Using background threads for long processes in MFC: Background Thread 1

15 Apr 2003 11:51 mfc

We add another button to the dialog:

void CBgthreadDlg::OnBackground()
    // Run on a background thread. Don't wait for the thread to exit.
    TaskThread t;

This uses the code from here to implement the thread class:

class TaskThread : public Thread
    virtual unsigned Run()
        Task task(NULL);

        return 0;

The source so far:

This works fine: the task is run on the background thread, making the dialog totally reponsive to the user. It has several of its own problems:

  • There’s no indication that it’s doing anything – we want to report progress to the user, but still have a responsive application.
  • The user can spawn multiple background threads. Depending on your application, this might be fine. We’ll assume for the moment that it’s not.
  • If the user closes the dialog, the application terminates the background thread with extreme prejudice.

More than that, however, there’s a problem with stability: About 80% of the time, the thread fails to start, failing with an access violation or a pure virtual function call.

This is caused by the destructor for TaskThread being called from the CBgthreadDlg::OnBackground() function before the background thread has had a chance to call the virtual Run function from StaticThreadRoutine.

We can fix this by allocating the thread object on the heap, rather than the stack. It’s then necessary to clean up the thread object properly, otherwise we get a memory leak. We’ll deal with this in the next post.