[LAD] making sense of Jack MIDI; or, is this an appropriate use for Jack?

Paul Coccoli pcoccoli at gmail.com
Sun Feb 17 21:20:46 UTC 2013

On Sun, Feb 17, 2013 at 10:35 AM, Harry van Haaren
<harryhaaren at gmail.com> wrote:
> On Sun, Feb 17, 2013 at 3:57 AM, M Donalies <ingeniousnebbish at cox.net>
> wrote:
>> No locks or mutexes in a callback function. I need to think about that
>> one.
> This is indeed a "lovely" topic for debate. I'm bound to say that, I'm
> currently doing a final-year project for college on the topic.
> I've came up with a solution, which I feel is the best balance between C++-y
> style Event classes, and C-style simple code. Simple is great for
> RT situations: its easy to see if something is RT or not.
> In essence it boils down to this:
> -Create an "EventBase" class            with pure virtual type() and size()
> functions
> -Derive actual events like "EventPlay"  overriding type() and size()
> -Create a ringbuffer                    checkout the example, it uses the
> JACK ringbuffer
> -Create events *on the stack*           ie EventPlay eventPlay();
> -Write events into the ringbuffer using jack_ringbuffer_write();
> The last two steps are key, as they enfore the event gets *copied* into the
> ringbuffer, and the original goes out
> of scope (and gets destroyed).
> This solution is very workable, on the condition that only fundamental C
> datatypes are passed trough the Event class.
> Things will get nasty if you pass std::shared_ptr<> objects trough it, I've
> made that mistake already :)
> The following code shows communication from the GUI thread to the RT thread.
> Create another ringbuffer and functions
> for communicating in the other direction.
> https://github.com/harryhaaren/realtimeAudioThreading
> HTH, -Harry

This scheme sounds error prone.  In general, copying C++ objects via
memcpy (or writing them 1 byte at a time into the ringbuffer, which is
what I think you're proposing) is a bad idea.  JACK ringbuffers are
ideally suited to passing simple types (like floats), and not vairable
sized things (like different derived Event classes).  Your enum for
event types is a bit of a red flag, too.  While its perfectly valid,
"type flags" like this more often than not accompany inflexible,
tightly coupled code (which may be fine in a small audio app, but few
apps stay small).

What about passing pointers via the ringbuffer?  To free the event
objects, you could pass them back via a second ringbuffer so the RT
threads aren't responsible for deleting them.

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