Unable to poll() or select() with PCM capture device

["Christophe Osuna" <christophe.osuna@xxxxxxxxx>]

Hi,

I am trying to use select() on ALSA devices but it does not work. I
must be missing something. Does anyone have an idea of what is wrong
with my code?

The following function works (it prints '+' whenever a new frame is ready):

void capture (snd_pcm_t *capture_handle, short *buffer,
			     int samples)
{
	while (1) {
		snd_pcm_readi (capture_handle, buffer, samples);
		printf ("+");
		fflush (stdout);
	}
}


The following uses select(). I can't get any '+' to be printed. If I
'ls -l' the file descriptor in /proc/<pid>/fd/ I can see that it is
the file descriptor of the capture device (as expected). If I use a
playback device (SND_PCM_STREAM_PLAYBACK) and the writefds of select
() the program is not blocked in the select ().

void capture_with_select (snd_pcm_t *capture_handle, short *buffer,
			  int samples)
{
	struct pollfd ufds;
	fd_set fds;

	int count = snd_pcm_poll_descriptors_count (capture_handle);
	
	if (snd_pcm_poll_descriptors(capture_handle, &ufds, count) < 0) {
		printf ("Can't get poll descriptor\n");
		exit (EXIT_FAILURE);
	}

	printf ("File descriptor: %d\n", ufds.fd);

	while (1) {
		FD_ZERO (&fds);
		FD_SET (ufds.fd, &fds);

		select (ufds.fd + 1, &fds, NULL, NULL, NULL);

		snd_pcm_readi (capture_handle, buffer, samples);
		printf ("+");
		fflush (stdout);
	}
}


Using poll() gives the same results; the program is blocked in poll()
when using a capture device.

void capture_with_poll (snd_pcm_t *capture_handle, short *buffer,
			int samples)
{
	struct pollfd ufds;
	unsigned short revents;

	int count = snd_pcm_poll_descriptors_count (capture_handle);
	
	if (snd_pcm_poll_descriptors(capture_handle, &ufds, count) < 0) {
		printf ("Can't get poll descriptor\n");
		exit (EXIT_FAILURE);
	}

	printf ("File descriptor: %d\n", ufds.fd);

	while (1) {
		poll (&ufds, count, -1);
		snd_pcm_poll_descriptors_revents (capture_handle, &ufds, count,
						  &revents);

		if (revents & POLLIN) {
			snd_pcm_readi (capture_handle, buffer, samples);
			printf ("+");
			fflush (stdout);
		}
		else {
			printf ("?");
			fflush (stdout);
		}
	}
}

Tested on:
_ Mandriva 2006, ALSA 1.0.9, USB sound device
_ SUSE 10.1, ALSA 1.0.11, USB sound device, SB Live! and ICH7 AC'97

Any idea of what is wrong and why it works with playback device but
not capture device?

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