How to get CardId for pseudo soundcards like dmix and dsnoop

[Gregor Jasny <jasny@vidsoft.de>]

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Hi,

In my application the user can specify the pcm devices as a string (i.e. 
hw:0,0, plughw:1, dmix, default).

Now I want to open the corresponding mixer device. At the moment I fetch 
the cardid via snd_pcm_info_get_card. Then I build a string hw:<cardid> 
and attach to that string. This works fine for hw and plughw devices. 
Doing this for dmix or dsnoop devices fails because the cardid is -1.

Another way I tried is to attach to the user supplied pcm string 
directly. But this does not work for dmix and dsnoop.

How do I get the right mixer for a opened pcm device?

Thank you for answering.

Gregor

PS: I've attached a small test case

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#include <stdio.h>
#include <stdlib.h>

#include <alsa/asoundlib.h>



int main (int argc, char *argv[])
{
	snd_pcm_t *pcm;
	snd_mixer_t *mixer;
	int err;
	int incoming = 1;
	snd_pcm_info_t *pcm_info;
	int cardid;
	char dev[64];

	if (argc < 2)
		exit (EXIT_FAILURE);
	
	if ((err = snd_pcm_open (&pcm,
	                         argv[1],
	                         (incoming?SND_PCM_STREAM_CAPTURE:SND_PCM_STREAM_PLAYBACK),
	                         SND_PCM_NONBLOCK)) < 0) {
		fprintf (stderr, "Can not determine card id: %s (%s)\n", argv[1], snd_strerror (err));
		return EXIT_FAILURE;
	}

	snd_pcm_info_alloca(&pcm_info);
	if ((err = snd_pcm_info( pcm, pcm_info )) < 0) {
        	fprintf (stderr, "Can not get pcm info: %s (%s)\n", argv[1], snd_strerror (err));
		snd_pcm_close(pcm);
		return EXIT_FAILURE;
	}

	cardid = snd_pcm_info_get_card( pcm_info );

	printf ("CardId: %i\n", cardid);

	sprintf(dev, "hw:%i", cardid); /* Option 1 */
//	strcpy (dev, argv[1]); /* Option 2 */

	if ((err = snd_mixer_open(&mixer, 0)) < 0) {
		fprintf (stderr, "snd_open_mixer(): %s\n", snd_strerror(-err));
		return EXIT_FAILURE;
	}
	if ((err = snd_mixer_attach(mixer, dev)) < 0) {
		fprintf (stderr, "snd_mixer_attach(%s): %s\n", dev, snd_strerror(-err));
		return EXIT_FAILURE;
	}
	printf("Attached to mixer: %s\n", dev);
	
	return EXIT_SUCCESS;
}