( thread_queue const &other );thread_queue &operator =( thread_queue &&other );bool empty() const;std::size_t size() const;void shrink_to_fit();void clear();template<typename ... Args>requires std::is_constructible_v<typename QueueType::value_type, Args ...>void enque( Args &&... args );template<typename Producer>requires requires( Producer producer ) { { producer() } -> std::same_as<std::pair<bool, typename QueueType::value_type>>; }void enqueue_multiple( Producer producer );template<typename Consumer>requires requires( Consumer consumer, typename QueueType::value_type value ) { { consumer( std::move( value ) ) } -> std::same_as<bool>; }void dequeue_multiple( Consumer consumer );typename QueueType::value_type dequeue();void swap( thread_queue &other );private:mutable std::mutex m_mtx;mutable std::condition_variable m_cv;QueueType m_queue;};template<typename QueueType>requires thread_queue_concept<QueueType>thread_queue<QueueType>::thread_queue(){}template<typename QueueType>requires thread_queue_concept<QueueType>thread_queue<QueueType>::thread_queue( typename QueueType::allocator_type const &alloc ) :m_queue( alloc ){}template<typename QueueType>requires thread_queue_concept<QueueType>thread_queue<QueueType>::thread_queue( thread_queue &&other ){using namespace std;lock_guard lock( other.m_mtx );m_queue = move( other.m_queue );}template<typename QueueType>requires thread_queue_concept<QueueType>thread_queue<QueueType> &thread_queue<QueueType>::thread_queue::operator =( thread_queue const &other ){std::lock_guardourLock( m_mtx ),otherLock( other.m_mtx );m_queue = other.m_queue;return *this;}template<typename QueueType>requires thread_queue_concept<QueueType>thread_queue<QueueType> &thread_queue<QueueType>::thread_queue::operator =( thread_queue &&other ){using namespace std;lock_guardourLock( m_mtx ),otherLock( other.m_mtx );m_queue = move( other.m_queue );return *this;}template<typename QueueType>requires thread_queue_concept<QueueType>bool thread_queue<QueueType>::thread_queue::empty() const{std::lock_guard lock( m_mtx );return m_queue.empty();}template<typename QueueType>requires thread_queue_concept<QueueType>std::size_t thread_queue<QueueType>::thread_queue::size() const{std::lock_guard lock( m_mtx );return m_queue.size();}template<typename QueueType>requires thread_queue_concept<QueueType>void thread_queue<QueueType>::thread_queue::shrink_to_fit(){std::lock_guard lock( m_mtx );return m_queue.shrink_to_fit();}template<typename QueueType>requires thread_queue_concept<QueueType>void thread_queue<QueueType>::thread_queue::clear(){std::lock_guard lock( m_mtx );m_queue.clear();}template<typename QueueType>requires thread_queue_concept<QueueType>template<typename ... Args>requires std::is_constructible_v<typename QueueType::value_type, Args ...>void thread_queue<QueueType>::thread_queue::enque( Args &&... args ){using namespace std;unique_lock lock( m_mtx );m_queue.emplace_front( forward<Args>( args ) ... );m_cv.notify_one();}template<typename QueueType>requires thread_queue_concept<QueueType>typename QueueType::value_type thread_queue<QueueType>::thread_queue::dequeue(){using namespace std;unique_lock lock( m_mtx );while( m_queue.empty() )m_cv.wait( lock );value_type value = move( m_queue.back() );m_queue.pop_back();return value;}template<typename QueueType>requires thread_queue_concept<QueueType>template<typename Producer>requires requires( Producer producer ) { { producer() } -> std::same_as<std::pair<bool, typename QueueType::value_type>>; }void thread_queue<QueueType>::enqueue_multiple( Producer producer ){using namespace std;lock_guard lock( m_mtx );for( std::pair<bool, value_type> ret; (ret = move( producer() )).first; )m_queue.emplace_front( move( ret.second ) ),m_cv.notify_one();}template<typename QueueType>requires thread_queue_concept<QueueType>template<typename Consumer>requires requires( Consumer consumer, typename QueueType::value_type value ) { { consumer( std::move( value ) ) } -> std::same_as<bool>; }void thread_queue<QueueType>::dequeue_multiple( Consumer consumer ){using namespace std;unique_lock lock( m_mtx );for( ; ; ){while( m_queue.empty() )m_cv.wait( lock );try{bool cont = consumer( move( m_queue.back() ) );m_queue.pop_back();if( !cont )return;}catch( ... ){m_queue.pop_back();throw;}}}template<typename QueueType>requires thread_queue_concept<QueueType>void thread_queue<QueueType>::thread_queue::swap( thread_queue &other ){std::lock_guardourLock( m_mtx ),otherLock( other.m_mtx );m_queue.swap( other.m_queue );}The only template-parameter is BaseType, which can be a std::deque type or std::list type, restricted with thread_queue_concept. This class uses this type as the internal queue type. Chose that BaseType that is most efficient for your application. I might have restricted the class on a more differentiated thread_queue_concepts that checks for all the used parts of BaseType so that this class might apply for other types compatible to std::list<> or std::deque<> but I was too lazy to implement that for the unlikely case that someone implements something like that on his own.One advantage of this code are enqueue_multiple and dequeue_multiple. These functions are given a function-object, usually a lambda, which can enqueue or dequeue multiple items with only one locking step. For enqueue this always holds true, for dequeue this depends on if the queue has elements to fetch or not.
enqueue_multiple usually makes sense if you have one producer and multiple consumers. It results in longer periods holding the lock and therefore it makes sense only if the items can be produced or move fast.
dequeue_multiple usually makes sense if you have multiple producers and one consumer. Here we also have longer locking periods, but as objects are usually only have fast moves here, this normally doesn't hurt.
If the consumer function object of the dequeue_multiple throws an exception while consuming, the exception is caugt and the element provided to the consumer (rvalue-refernce inside the underlying queue types object) is removed.
If you like to use this class with C++11 you have to remove the concepts or disable them with #if defined(__cpp_concepts).
You may like lfqueue, https://github.com/Taymindis/lfqueue.It’s lock free concurrent queue. I’m currently using it to consuming the queue from multiple incoming calls and works like a charm.