On Tue, Dec 20, 2016 at 11:09 AM, ROBERT WOLF <robert.wolf@hercsmusicsystems.com.au> wrote:
Dear All,

Hi Robert,
What exactly is the difference between jack_midi_event_reserve and jack_midi_event_write ?

From my understanding:
- event_reserve() takes "size" number of bytes from JACKs internal MIDI buffers for a particular port, and returns the address to that buffer. Your application can now write to that pointer, and once the RT callback is done, JACK will transmit the data.

- event_write() takes a pointer from your application, and copies the data pointed to into the JACK internal midi buffer. Your application can re-use or delete the buffer you passed to the event_write() function now, as JACK has a copy of the data.
In particular, if I call jack_midi_event_write, does it actually copy the data from the buffer, which I provide?
Or does it use that same buffer.

See above.
Another question is how can I send midi events generated from GUI, (which does not run in the audio callback)?
My guess is to cache them somewhere and transmit them inside the callback?
Am I guessing correctly?

In short: Yes. You need to transfer the MIDI data to be sent between two threads - this can be trivial with mutexs/semaphors/locking systems, but keep in mind that your real-time thread must *never* block. Personally, I am a big proponent of message passing, instead of locking based systems.

To answer your question, I would use a ringbuffer (jack has a ringbuffer t) to move a message (struct with some metadata) to transfer an event (midi note one) between the GUI to JACK thread.

Hope that helps, -Harry