spotify session callback get_audio_buffer_stats

I'm trying to make a program in Spotify that collects the audio data. I saw in the API that there is a callback get_audio_buffer_stats, which has stutter and samples. I tried adding that to the program (I am just modifying the jukebox example), but it only ever prints 0 for stutter and samples, even when I turn off the wifi and wait for the song to stop playing. And by adding the code, I mean that I made a callback function for it, and I added it to the session callbacks. Am I missing something? Can anyone help me to get this callback to work? Thanks! My code is below:

static void get_audio_buffer_stats(sp_session *sess, sp_audio_buffer_stats *stats)
{
pthread_mutex_lock(&g_notify_mutex);

//log session data
stuttervariable = stats->stutter;
samplesvariable = stats->samples;

printf("stutter, %d\n", stuttervariable);
printf("samples, %d\n", samplesvariable);

pthread_cond_signal(&g_notify_cond);
pthread_mutex_unlock(&g_notify_mutex);

}

/**
 * The session callbacks
 */
static sp_session_callbacks session_callbacks = {
.logged_in = &logged_in,
.notify_main_thread = &notify_main_thread,
.music_delivery = &music_delivery,
.metadata_updated = &metadata_updated,
.play_token_lost = &play_token_lost,
.log_message = NULL,
.end_of_track = &end_of_track,
.get_audio_buffer_stats = &get_audio_buffer_stats,
};

I think the idea with get_audio_buffer_stats is that you are supposed to tell libspotify if you've suffered stuttering and how many samples are left in your buffer. When it calls get_audio_buffer_stats, it passes a pointer to a struct that you are supposed to fill in. Presumably if you tell libspotify that you're suffering stutter it will try to send you a bit more data to keep your buffer more full. By telling libspotify how full your buffer is, it can accommodate for drift in your clock causing you to consume audio slightly faster or slower than it expects.