A very simple square wave tone generator I made to create sound effects for a game using the SDL cross-platform multimedia library, with no dynamic allocations or VLAs.

tonegen.c

// A very simple square wave tone generator I made to create sound effects for
// a game using the SDL cross-platform multimedia library, with no dynamic
// allocations or VLAs.
//
// cc tonegen.c -std=c99 -Wall -Wextra -pedantic -lm $(sdl2-config --cflags
// --libs)
#include <SDL.h>

// Unnecessary if you're compiling with GNU extensions or MSVC with
// _USE_MATH_DEFINES.
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif

#define TONEGEN_SAMPLING_RATE 44100         // samples per second
#define TONEGEN_FORMAT_SIZE sizeof(int16_t) // sample format
#define TONEGEN_BUF_MAX_LENGTH (TONEGEN_SAMPLING_RATE / 10)

static const int FORMAT_MAX_VALUE = INT16_MAX; // determ. by TONEGEN_FORMAT_SIZE
static const int AMPLITUDE = 0.025 * FORMAT_MAX_VALUE; // volume

struct tonegen_tone {
    int freq;
    int duration; // in ms
};

struct tonegen {
    int freq;
    int sample_idx;
    int remaining_samples; // samples yet to be generated
    int16_t buf[TONEGEN_BUF_MAX_LENGTH];
    size_t buf_size;
};

void tonegen_set_tone(struct tonegen *gen, struct tonegen_tone tone);
void tonegen_generate(struct tonegen *gen);
void tonegen_queue(struct tonegen *gen, SDL_AudioDeviceID device_id);

int main() {
    if (SDL_Init(SDL_INIT_AUDIO) < 0) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                     "Couldn't initialize SDL: %s", SDL_GetError());
        return EXIT_FAILURE;
    }

    SDL_AudioDeviceID device_id =
        SDL_OpenAudioDevice(NULL, 0,
                            &(SDL_AudioSpec){.freq = TONEGEN_SAMPLING_RATE,
                                             .format = AUDIO_S16SYS,
                                             .channels = 1,
                                             .samples = 2048},
                            NULL, 0);
    if (device_id == 0) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                     "Couldn't open an audio device: %s", SDL_GetError());
        return EXIT_FAILURE;
    }

    // Unpause the audio device because it is paused by default.
    SDL_PauseAudioDevice(device_id, 0);

    // Some tones from the original Pong game.
    struct tonegen_tone score_tone = {240, 500};
    struct tonegen_tone paddle_hit_tone = {480, 35};
    struct tonegen_tone wall_hit_tone = {240, 20};

    struct tonegen gen = {0};

    tonegen_set_tone(&gen, score_tone);

    // Queues all three tunes, pausing for 1 second after each tune is
    // completely queued.
    int tone_idx = 0;
    while (tone_idx < 3) {
        tonegen_generate(&gen);
        tonegen_queue(&gen, device_id);

        if (gen.remaining_samples == 0) {
            SDL_Delay(1000);

            if (tone_idx == 0) {
                tonegen_set_tone(&gen, paddle_hit_tone);
            } else if (tone_idx == 1) {
                tonegen_set_tone(&gen, wall_hit_tone);
            }
            tone_idx++;
        }
    }

    SDL_CloseAudioDevice(device_id);

    SDL_Quit();

    return EXIT_SUCCESS;
}

void tonegen_set_tone(struct tonegen *gen, struct tonegen_tone tone) {
    gen->freq = tone.freq;
    gen->sample_idx = 0;
    gen->remaining_samples = (tone.duration / 1000.0) * TONEGEN_SAMPLING_RATE;
}

static int square_wave_sample(int idx, int freq) {
    if (sin(2.0 * M_PI * freq * (idx / (double)TONEGEN_SAMPLING_RATE)) >= 0.0) {
        return AMPLITUDE;
    }
    return -AMPLITUDE;
}

void tonegen_generate(struct tonegen *gen) {
    int len = gen->remaining_samples;
    if (len > TONEGEN_BUF_MAX_LENGTH) {
        len = TONEGEN_BUF_MAX_LENGTH;
    }
    gen->remaining_samples -= len;
    gen->buf_size = len * TONEGEN_FORMAT_SIZE;

    for (int i = 0; i < len; i++) {
        gen->buf[i] = square_wave_sample(gen->sample_idx + i, gen->freq);
    }
    gen->sample_idx += len;
}

void tonegen_queue(struct tonegen *gen, SDL_AudioDeviceID device_id) {
    if (gen->buf_size > 0) {
        if (SDL_QueueAudio(device_id, gen->buf, gen->buf_size) < 0) {
            SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                         "Couldn't queue audio: %s", SDL_GetError());
        }
    }
}