QByteArray dequeue() crash

Abhi_Varma

Hi, I'm new to qt. I'm facing a small issue with dequeue operation of qt for the following scenario. I have incoming messages coming from server, whenever a new message is sent the following method is invoked and the new message is enqueued to a QQueue<QByteArray> Object called "qbaServerMessages".

void OnMessageReceieved(const QByteArray& baServerMessage)
{
     if(!baServerMessage.isEmpty())
     {
           qbaServerMessages.enqueue(baServerMessage);
     }
}

In the following method I dequeue the "qbaServerMessages" and handle each message separately.

void HandleServerMessages()
{
   while(!qbaServerMessages.isEmpty())
   {
      try
      {
          QByteArray baMessage = qbaServerMessages.dequeue();
          //.......
          //Code to handle the message
      }
      catch(...)
      {
         //Code to handle exception.
      }
   }
}

I'm running this code on vscode. Once in a while its showing a crash at QByteArray.erase(). When the crash happened here the call stack was like this.

QList::erase();
QList::removeFirst() 
QList<T>::takeFirst()
QQueue::dequeue()
void HandleServerMessages()

it was crashing at erase() in qlist. My question why isn't my exception handling working and how can I resolve this.

SGaist

Hi,

Is this a multithreaded application ?
If so, you need to protect the access to your queue.

Abhi_Varma

@SGaist Yes. It is multithreaded app. and how can I protect the access to the queue. (I have added few more comments to the post at the end)

J.Hilk

@Abhi_Varma said in QByteArray dequeue() crash:

@SGaist Yes. It is multithreaded app. and how can I protect the access to the queue.

If you're unsure about this, maybe you shouldn't do multi threading to start with, especially as Qt-Classes/Modules have almost always asynchronous apis.

For completeness however take a look at mutexes
https://doc.qt.io/qt-5/qmutex.html
https://en.cppreference.com/w/cpp/thread/mutex

(I have added few more comments to the post at the end)

My question why isn't my exception handling working and how can I resolve this.

because c++ is not js or python, you can't move everything in a try catch block and be done with it. That only works when exceptions are actually thrown. 😉

An invalid pointer/instance access does not fall in that category :D