[alsa-devel] ALSA

Latha Prabhu lvprabhu at gmail.com
Fri Sep 9 08:52:08 CEST 2011

I wrote a sample program as follows to OPEN ALSA sound device and set its hw
params -


    /* Open PCM device for playback. */
    rc = snd_pcm_open(&handle, "default",
                    SND_PCM_STREAM_PLAYBACK, 0);
    if (rc < 0) {
            "unable to open pcm device: %s\n",
      return 1;

    /* Allocate a hardware parameters object. */

    /* Fill it in with default values. */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params,

    /* Signed 16-bit little-endian format */
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);

    /* One channel (mono) */
    snd_pcm_hw_params_set_channels(handle, params, 1);

    /* 8000 bits/second sampling rate  */
    val = 8000;
    snd_pcm_hw_params_set_rate_near(handle, params,
                                  &val, &dir);

    /* Set period size to 32 frames. */
    frames = 32;
                              params, &frames, &dir);

    /* Write the parameters to the driver */
    rc = snd_pcm_hw_params(handle, params);

Above lines of code works fine if I run it as an independent test program.
When I use the same lines of code in my application, it is failing in
snd_pcm_hw_params_any(). It returns "Operation not permitted". So I changed
the device name to "plughw:0,0", instead of "default".
With this change, its failing at snd_pcm_hw_params_set_rate_near(). It
returns some huge value 192000. Also, following
snd_pcm_hw_params_set_period_size() also fails with 699050.

Any idea, why I'm getting this error? Is link to some other library or some
header file inclusion, in my application causing this error? I am trying to
play 8k, 16 bit PCM audio.

Then I changed snd_pcm_hw_set_rate_near() to snd_pcm_hw_set_rate() and also
snd_pcm_hw_params_set_period_size_near() to
snd_pcm_hw_params_set_period_size(). These 2 calls succeed. But when I check
period_size after calling snd_pcm_hw_params(), it is different. My set value
was 32 but I got it as 1. Then call to snd_pcm_writei() crashes with -
 pcm.c:1240: snd_pcm_writei: Assertion `pcm' failed.

Please note my application is  multi threaded but opening the device and
setting hardware parameters are under one place.

I appreciate your help on this.


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