Re: Most simple use of sequencer in ALSA

[Takashi Iwai <tiwai@suse.de>]

At Fri, 15 Feb 2002 07:21:24 +0100 (MET),
Guenther Sohler wrote:
> 
> I have simplified the seq.c in the alsa-driver-test direcory for me
> to also understand.
> 
> The program is now quite short and outputs a note and quits
> 
> here is it
> -----------
> 
> #include <stdio.h>
> #include "alsa/asoundlib.h"
> 
> 
> int main(int argc, char *argv[])
> {
>         snd_seq_t *handle;
>         snd_seq_event_t ev;
> 
>         snd_seq_open(&handle, "hw", SND_SEQ_OPEN_DUPLEX, 0);
>         snd_seq_alloc_queue(handle);
>         
>         bzero(&ev, sizeof(ev));
>         ev.dest.client=65;
>         ev.dest.port=0;
>         ev.type = SND_SEQ_EVENT_NOTEON;
>         ev.data.note.channel = 0;
>         ev.data.note.note = 64 ;
>         ev.data.note.velocity = 127;
>         snd_seq_event_output_direct(handle, &ev);
>         sleep(1);
>         ev.type = SND_SEQ_EVENT_NOTEOFF;
>         snd_seq_event_output_direct(handle, &ev);
>         snd_seq_drain_output(handle);
>         return 0;
> }
> 
> There are two questions ?
> 
> Why doesn't it work if I allocate no queue ? There is no need for a queue as i
> directly output events

No, you don't need to allocate a queue if you schedule by yourself.
your program lack the schedule of events, i.e.

	snd_seq_ev_set_direct(&ev);

before output the event packet.  this is equivalent with

	ev.queue = SND_SEQ_QUEUE_DIRECT;

if this is specified, the event is sent immediately to the
destination, so you don't need any queue.
as default, the event is scheduled using a specified queue, and as
default, the queue is #0.  that's why you had to allocate a queue.

one more note: you don't need to call snd_seq_drain_output() when you
send events via snd_seq_event_output_direct().  the drain_output() is
necessary for buffered outputs via normal snd_seq_event_output()
functions.


> What does "hw" mean ? what are the other possibilities ?

originally it means "hardware".
in future it can be other ones, such like "network:foo", etc.
(well, we can dream :)


ciao,

Takashi

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