Alsa buffer underrun

Question

I am trying to write random noise to to a device and allow my loop to sleep when I have written enough data. My understanding is that for each call to snd_pcm_writei I am writing 162 bytes (81 frames) which at 8khz rate and 16bit format it should be enough audio for ~10ms. I have verified that alsa does tell me I have written 81 frames.

I would expect that I can then sleep for a short amount of time before waking up and pushing the next 10 ms worth of data. However when I sleep for any amount - even a single ms - I start to get buffer underrun errors.

Obviously I have made an incorrect assumption somewhere. Can anyone point me to what I may be missing? I have removed most error checking to shorten the code - but there are no errors initializing the alsa system on my end. I would like to be able to push 10ms of audio and sleep (even for 1 ms) before pushing the next 10ms.

#include <alsa/asoundlib.h>
#include <spdlog/spdlog.h>

int main(int argc, char **argv) {
    snd_pcm_t* handle;
    snd_pcm_hw_params_t* hw;

    unsigned int rate = 8000;
    unsigned long periodSize = rate / 100;  //period every 10 ms

    int err = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
    snd_pcm_hw_params_malloc(&hw);
    snd_pcm_hw_params_any(handle, hw);
    snd_pcm_hw_params_set_access(handle, hw, SND_PCM_ACCESS_RW_INTERLEAVED);
    snd_pcm_hw_params_set_format(handle, hw, SND_PCM_FORMAT_S16_LE);
    snd_pcm_hw_params_set_rate(handle, hw, rate, 0);
    snd_pcm_hw_params_set_channels(handle, hw, 1);

    int dir = 1;
    snd_pcm_hw_params_set_period_size_near(handle, hw, &periodSize, &dir);
    snd_pcm_hw_params(handle, hw);

    snd_pcm_uframes_t frames;
    snd_pcm_hw_params_get_period_size(hw, &frames, &dir);
    int size = frames * 2; // two bytes a sample
    char* buffer = (char*)malloc(size);
    unsigned int periodTime;
    snd_pcm_hw_params_get_period_time(hw,&periodTime, &dir);
    snd_pcm_hw_params_free(hw);
    snd_pcm_prepare(handle);

    char* randomNoise = new char[size];
    for(int i = 0; i < size; i++)
        randomNoise[i] = random() % 0xFF;

    while(true) {
        err = snd_pcm_writei(handle, randomNoise, size/2);
        if(err > 0) {
            spdlog::info("Write {} frames", err);
        } else {
            spdlog::error("Error write {}\n", snd_strerror(err));
            snd_pcm_recover(handle, err, 0);
            continue;
        }
        usleep(1000); // <---- This is what causes the buffer underrun
    }
}

Answer 1

Try to put in /etc/pulse/daemon.conf :

default-fragments = 5
default-fragment-size-msec = 2

and restart linux.

Answer 2

What I don't understand is why you write a buffer of size "size" to the device, and in the approximate calculations of time you rely on the "periodSize" declared by you. Then write a buffer with the size "periodSize" to the device.