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;
 }

---

Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel