ALSA playback interrupted without snd_pcm_hw_params_get_* calls - audio

Question

I'm finding that a simple ALSA playback program behaves differently when I put in some calls to snd_pcm_hw_params_get_* functions. My program plays a sine wave from a buffer repeatedly. When I include the calls, I get a pure tone as I expected. When I remove the calls, however, I get a series of beeps. This worries me, because I would not expect calls that retrieve data to have anything to do with how the sound is played. I get this behavior both on a cheap USB sound card and my (presumably nicer) internal sound card.
Here is the code:
#define GETPARAMS
int main() {
snd_pcm_t *handle;
snd_pcm_hw_params_t *params;
const char name[] = "hw:0,0";
int dir;
snd_pcm_stream_t stream = SND_PCM_STREAM_PLAYBACK;
snd_pcm_access_t access = SND_PCM_ACCESS_RW_INTERLEAVED;
snd_pcm_format_t format = SND_PCM_FORMAT_S16_LE;
unsigned int rate = 48000;
unsigned int channels = 2;
unsigned int periods = 4;
snd_pcm_uframes_t periodsize = 2048;
int num_frames = 2*periodsize;
snd_pcm_hw_params_alloca(&params);
snd_pcm_open(&handle, name, stream, 0);
snd_pcm_hw_params_any(handle, params);
#ifdef GETPARAMS
printf("\nparameters before setting:\n");
snd_pcm_hw_params_get_rate(params, &rate, &dir);
printf(" rate = %d, dir = %d\n", rate, dir);
snd_pcm_hw_params_get_channels(params, &channels);
printf(" channels = %d\n", channels);
snd_pcm_hw_params_get_periods(params, &periods, &dir);
printf(" periods = %d, dir = %d\n", periods, dir);
snd_pcm_hw_params_get_buffer_size(params, &periodsize);
printf(" periodsize = %ld\n", periodsize);
#endif
snd_pcm_hw_params_set_access(handle, params, access);
snd_pcm_hw_params_set_format(handle, params, format);
snd_pcm_hw_params_set_rate_near(handle, params, &rate, &dir);
snd_pcm_hw_params_set_channels(handle, params, 2);
snd_pcm_hw_params_set_periods(handle, params, periods, 0);
snd_pcm_hw_params_set_buffer_size(handle, params, num_frames);
snd_pcm_hw_params(handle, params);
#ifdef GETPARAMS
printf("\nparameters after setting:\n");
snd_pcm_hw_params_get_rate(params, &rate, &dir);
printf(" rate = %d, dir = %d\n", rate, dir);
snd_pcm_hw_params_get_channels(params, &channels);
printf(" channels = %d\n", channels);
snd_pcm_hw_params_get_periods(params, &periods, &dir);
printf(" periods = %d, dir = %d\n", periods, dir);
snd_pcm_hw_params_get_buffer_size(params, &periodsize);
printf(" periodsize = %ld\n\n", periodsize);
#endif
int16_t *data = (int16_t*)calloc(2*periodsize, sizeof(int16_t));
loadpage(data, 2*periodsize);
snd_pcm_sframes_t frames;
snd_pcm_prepare(handle);
for (int i=0; i<8; i++) {
frames = snd_pcm_writei(handle, data, num_frames);
if (frames < 0)
frames = snd_pcm_recover(handle, frames, 0);
if (frames < 0) {
printf("snd_pcm_writei failed: %s\n", snd_strerror(frames));
}
if (frames > 0 && frames < num_frames)
printf("short write (expected %d, write %li)\n", num_frames, frames);
}
snd_pcm_close(handle);
free(data);
}
loadpage() fills the buffer. When I comment out the #define GETPARAMS I get a series of short beeps. When I include it I get a pure tone.
Here is the output when GETPARAMS is defined:
parameters before setting:
rate = 48000, dir = 32766
channels = 2
periods = 4, dir = 32766
periodsize = 2048
parameters after setting:
rate = 48000, dir = 0
channels = 2
periods = 4, dir = 0
periodsize = 4096

Answer 1

You must not call the snd_pcm_hw_param_get_*() functions if the parameters have not yet been set because at that time, the configuration space contains multiple potential values for the parameters.
To print the current state of the hw_params container, use snd_pcm_hw_params_dump():
snd_output_t *output;
snd_output_stdio_attach(&output, stdout, 0);
...
snd_pcm_hw_params_dump(params, output);
...
snd_output_close(output);
Anyway, the problem is that the initial values of periods, periodsize, and num_frames are inconsistent, and that the _get_ calls overwrite these variables with other values that happen to be consistent.
I do not know what values you actually want to use, but note that the period size and the buffer size are measured in frames, and that one frame contains all samples of all channels, i.e., in this case, one frame has four bytes.