QSlider Update Problem

[December]

Well, even if I push the fader up real slow.. say 6 seconds from bottom to top, the same problem occurs.

That would mean around 16 messages per second.The code in the MIDI thread is actually still a bit of a mystery to me.. as its taken from some other o/s projects

void midiThread::run()
{
  snd_seq_t *seq_handle;
  int npfd;
  struct pollfd *pfd;
 
  seq_handle = open_seq();
  npfd = snd_seq_poll_descriptors_count(seq_handle, POLLIN);
  pfd = (struct pollfd *)alloca(npfd * sizeof(struct pollfd));
  snd_seq_poll_descriptors(seq_handle, pfd, npfd, POLLIN);
  while (1) {
    if (poll(pfd, npfd, 100000) > 0) {
      	 midi_action(seq_handle);
		}  
  	}
}

void midiThread::run()
{
  snd_seq_t *seq_handle;
  int npfd;
  struct pollfd *pfd;
    
  seq_handle = open_seq();
  npfd = snd_seq_poll_descriptors_count(seq_handle, POLLIN);
  pfd = (struct pollfd *)alloca(npfd * sizeof(struct pollfd));
  snd_seq_poll_descriptors(seq_handle, pfd, npfd, POLLIN);
  while (1) {
    if (poll(pfd, npfd, 100000) > 0) {
      	 midi_action(seq_handle);
		}  
  	}
}

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This is the run part of the QThread. The other relevant bit is:

void midiThread::midi_action(snd_seq_t *seq_handle) {
 
  snd_seq_event_t *ev;
  MyDataEvent dataEvent(0,0);
 
  do {
    snd_seq_event_input(seq_handle, &ev);
    switch (ev->type) {
      case SND_SEQ_EVENT_CONTROLLER: 
		dataEvent.setParam(ev->data.control.param); 
		dataEvent.setValue(ev->data.control.value);
 		QApplication::sendEvent( parent, &dataEvent );
       break;
    }
 
    snd_seq_free_event(ev);
  } while (snd_seq_event_input_pending(seq_handle, 0) > 0);
 
}

void midiThread::midi_action(snd_seq_t *seq_handle) {

  snd_seq_event_t *ev;
  MyDataEvent dataEvent(0,0);

  do {
    snd_seq_event_input(seq_handle, &ev);
    switch (ev->type) {
      case SND_SEQ_EVENT_CONTROLLER: 
		dataEvent.setParam(ev->data.control.param); 
		dataEvent.setValue(ev->data.control.value);
 		QApplication::sendEvent( parent, &dataEvent );
       break;
    }

    snd_seq_free_event(ev);
  } while (snd_seq_event_input_pending(seq_handle, 0) > 0);

}

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If this is a horrible way to do things.. I would much appreciate any ideas on how to handle MIDI better.

On windows I used Borland C++ Builder.. and found a great component that pretty much did it all for me, passing the messages to my callback function. I guess I have to learn more C++ to program on Linux ;-)

[jacek]

That would mean around 16 messages per second.

Then it shouldn't be a problem, unless you do a busy-wait somewhere. You check with top if your program doesn't eat whole CPU power.

poll() is a good way to go, but in midi_action() you have a problem --- you use sendEvent(), but only postEvent() is thread-safe.

Try:

void midiThread::midi_action(snd_seq_t *seq_handle) {
  ...
  do {
    snd_seq_event_input(seq_handle, &ev);
    switch (ev->type) {
      case SND_SEQ_EVENT_CONTROLLER: 
		MyDataEvent *dataEvent = new MyDataEvent(0,0);
		dataEvent.setParam(ev->data.control.param); 
		dataEvent.setValue(ev->data.control.value);
		QApplication::postEvent( parent, dataEvent ); // QApplication takes ownership of the event
      break;
    }
  ...

void midiThread::midi_action(snd_seq_t *seq_handle) {
  ...
  do {
    snd_seq_event_input(seq_handle, &ev);
    switch (ev->type) {
      case SND_SEQ_EVENT_CONTROLLER: 
		MyDataEvent *dataEvent = new MyDataEvent(0,0);
		dataEvent.setParam(ev->data.control.param); 
		dataEvent.setValue(ev->data.control.value);
		QApplication::postEvent( parent, dataEvent ); // QApplication takes ownership of the event
      break;
    }
  ...

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[December]

Wow, that was the problem.

Changing from using sendEvent to postEvent solved it. I feel stupid now, I even remember reading that sendEvent is not thread safe a while back

Thanks for the help.. I think QT is going to be a lot of fun with such knowledgeable people around