[alsa-devel] Segmentation fault during snd_pcm_prepare()

[David Henningsson]

On 05/15/2013 06:47 AM, Shane Loretz wrote:
> I'm interested in writing an application using alsa. I've chosen to
> explore the library by writing a program to echo sound data from a
> microphone back to my speakers.
>
> I'm only interested in mono sound.
>
> When running this program, the first call to snd_pcm_readi() fails,
> and a segmentation fault occurs inside of the snd_pcm_prepare()
> function according to a backtrace from gdb. I suspect I'm not
> allocating enough space for the pointer capdata, but I don't know how
> to determine how much space I need.

S16_LE = 16 bits = 2 bytes per frame.

So if you're trying to read num_frames frames, the amount allocated must 
be at least num_frames * 2. (Also multiplied with the number of 
channels, which is 1 in this case.)

Also snd_pcm_recover is recommended to use to recover from error (rather 
than snd_pcm_prepare).

>
> Here is the code
>
> // Test out the alsa library
> //
> #include <alsa/asoundlib.h>
> #include <cstdio>
> #include <unistd.h>
>
> bool getMonoDevice(snd_pcm_t *pcm_handle, snd_pcm_stream_t stream,
> unsigned int rate, int periods, snd_pcm_uframes_t periodsize);
>
> int main()
>      {
>      snd_pcm_t *pcm_handle_read;
>      snd_pcm_t *pcm_handle_write;
>      snd_pcm_stream_t stream = SND_PCM_STREAM_CAPTURE;
>
>      unsigned int rate = 44100;
>      int periods = 2;
>      snd_pcm_uframes_t periodsize = 8192;
>
>      if (!getMonoDevice(pcm_handle_write,
> SND_PCM_STREAM_PLAYBACK,rate,periods,periodsize))
>          {
>          fprintf(stderr, "Error couldn't open speakers\n");
>          return -1;
>          }
>      if (!getMonoDevice(pcm_handle_read,
> SND_PCM_STREAM_CAPTURE,rate,periods,periodsize))
>          {
>          fprintf(stderr, "Error couldn't open microphone\n");
>          return -1;
>          }
>
>      //read frames from device
>      //and echo them out
>      unsigned char *capdata;
>      int pcmreturn;
>      int num_frames;
>      int error;
>
>      capdata = (unsigned char *)malloc(periodsize);
>      num_frames = periodsize * 2;
>      while (true)
>          {
>          while ((pcmreturn = snd_pcm_readi(pcm_handle_read, capdata,
> num_frames)) < 0)
>              {
>              usleep(10);
>              error = snd_pcm_prepare(pcm_handle_read);
>              if (error < 0)
>                  {
>                  fprintf(stderr, "Unrecoverable mic error
> %s\n",snd_strerror(error));
>                  return error;
>                  }
>              }
>          while ((pcmreturn = snd_pcm_writei(pcm_handle_write, capdata,
> num_frames)) < 0)
>              {
>              usleep(10);
>              error = snd_pcm_prepare(pcm_handle_write);
>              if (error < 0)
>                  {
>                  fprintf(stderr, "Unrecoverable speaker error
> %s\n",snd_strerror(error));
>                  return error;
>                  }
>              }
>          }
>
>      //stop pcm device and drop pending frames
>      snd_pcm_drop(pcm_handle_read);
>      snd_pcm_drain(pcm_handle_read);
>      snd_pcm_drop(pcm_handle_write);
>      snd_pcm_drain(pcm_handle_write);
>      }
>
>
> bool getMonoDevice(snd_pcm_t *pcm_handle, snd_pcm_stream_t stream,
> unsigned int rate, int periods, snd_pcm_uframes_t periodsize)
>      {
>      snd_pcm_hw_params_t *hwparams;
>
>      char *pcm_name;
>
>      pcm_name = strdup("plughw:0,0");
>
>      snd_pcm_hw_params_alloca(&hwparams);
>
>      if (snd_pcm_open(&pcm_handle, pcm_name, stream, 0) < 0)
>          {
>          fprintf(stderr, "Error opening PCM device %s\n", pcm_name);
>          return false;
>          }
>
>      if (snd_pcm_hw_params_any(pcm_handle, hwparams) < 0 )
>          {
>          fprintf(stderr, "Can not configure this PCM device.\n");
>          return false;
>          }
>
>      unsigned int exact_rate;
>      int dir;
>
>      if (snd_pcm_hw_params_set_access(pcm_handle, hwparams,
> SND_PCM_ACCESS_RW_INTERLEAVED) < 0)
>          {
>          fprintf(stderr, "Error setting access.\n");
>          return false;
>          }
>
>      if (snd_pcm_hw_params_set_format(pcm_handle, hwparams,
> SND_PCM_FORMAT_S16_LE) < 0)
>          {
>          fprintf(stderr, "Error setting format.\n");
>          return false;
>          }
>
>      exact_rate = rate;
>      if (snd_pcm_hw_params_set_rate_near(pcm_handle, hwparams,
> &exact_rate, 0) < 0)
>          {
>          fprintf(stderr, "Error setting rate.\n");
>          return false;
>          }
>      if (rate != exact_rate)
>          {
>          fprintf(stderr, "The rate %d is not supported by your
> hardware.\n Using %d Hz instead.\n", rate, exact_rate);
>          }
>
>      //Configure mono sound
>      if (snd_pcm_hw_params_set_channels(pcm_handle, hwparams, 1) < 0)
>          {
>          fprintf(stderr, "Error setting channels.\n");
>          return false;
>          }
>
>      if (snd_pcm_hw_params_set_periods(pcm_handle, hwparams, periods, 0) < 0)
>          {
>          fprintf(stderr, "Error setting periods.\n");
>          return false;
>          }
>
>      if (snd_pcm_hw_params_set_buffer_size(pcm_handle, hwparams,
> (periodsize * periods) >> 2) < 0 )
>          {
>          fprintf(stderr, "Error setting buffersize.\n");
>          return false;
>          }
>
>      if (snd_pcm_hw_params(pcm_handle, hwparams) < 0)
>          {
>          fprintf(stderr, "Error setting HW params.\n");
>          return false;
>          }
>
>      //device is finally configured
>      return true;
>      }
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>



-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic