[twobob]

Another interrupt driven example that fails:

http://equalarea.com/paul/alsa-audio.html

Title "A Minimal Interrupt-Driven Program" ( 3rd on page)

THE FAIL: straced.

Spoiler:

ioctl(4, SNDRV_PCM_IOCTL_SW_PARAMS, 0xbed9eb48) = 0         
ioctl(4, SNDRV_PCM_IOCTL_PREPARE, 0)    = 0                          
ioctl(4, SNDRV_PCM_IOCTL_SYNC_PTR, 0xe8240) = 0                       
ioctl(4, SNDRV_PCM_IOCTL_SW_PARAMS, 0xe8170) = 0              
ioctl(4, SNDRV_PCM_IOCTL_PREPARE, 0)    = 0
ioctl(4, SNDRV_PCM_IOCTL_SYNC_PTR, 0xe8240) = 0
poll([{fd=4, events=POLLOUT|POLLERR|POLLNVAL}], 1, 1000) = 0 (Timeout)
ioctl(4, SNDRV_PCM_IOCTL_SYNC_PTR, 0xe8240) = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
write(1, "playback callback called with 40"..., 42) = 42
ioctl(4, SNDRV_PCM_IOCTL_WRITEI_FRAMES, 0xbed9eb7c) = 0               
ioctl(4, SNDRV_PCM_IOCTL_SYNC_PTR, 0xe8240) = 0             
poll([{fd=4, events=POLLOUT|POLLERR|POLLNVAL}], 1, 1000) = 1 ([{fd=4,
ioctl(4, SNDRV_PCM_IOCTL_SYNC_PTR, 0xe8240) = 0
write(2, "poll failed (No such device or a"..., 40) = 40
ioctl(4, SNDRV_PCM_IOCTL_DROP, 0xdbe3c) = 0
ioctl(4, SNDRV_PCM_IOCTL_HW_FREE, 0x2b018) = 0

The Code

Spoiler:

	#include <stdio.h>
	#include <stdlib.h>
	#include <errno.h>
	#include <poll.h>
	#include <alsa/asoundlib.h>

	snd_pcm_t *playback_handle;
	short buf[4096];

	int
	playback_callback (snd_pcm_sframes_t nframes)
	{
		int err;

		printf ("playback callback called with %u frames\n", nframes);

		/* ... fill buf with data ... */

		if ((err = snd_pcm_writei (playback_handle, buf, nframes)) < 0) {
			fprintf (stderr, "write failed (%s)\n", snd_strerror (err));
		}

		return err;
	}

	main (int argc, char *argv[])
	{

		snd_pcm_hw_params_t *hw_params;
		snd_pcm_sw_params_t *sw_params;
		snd_pcm_sframes_t frames_to_deliver;
		int nfds;
		int err;
		struct pollfd *pfds;

		if ((err = snd_pcm_open (&playback_handle, argv[1], SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
			fprintf (stderr, "cannot open audio device %s (%s)\n",
				 argv[1],
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params_malloc (&hw_params)) < 0) {
			fprintf (stderr, "cannot allocate hardware parameter structure (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params_any (playback_handle, hw_params)) < 0) {
			fprintf (stderr, "cannot initialize hardware parameter structure (%s)\n",
				 snd_strerror (err));
			exit (1);
		}
   //     printf("here");
		if ((err = snd_pcm_hw_params_set_access (playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
			fprintf (stderr, "cannot set access type (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params_set_format (playback_handle, hw_params, SND_PCM_FORMAT_S16_LE)) < 0) {
			fprintf (stderr, "cannot set sample format (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params_set_rate (playback_handle, hw_params, 44100, 0)) < 0) {
			fprintf (stderr, "cannot set sample rate (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params_set_channels (playback_handle, hw_params, 2)) < 0) {
			fprintf (stderr, "cannot set channel count (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		if ((err = snd_pcm_hw_params (playback_handle, hw_params)) < 0) {
			fprintf (stderr, "cannot set parameters (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		snd_pcm_hw_params_free (hw_params);

		/* tell ALSA to wake us up whenever 4096 or more frames
		   of playback data can be delivered. Also, tell
		   ALSA that we'll start the device ourselves.
		*/

		if ((err = snd_pcm_sw_params_malloc (&sw_params)) < 0) {
			fprintf (stderr, "cannot allocate software parameters structure (%s)\n",
				 snd_strerror (err));
			exit (1);
		}
		if ((err = snd_pcm_sw_params_current (playback_handle, sw_params)) < 0) {
			fprintf (stderr, "cannot initialize software parameters structure (%s)\n",
				 snd_strerror (err));
			exit (1);
		}
		if ((err = snd_pcm_sw_params_set_avail_min (playback_handle, sw_params, 4096)) < 0) {
			fprintf (stderr, "cannot set minimum available count (%s)\n",
				 snd_strerror (err));
			exit (1);
		}
		if ((err = snd_pcm_sw_params_set_start_threshold (playback_handle, sw_params, 0U)) < 0) {
			fprintf (stderr, "cannot set start mode (%s)\n",
				 snd_strerror (err));
			exit (1);
		}
		if ((err = snd_pcm_sw_params (playback_handle, sw_params)) < 0) {
			fprintf (stderr, "cannot set software parameters (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		/* the interface will interrupt the kernel every 4096 frames, and ALSA
		   will wake up this program very soon after that.
		*/

		if ((err = snd_pcm_prepare (playback_handle)) < 0) {
			fprintf (stderr, "cannot prepare audio interface for use (%s)\n",
				 snd_strerror (err));
			exit (1);
		}

		while (4) {

			/* wait till the interface is ready for data, or 1 second
			   has elapsed.
			*/

			if ((err = snd_pcm_wait (playback_handle, 1000)) < 0) {
			        fprintf (stderr, "poll failed (%s)\n", strerror (errno));
			        break;
			}

			/* find out how much space is available for playback data */

			if ((frames_to_deliver = snd_pcm_avail_update (playback_handle)) < 0) {
				if (frames_to_deliver == -EPIPE) {
					fprintf (stderr, "an xrun occured\n");
					break;
				} else {
					fprintf (stderr, "unknown ALSA avail update return value (%d)\n",
						 frames_to_deliver);
					break;
				}
			}

			frames_to_deliver = frames_to_deliver > 4096 ? 4096 : frames_to_deliver;

			/* deliver the data */

			if (playback_callback (frames_to_deliver) != frames_to_deliver) {
			        fprintf (stderr, "playback callback failed\n");
				break;
			}
		}

		snd_pcm_close (playback_handle);
		exit (0);
	}

So issues with that one... Polling doesn't look like it wants to play nice.
It's only an example. I prefer the previous posts design in oh so many ways.