Alsa snd_pcm_avail always returning a 0 - alsa

Question

I am trying to read data from my codec. For reasons in my project I would like to do nonblocking, but every time I read the number of bytes available on my codec it says zero.
The algorithm is pretty simple: wait 1ms then check to see if there are 160+ samples available in the codec to read and then read the samples. But every time I do a read it says the sample count is zero.
Can someone help me understand why "rc = snd_pcm_avail(inputCodecHandle);" is always returning a zero?
Here is the thread with the code in it.
void CRadioStack::rcvThread() {
ChannelBuffer_t *buffer_p = NULL;
int8_t *inputBuf_p;
int rc;
int16_t *inputBuf16_p;
int samplesToRead;
const int rxFrameSize = 160;
snd_pcm_sframes_t delay;
snd_pcm_nonblock(inputCodecHandle, 1);
snd_pcm_prepare(inputCodecHandle);
while (true) {
TWTime::msleep(1);
// get the number of samples available
snd_pcm_delay(inputCodecHandle, &delay);
rc = snd_pcm_avail(inputCodecHandle);
if (rc < 0) {
myLog->warn("Error in getting sample count: %s", snd_strerror(rc));
snd_pcm_prepare(outputCodecHandle);
continue;
}
samplesToRead = rc;
// if number of samples > 160 then get 160 samples
if (samplesToRead <= rxFrameSize) {
continue;
}
// read the from the codec into the Channel Buffer.
rc = snd_pcm_readi(inputCodecHandle, inputBuf_p, rxFrameSize);
if (rc < 0) {
myLog->warn("Error reading Codec: %s", snd_strerror(rc));
continue;
} else if (rc != rxFrameSize) { // nothing to get
myLog->warn("Input samples on codec not 160");
}
pushToInputQueue(inputBuf_p);
}
}
And here is the code to open the codec.
bool CRadioStack::openInputCodec()
{
unsigned int val;
int dir;
const int NUM_OF_CHAN = 1;
codecRunning = false;
snd_pcm_uframes_t frames;
int rc;
snd_pcm_t *handle;
snd_pcm_hw_params_t *params;
inputCodecHandle = nullptr;
// Open pcm device for output
rc = snd_pcm_open(&handle, "hw:0,0", SND_PCM_STREAM_CAPTURE, 0);
if (rc < 0) {
myLog->error("Unable to open input codec: %s", snd_strerror(rc));
return false;
}
// allocate a hardware parameters object
snd_pcm_hw_params_alloca(&params);
// fill with default values
snd_pcm_hw_params_any(handle, params);
// now setup the hardware paramters
snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED); // interleaved
snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE); // 16bin linear little-endian
snd_pcm_hw_params_set_channels(handle, params, NUM_OF_CHAN); // one channel
val = 0;
snd_pcm_hw_params_set_channels_near(handle, params, &val); // one channel
val = 8000;
dir = 0;
snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir); // 8k sample rate.
frames = 160;
snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir); // period size = 160 frames
// save the hardware parameters
rc = snd_pcm_hw_params(handle, params);
if (rc < 0) {
myLog->error("Unable to save hardware parameters to output codec.");
return false;
}
// ready to write to output codec.
// so save the handle so that it can be used elsewhere.
inputCodecHandle = handle;
return true;
}
Thanks!

Answer 1

The device was never started.
This would happen automatically with the first snd_pcm_read*() call, but can also be done explicitly with snd_pcm_start().