Why does MMAP access ignore start_threshold?

25 Feb 2026


      It seems that start_threshold (swparams) is not honoured if MMAP access is 
used.
Whereas I expect that once a total of enough bytes have been committed the 
device automatically starts the consuming of the buffer, as it does with 
snd_pcm_writei().
Is this a bug, or intentional design?
It does not seem to be the subject of any documentation -- yet for many 
years I seem to be be working around it.
Perhaps I am consistently doing something wrong, here's a test case below.
-- 
Mark


#include <alsa/asoundlib.h>

#define always(r) _always(r, __LINE__)
#define FORMAT SND_PCM_FORMAT_S16
#define ACCESS SND_PCM_ACCESS_MMAP_INTERLEAVED

static void _always(int r, unsigned line)
{
    if (r < 0) {
    	fprintf(stderr, "%d: %s\n", line, snd_strerror(r));
    	abort();
    }
}

static void set_hw(snd_pcm_t *pcm,
    	unsigned rate,
    	unsigned channels,
    	snd_pcm_uframes_t buffer_size,
    	snd_pcm_uframes_t period_size)
{
    int r;
    snd_pcm_hw_params_t *hw;

    snd_pcm_hw_params_alloca(&hw);

    r = snd_pcm_hw_params_any(pcm, hw);
    always(r);

    r = snd_pcm_hw_params_set_access(pcm, hw, ACCESS);
    always(r);

    r = snd_pcm_hw_params_set_format(pcm, hw, FORMAT);
    always(r);

    r = snd_pcm_hw_params_set_rate(pcm, hw, rate, 0);
    always(r);

    r = snd_pcm_hw_params_set_channels(pcm, hw, channels);
    always(r);

    r = snd_pcm_hw_params_set_buffer_size(pcm, hw, buffer_size);
    always(r);

    r = snd_pcm_hw_params_set_period_size(pcm, hw, period_size, 0);
    always(r);

    r = snd_pcm_hw_params(pcm, hw);
    always(r);
}

static void set_sw(snd_pcm_t *pcm,
    	snd_pcm_uframes_t start_threshold,
    	snd_pcm_uframes_t avail_min)
{
    int r;
    snd_pcm_sw_params_t *sw;

    snd_pcm_sw_params_alloca(&sw);

    r = snd_pcm_sw_params_current(pcm, sw);
    always(r);

    r = snd_pcm_sw_params_set_start_threshold(pcm, sw, start_threshold);
    always(r);

    r = snd_pcm_sw_params_set_avail_min(pcm, sw, avail_min);
    always(r);

    r = snd_pcm_sw_params(pcm, sw);
    always(r);
}

snd_pcm_uframes_t do_mmap(snd_pcm_t *pcm)
{
    int r;
    snd_pcm_uframes_t frames, offset;
    const snd_pcm_channel_area_t *area;

    r = snd_pcm_wait(pcm, 1000);
    always(r);

    r = snd_pcm_mmap_begin(pcm, &area, &offset, &frames);
    always(r);

    r = snd_pcm_mmap_commit(pcm, offset, frames);
    always(r);

    return frames;
}

snd_pcm_uframes_t do_write(snd_pcm_t *pcm, unsigned channels, snd_pcm_uframes_t frames)
{
    int r;
    void *buf;
    size_t len = channels * snd_pcm_format_size(FORMAT, frames);

    buf = alloca(len);
    bzero(buf, len);

    r = snd_pcm_writei(pcm, buf, frames);
    always(r);

    return frames;
}

int main(int argc, char *argv[])
{
    int r;
    snd_pcm_t *pcm;

    r = snd_pcm_open(&pcm, argv[1], SND_PCM_STREAM_PLAYBACK, 0);
    always(r);

    set_hw(pcm, 48000, 2, 2048, 512);
    set_sw(pcm, 1024, 512);

    r = snd_pcm_prepare(pcm);
    always(r);

    for (;;) {
    	snd_pcm_uframes_t frames;

    	if (ACCESS == SND_PCM_ACCESS_MMAP_INTERLEAVED)
    		frames = do_mmap(pcm);
    	else
    		frames = do_write(pcm, 2, 512);

    	fprintf(stderr, "written %lu\n", frames);
    }

    r = snd_pcm_close(pcm);
    always(r);

    return 0;
}

Mark Hills

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