Function outputting sound through alsa not working when called via pthread create: no sound, 100% CPU usage

Question

I have a program that receives messages via a socket and starts or stops playing a certain sound file depending on the message. In order for the "stop" message to work, I need the sound to play from a separate thread. My solution is to play the sound using alsa from a function I invoke using pthread_create(), and upon receiving a stop message I end the thread using pthread_cancel(). The function that plays the sound is called play_sound(void *args);

Here's what works:

struct args *args;

args->fp     = fopen("path/to/soundfile.wav", "r");
args->volume = 1;

play_sound((void *) args);

but as soon as I try to run the function from within a new thread, I get no sound and 100% CPU usage on both my threads:

struct args *args;
int sound_thread;

args->fp     = fopen("path/to/soundfile.wav", "r");
args->volume = 1;

pthread_create(&sound_thread, NULL, (void *) play_sound, (void *) args);

I have no idea where to even begin troubleshooting.

My code looks as follows:

#include <alsa/asoundlib.h>
#include <alsa/mixer.h>
#include <stdbool.h>
#include <pthread.h>

#include "server.h"
#include "sound.h"
//#include "log.h" 



int sound_thread;




struct args {
    FILE *fp;
    float volume;
};




void init_sound ()
{
    sound_thread = -1;
}




void stop_sound ()
{
    if (sound_thread != -1) {
        pthread_cancel(sound_thread);
        keep_playing = false;
        sound_thread = -1;
    }
}




void dispatch_sound (FILE *fp, float volume)
{

    // this function serves to create a new thread for the
    // sound to be played from. This is what's giving me
    // headaches.


    if (sound_thread != -1) {
        stop_sound();
    }

    struct args *args = (struct args *) malloc(sizeof(struct args));

    args->fp     = fp;
    args->volume = volume;

    if (pthread_create(&sound_thread, NULL, (void *) play_sound, args) != 0) 
        sound_thread = -1;
    }
}




bool play_sound (void *args)
{

    // This function actually plays the sound using functions
    // from the alsa lib. it works when invoked regularly without
    // threading.


    keep_playing = true;

    FILE                *fp;
    int                 volume;
    bool                success;
    unsigned int        samplerate;
    int                 bufsz;
    char                *buf;
    snd_pcm_t           *pcm;
    snd_pcm_hw_params_t *params;
    snd_pcm_uframes_t   frames;

    samplerate = SAMPLERATE;


    fp     = ((struct args *) args)->fp;
    volume = ((struct args *) args)->volume;

    // volume is not actually used right now, since I took out
    // the function that sets the volume before playing the
    // audio in order to make it easier to pinpoint the issue.


    if (snd_pcm_open(&pcm, PCM_DEVICE, SND_PCM_STREAM_PLAYBACK, 0) < 0) {
        success = false;
    }


    snd_pcm_hw_params_alloca(&params);
    snd_pcm_hw_params_any(pcm, params);

    if (snd_pcm_hw_params_set_access(pcm, params, SND_PCM_ACCESS_RW_INTERLEAVED) < 0) {
        success = false;
    }

    if (snd_pcm_hw_params_set_format(pcm, params, SND_PCM_FORMAT_S16_LE) < 0) {
        success = false;
    }

    if (snd_pcm_hw_params_set_channels(pcm, params, CHANNELS) < 0) {
        success = false;
    }

    if (snd_pcm_hw_params_set_rate_near(pcm, params, &samplerate, 0) < 0) {
        success = false;
    }´

    if (snd_pcm_hw_params(pcm, params) < 0) {
        success = false;
    }


    snd_pcm_hw_params_get_period_size(params, &frames, 0);
    bufsz = frames * CHANNELS * SAMPLE_SIZE;
    buf   = (char *) malloc(bufsz);


    while (keep_playing) {
        while (fread(buf, bufsz, 1, fp) != 0 && keep_playing) {

            int err;

            if ((err = snd_pcm_writei(pcm, buf, frames)) == -EPIPE) {
                snd_pcm_prepare(pcm);
            }
        }

        rewind(fp);
    }


    snd_pcm_drain(pcm);
    snd_pcm_close(pcm);
    free(buf);


    return success;
}

Answer 1

From the man page of pthread_cancel:

On Linux, cancellation is implemented using signals. Under the NPTL threading implementation, the first real-time signal (i.e., signal 32) is used for this purpose. On LinuxThreads, the second real-time signal is used, if real-time signals are available, otherwise SIGUSR2 is used.

In your while(keep_playing) loop, you aren't yielding the thread enough to handle the cancel signal; in your main thread; you aren't waiting for the result of the cancel request, ergo both threads hog the cpu.

A small delay before you restart playing the sound and pthread_join() after you call pthread_cancel should fix your problem.