SDL - How to play audio asynchronously in C++ without stopping code execution?

I am developing a clone of Asteroid in pure C++ and for that purpose, I need to add sounds to different events such as when a bullet is fired and when an explosion occurs. The issue however is that I don't have any experience with audio libraries.

I am using Simple DirectMedia Layer (SDL) and wrote a function named playsound() to play a sound in case a certain event occurs. The problem however is the fact that if an event occurs, playsound() is called and the code execution stops until the sound is wholly played out or until I return from the function (I delay the return using delay func).

What I would want to do is that the sound plays in the background without creating any lag for the rest of the Game. I am developing on Ubuntu 16.04 and can't use Windows PlaySound() either to call in the ASYNC flag.

Here is the function:

void playsound(string path) {
    // Initialize SDL.
        if (SDL_Init(SDL_INIT_AUDIO) < 0)
                return;

        // local variables
        Uint32 wav_length; // length of our sample
        Uint8 *wav_buffer; // buffer containing our audio file
        SDL_AudioSpec wav_spec;
        if(SDL_LoadWAV(path.c_str(), &wav_spec, &wav_buffer, &wav_length) == NULL){
          return;
        }
        SDL_AudioDeviceID deviceId = SDL_OpenAudioDevice(NULL, 0, &wav_spec, NULL, 0);
        SDL_QueueAudio(deviceId, wav_buffer, wav_length);
        SDL_PauseAudioDevice(deviceId, 0);
        SDL_Delay(50);
        SDL_CloseAudioDevice(deviceId);
        SDL_FreeWAV(wav_buffer);
        SDL_Quit();
}

Your delay is stopping your code from executing, 50ms of delay is almost 2 frames at 33ms per frame or 3 frames at 16ms per frame, having a frame drop here and there might not be a problem, but you could see how calling several sounds in succession will slow your program down.

This is how I play sounds in my engine, using SDL2_mixer, (short sounds, for music you have another method called Mix_PlayMusic), it might be helpful to you. I have no lag (and I don't use any sleep or delays in my code). Once you call play() the sound should be played in full, unless there is something else pausing your code.

#pragma once
#include <string>
#include <memory>
#include <SDL2/SDL_mixer.h>

class sample {
public:
    sample(const std::string &path, int volume);
    void play();
    void play(int times);
    void set_volume(int volume);

private:
    std::unique_ptr<Mix_Chunk, void (*)(Mix_Chunk *)> chunk;
};

And the cpp file

#include <sample.h>

sample::sample(const std::string &path, int volume)
    : chunk(Mix_LoadWAV(path.c_str()), Mix_FreeChunk) {
    if (!chunk.get()) {
        // LOG("Couldn't load audio sample: ", path);
    }

    Mix_VolumeChunk(chunk.get(), volume);
}

// -1 here means we let SDL_mixer pick the first channel that is free
// If no channel is free it'll return an err code.
void sample::play() {
    Mix_PlayChannel(-1, chunk.get(), 0);
}

void sample::play(int times) {
    Mix_PlayChannel(-1, chunk.get(), times - 1);
}

void sample::set_volume(int volume) {
    Mix_VolumeChunk(chunk.get(), volume);
}

Notice that I don't need to thread my model, every time something triggers a sound play the program keeps execution. (I guess SDL_Mixer plays in the main SDL thread).

For this to work, where you init SDL you'll also have to init the mixer as

if (Mix_OpenAudio(44100, MIX_DEFAULT_FORMAT, 2, 1024) < 0) {
    // Error message if can't initialize
}

// Amount of channels (Max amount of sounds playing at the same time)
Mix_AllocateChannels(32);

And an example of how to play a sound would be

// at some point loaded a sample s with sample(path to wave mp3 or whatever)
s.play();

A few remarks, you don't need to use, but can, the code as it is, it is more of a simple example of using SDL2_mixer.

This mean functionality is lacking, you might want a tighter handling of sound, for example to stop a sound mid play (for some reason), you can do this if you play your sounds in different channels with the Mix_HaltChannel function, and the play() function could receive the channel where you want it to be played.

All these functions return error values, for example if no unreserved channel is available Mix_PlayChannel will return an error code.

Another thing you want to keep in mind is if you play the same sound several times it'll start to get blurry/you would not notice if the same sound is being played again. So you could add an integer to sample to count how many times a sample can be played.

If you REALLY want to thread your mixer/audio from the main SDL thread (and still only use SDL), you can just spawn a new SDL context in a thread and send in some way signals to play audio.