重绘小部件时Qt崩溃

Question

I have a custom-built player which can "play" recoded media samples (sensor data with timestamps).

What I am trying to do is couple this player to an Qt UI. I have created a Widget and coupled the Player and the Widget through Signals and slots (Qt::DirectConnection).

The problem I'm experiencing is that the program crashes with an exception thrown. The moment when it crashes is completely random, and sometimes it may not crash. As far as I can tell, the problem lies in that the update in the widget part is not thread-safe. I have tried restricting access to it with a binary semaphore (immediately return if semaphore true, continue if not). The error still doesn't go away.

My question is: is the Qt Painter running on the same thread as the Widget updater? I suspect I'm calling the painter before the previous paint operation has finished, which is why it still crashes. I suspect the widget only somehow posts an event to the painter (non-blocking)..

What else could be the problem? Unfortunately, the code is too complex to paste here. However, I am attaching a screenshot of the crash.

Cheers, Mihai

Answer 1

Thank you for your precious feedback. I have managed to find the root cause for the problem. It was tied to the Qt::DirectConnection . As described in the API documentation, this will be executed:

"in the emitter's thread, which is not necessarily the receiver's thread."

In my case, the emitter is the Player and the receiver is the UI thread. Which means I'm manipulating elements in the UI outside of the UI Thread. This is the root cause.

I have changed the implementation to used Qt::QueuedConnection and now it runs smoothly.

As to why the original implementation with Qt::QueuedConnection did not work initially, I put a bool status = connect() which always returned false , meaning the connection failed. I changed the name of the signal method to be different from the name of the slot one (initially, they were the same), + I defaulted all the input parameters to standard types ( unsigned int instead of tUInt32 ) and it worked. connect() now returns true . I suspect that somehow the connect() got confused as to the wiring..