C 11 thread-safe queue

Question

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 queue  void FileQueue  enqueue std  string amp  amp  filename        std  lock guard lt std  mutex gt  lock qMutex       q push std  move filename            Notify anyone waiting for additional files that more have arrived     populatedNotifier notify one       std  string FileQueue  dequeue const std  chrono  milliseconds amp  timeout        std  unique lock lt std  mutex gt  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

User · Accepted Answer

According 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  speci ed by abs time  or spuriously      Returns  cv status  timeout if the absolute timeout  30 2 4  speci edby abs time expired  other-ise cv status  no timeout

User · Answer

I would rewrite your dequeue function as   std  string FileQueue  dequeue const std  chrono  milliseconds amp  timeout        std  unique lock lt std  mutex gt  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

User · Answer

This is probably how you should do it   void push std  string amp  amp  filename                  std  lock guard lt std  mutex gt  lock qMutex            q push std  move filename               populatedNotifier notify one       bool try pop std  string amp  filename  std  chrono  milliseconds timeout        std  unique lock lt std  mutex gt  lock qMutex        if  populatedNotifier wait for lock  timeout   this    return  q empty                return false       filename   std  move q front         q pop         return true

User · Answer

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

User · 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  lt queue gt   include  lt mutex gt   include  lt condition variable gt      A threadsafe-queue  template  lt class T gt  class SafeQueue   public    SafeQueue void        q         m         c            SafeQueue void             Add an element to the queue    void enqueue T t          std  lock guard lt std  mutex gt  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 lt std  mutex gt  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 lt T gt  q    mutable std  mutex m    std  condition variable c      endif

User · Answer

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

User · Answer

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

User · Answer

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

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