snd_pcm_writei
takes in 3 parameters, as you already know:
- PCM handle
- frames containing buffer
- frames to be written
You have to read()
the frames from the PCM into the buffer and then play them back
Here is a short program (reformatted from here, listing 3) that plays back the raw PCM data:
/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API
#include <alsa/asoundlib.h>
int main() {
long loops;
int rc;
int size;
snd_pcm_t *handle;
snd_pcm_hw_params_t *params;
unsigned int val;
int dir;
snd_pcm_uframes_t frames;
char *buffer;
/* Open PCM device for playback. */
rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
if (rc < 0)
{
fprintf(stderr, "Unable to open PCM device: %s\n", snd_strerror(rc));
exit(1);
}
/* Allocate a hardware parameters object. */
snd_pcm_hw_params_alloca(¶ms);
/* 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, SND_PCM_ACCESS_RW_INTERLEAVED);
/* Signed 16-bit little-endian format */
snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
/* Two channels (stereo) */
snd_pcm_hw_params_set_channels(handle, params, 2);
/* 44100 bits/second sampling rate (CD quality) */
val = 44100;
snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
/* Set period size to 32 frames. */
frames = 32;
snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
/* Write the parameters to the driver */
rc = snd_pcm_hw_params(handle, params);
if (rc < 0)
{
fprintf(stderr, "Unable to set HW parameters: %s\n", snd_strerror(rc));
exit(1);
}
/* Use a buffer large enough to hold one period */
snd_pcm_hw_params_get_period_size(params, &frames, &dir);
size = frames * 4; /* 2 bytes/sample, 2 channels */
buffer = (char *) malloc(size);
/* We want to loop for 5 seconds */
snd_pcm_hw_params_get_period_time(params, &val, &dir);
/* 5 seconds in microseconds divided by period time */
loops = 5000000 / val;
while (loops > 0)
{
loops--;
rc = read(0, buffer, size); /* ### Retrieve the buffer ### */
if (rc == 0)
{
fprintf(stderr, "End of file on input\n");
break;
} else if (rc != size)
{
fprintf(stderr, "Short read: read %d bytes\n", rc);
}
rc = snd_pcm_writei(handle, buffer, frames);
if (rc == -EPIPE)
{
/* EPIPE means underrun */
fprintf(stderr, "Underrun occurred\n");
snd_pcm_prepare(handle);
} else if (rc < 0)
{
fprintf(stderr, "Error from writei: %s\n", snd_strerror(rc));
} else if (rc != (int)frames)
{
fprintf(stderr, "Short write, write %d frames\n", rc);
}
}
snd_pcm_drain(handle);
snd_pcm_close(handle);
free(buffer);
return 0;
}