Re: [fluid-dev] Missing function fluid_midi_event_get_sysex

[Reinhold Hoffmann]

Tom,

Thanks. Now, I understand the reason why which makes sense to me.

Our software offers a feature called "recording" where the content of a midi
event is presented to the user for manipulating, changable by tools etc.
E.g. for a Note_On event the user can change the velocity of a single note
resp. a single Note_On midi event. For sysex events advanced users can
manipulate the sysex event content as well. So far, for recording we use the
Windows mmeapi interface as midi input interface. 

Currently, we enhance the software with the jack interface where we also use
fluidsynth's jack capabilities. The feature for the user is the perform the
same recording for jack which is available from the Windows mmeapi software.
This lead to the question how to access sysex events when a midi events
comes in from the fluid jack midi input.

The relevant buffer sizes are pretty small (max. ~ 30 bytes). A typical user
scenario for a Yamaha keyboard user is to switch instruments on his keyboard
where in addition to a controller change and program change sysex events
determine the changed sound. In those scenarios the longest sysex event
which I have ever seen is in those ~30 bytes range.

For the time being we have created a trivial workaround by casting the
equivalent fluid_midi_event_t structure and accessing paramptr and the
param1 for the length. 

I understand that a general solution is more complex and would be an
overkill for our needs. Our workaround is so simple that we can live with
that.

Reinhold  

-----Ursprüngliche Nachricht-----
Von: Tom M. [mailto:tom.mbrt@googlemail.com] 
Gesendet: Mittwoch, 12. Oktober 2022 17:54
An: FluidSynth mailing list; Reinhold Hoffmann
Betreff: Re: [fluid-dev] Missing function fluid_midi_event_get_sysex

I think I've seen this question already 4 years ago. I didn't know back then
why it was done that way, neither do I know now. I assumed that this
function wasn't made public because fluidsynth is not supposed to be a MIDI
parser. Particularly, there is an internal MIDI parser, but this one has a
limited buffer, causing potentially too long SysEx messages to be cut off:

https://github.com/FluidSynth/fluidsynth/blob/1fc6f0935bacc6fb3e38b7bebe8b24
f6cf110dab/src/midi/fluid_midi.c#L2744-L2749

This "parser" may or may not be used depending on where the
fluid_midi_event_t is coming from.

I understand that there is a corresponding setter function
fluid_midi_event_set_sysex(), but unfortunately, this function is broken by
design: A dynamically malloc()ed and user provided buffer could be freed
internally by fluidsynth's version of free(). Allocating some data in one
domain, carrying it in a different domain and freeing it with different
means could lead to heap corruption, when two different memory runtimes are
in use. This is a common problem on Windows.

So my question would be, for what purpose you intend to use
fluid_midi_event_get_sysex()? And whether or not you would be happy only
getting the first 1024 bytes of the SysEx message?

Tom