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

Paul Coccoli pcoccoli at gmail.com
Sun Feb 17 22:03:52 UTC 2013

On Sun, Feb 17, 2013 at 4:41 PM, Harry van Haaren <harryhaaren at gmail.com> wrote:
> On Sun, Feb 17, 2013 at 9:20 PM, Paul Coccoli <pcoccoli at gmail.com> wrote:
>> 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.
> Nope, write them one   sizeof( event->size() ) at a time.

But you're not guaranteed to write that many bytes at a time.  If the
write of a chunk of bytes (your event object) to a ringbuffer wraps,
you probably wind up with two writes.  A read from another thread may
get an incomplete object.  What happens then?

> I'm very interested in why copying C++ objects like this is a bad idea.
> Its been discussed on list before
> (http://linux-audio.4202.n7.nabble.com/Inter-thread-Communication-Design-Approach-td68710.html).
> This seemed to be the best simple RT safe solution. If you have suggestions
> / improvements I'd love to hear them.

I did make a suggestion.

>> 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?
> Pointers to an Event? Just makes it more hassle to send an Event from the RT
> thread.
> Involves taking X memory from a mem-pool, and then using placement new to
> construct
> the EventPlay(), and then send the pointer trough the ringbuffer. More
> complicated IMO.

My suggestion has nothing to do with mem-pools or placement new.  That
depends on your memory management strategy, which is orthogonal to
this discussion.

>> To free the event  objects, you could pass them back via a second
>> ringbuffer so the RT
>> threads aren't responsible for deleting them.
> Indeed, that would be necessary. Again, more complications. That said, it
> can be done,
> and would involve less "traffic" trough the ringbuffer, and also "fixed
> size" traffic": pointers to EventBase.

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