[LAD] Problem with small ALSA rawmidi app

Frank Neumann beachnase at web.de
Thu Oct 22 17:21:54 UTC 2009


I am trying to communicate with a "real" MIDI device through the ALSA RawMidi
API, but it doesn't quite do what I want.

I want to:
- Send a small SysEx package that asks a device for some data
- Receive the answer

What I basically do is (code excerpt / pseudo code):

snd_rawmidi_t *handle_in = 0, *handle_out = 0;
unsigned char ibuf[256];
unsigned char obuf[] = { 0xf0, .., 0xf7 }; /* 6 bytes sysex */

snd_rawmidi_open(&handle_in, NULL, "hw:2,0,0", SND_RAWMIDI_NONBLOCK);

snd_rawmidi_open(NULL, &handle_out, "hw:2,0,0", SND_RAWMIDI_NONBLOCK);

snd_rawmidi_write(handle_out, &obuf, 6);

// wait a little for the answer
// I know I should not do it this way, but for testing purposes..

num = snd_rawmidi_read(handle_in, ibuf, sizeof(ibuf));

I know that the data is sent out, but reading back the answer always gives
me -1 for num.

Trying to do the same thing with 2 instances of the "amidi" program works, though:

First terminal window:
amidi -p hw:2,0,0 -d

Second terminal window:
amidi -p hw:2,0,0 -S "F0.....F7"

Sending the second one out, I immediately get the desired answer on the first

I'm sure there is something very basic I am doing wrong, but I fail to see
that at the moment. 

Eventually, the whole thing will have to be rewritten for the ALSA sequencer API
anyway, but for a first quick test, I wanted to try it this way.
Any hints are most welcome.


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