[LAD] Problem with small ALSA rawmidi app

[Paul Davis]

On Thu, Oct 22, 2009 at 1:21 PM, Frank Neumann <beachnase at web.de> wrote:
>
> Hi,
>
> 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);
> snd_rawmidi_drain(handle_out);
>
> // wait a little for the answer
> // I know I should not do it this way, but for testing purposes..
> usleep(1000000);
>
> 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.

you opened both ports in NON_BLOCKing mode.

this means that a read or write will ALWAYS return immediately, no
matter whether there is data/space for the write/read or not.