SDL
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1/*
2* This example code creates an simple audio stream for playing sound, and
3* generates a sine wave sound effect for it to play as time goes on. Unlike
4* the previous example, this uses a callback to generate sound.
5*
6* This might be the path of least resistance if you're moving an SDL2
7* program's audio code to SDL3.
8*
9* This code is public domain. Feel free to use it for any purpose!
10*/
11
12#define SDL_MAIN_USE_CALLBACKS 1 /* use the callbacks instead of main() */
13#include <SDL3/SDL.h>
14#include <SDL3/SDL_main.h>
15
16/* We will use this renderer to draw into this window every frame. */
17static SDL_Window *window = NULL;
18static SDL_Renderer *renderer = NULL;
19static SDL_AudioStream *stream = NULL;
20static int total_samples_generated = 0;
21
22/* this function will be called (usually in a background thread) when the audio stream is consuming data. */
23static void SDLCALL FeedTheAudioStreamMore(void *userdata, SDL_AudioStream *astream, int additional_amount, int total_amount)
24{
25/* total_amount is how much data the audio stream is eating right now, additional_amount is how much more it needs
26than what it currently has queued (which might be zero!). You can supply any amount of data here; it will take what
27it needs and use the extra later. If you don't give it enough, it will take everything and then feed silence to the
28hardware for the rest. Ideally, though, we always give it what it needs and no extra, so we aren't buffering more
29than necessary. */
30additional_amount /= sizeof (float); /* convert from bytes to samples */
31while (additional_amount > 0) {
32float samples[128]; /* this will feed 128 samples each iteration until we have enough. */
33const int total = SDL_min(additional_amount, SDL_arraysize(samples));
34int i;
35
36for (i = 0; i < total; i++) {
37/* You don't have to care about this math; we're just generating a simple sine wave as we go.
38https://en.wikipedia.org/wiki/Sine_wave */
39const float time = total_samples_generated / 8000.0f;
40const int sine_freq = 500; /* run the wave at 500Hz */
41samples[i] = SDL_sinf(6.283185f * sine_freq * time);
42total_samples_generated++;
43}
44
45/* feed the new data to the stream. It will queue at the end, and trickle out as the hardware needs more data. */
46SDL_PutAudioStreamData(astream, samples, total * sizeof (float));
47additional_amount -= total; /* subtract what we've just fed the stream. */
48}
49}
50
51/* This function runs once at startup. */
52SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
53{
54SDL_AudioSpec spec;
55
56if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO) == -1) {
57SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL!", SDL_GetError(), NULL);
58return SDL_APP_FAILURE;
59}
60
61/* we don't _need_ a window for audio-only things but it's good policy to have one. */
62if (SDL_CreateWindowAndRenderer("examples/audio/simple-playback-callback", 640, 480, 0, &window, &renderer) == -1) {
63SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL);
64return SDL_APP_FAILURE;
65}
66
67/* We're just playing a single thing here, so we'll use the simplified option.
68We are always going to feed audio in as mono, float32 data at 8000Hz.
69The stream will convert it to whatever the hardware wants on the other side. */
70spec.channels = 1;
71spec.format = SDL_AUDIO_F32;
72spec.freq = 8000;
73stream = SDL_OpenAudioDeviceStream(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec, FeedTheAudioStreamMore, NULL);
74if (!stream) {
75SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create audio stream!", SDL_GetError(), window);
76return SDL_APP_FAILURE;
77}
78
79/* SDL_OpenAudioDeviceStream starts the device paused. You have to tell it to start! */
80SDL_ResumeAudioStreamDevice(stream);
81
82return SDL_APP_CONTINUE; /* carry on with the program! */
83}
84
85/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
86SDL_AppResult SDL_AppEvent(void *appstate, const SDL_Event *event)
87{
88if (event->type == SDL_EVENT_QUIT) {
89return SDL_APP_SUCCESS; /* end the program, reporting success to the OS. */
90}
91return SDL_APP_CONTINUE; /* carry on with the program! */
92}
93
94/* This function runs once per frame, and is the heart of the program. */
95SDL_AppResult SDL_AppIterate(void *appstate)
96{
97/* we're not doing anything with the renderer, so just blank it out. */
98SDL_RenderClear(renderer);
99SDL_RenderPresent(renderer);
100
101/* all the work of feeding the audio stream is happening in a callback in a background thread. */
102
103return SDL_APP_CONTINUE; /* carry on with the program! */
104}
105
106/* This function runs once at shutdown. */
107void SDL_AppQuit(void *appstate)
108{
109/* SDL will clean up the window/renderer for us. */
110}
111
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