[Jack-Devel] Fwd: Possible error codes for jack_midi_event_get()?

[Hanspeter Portner]

On 28.07.2018 16:55, Matthias Geier wrote:
> Dear list.
> 
> I'm the author of one of the Python bindings for JACK:
> https://jackclient-python.readthedocs.io/
> 
> I was a bit lazy when implementing a generator function that's
> supposed to yield all incoming MIDI events one-by-one.
> 
> Here's the code if somebody is interested (it's using CFFI and the
> _lib object is a wrapper for the JACK API):
> 
> 
>     def incoming_midi_events(self):
>         event = self._event
>         buf = _lib.jack_port_get_buffer(
>             self._ptr, self._client.blocksize)
>         for i in range(_lib.jack_midi_get_event_count(buf)):
>             err = _lib.jack_midi_event_get(event, buf, i)
>             # TODO: proper error handling if this ever happens:
>             assert not err, err
>             yield event.time, _ffi.buffer(event.buffer, event.size)
> 
> 
> As you can see, I didn't check the return value of
> jack_midi_event_get(), I just added an "assert" statement hoping that
> somebody would report it if the call to jack_midi_event_get() would
> ever fail.
> 
> I didn't hear anything for a long time, but recently I got a bug
> report where the assertion was violated:
> https://github.com/spatialaudio/jackclient-python/issues/54
> 
> This shows that jack_midi_event_get() can raise an error, even if I
> check with jack_midi_get_event_count() beforehand.
> 
> My question is now: how should I react to this error?

For JACK1 error is only returned if midi_event_index >= midi_event_count [1].

For JACK2 the same is true; additionally it can fail if the MIDI port buffer is
invalid (whatever that means...) [2].

[1] https://github.com/jackaudio/jack1/blob/master/libjack/midiport.c#L96
[2] https://github.com/jackaudio/jack2/blob/master/common/JackMidiAPI.cpp#L67

> 
> Shall I ...
> 
> * discard the current event and continue reading further events?
> 
> * discard the current and all following events in this block?
> 
> * raise an error?
> 
> * do something else?

Well, there is no valid event, so ignore it ?

> 
> Can I guarantee that no incoming MIDI events get lost?
> 

MIDI events can only get lost while multiplexed down from multiple ports into
one (and the latter can not take them all), iirc.

> Another related question: what error code is supposed to be returned?
> The documentation
> (http://www.jackaudio.org/api/group__MIDIAPI.html#ga838c794bd1451bfd47edde1c7cd1ff4f)
> mentions ENODATA, but according to my bug report, ENOBUFS (-105) was
> returned.
> 
> Is this a bug in JACK or an omission in the documentation?

It is in the documentation, that it can fail [3].

[3] http://jackaudio.org/api/group__MIDIAPI.html#ga838c794bd1451bfd47edde1c7cd1ff4f

> 
> I don't know which JACK version was used, the OS was Ubuntu 18.04 LTS 64 bits.
> 
> cheers,
> Matthias