save Shadertoy audio to wav file C code

shadertoy_audio_to_wav.c

// gcc shadertoy_audio_to_wav.c -lm
// ./a.out

// detailed info about shadertoy audio texture 
// https://gist.github.com/soulthreads/2efe50da4be1fb5f7ab60ff14ca434b8

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <assert.h>
#include <signal.h>
#include <math.h>
#include <stdint.h>

//----------------------------

// equal to shadertoy code
/*
vec2 mainSound( int samp, float time )
{
    // A 440 Hz wave that attenuates quickly overt time
    return vec2( sin(6.2831*440.0*fract(time))*exp(-3.0*fract(time)) );
}
*/

// mainsound func
double * Sound( float time ){
    const double two_pi = 6.283185307179586476925286766559;
    double frequency = 440.0; // 261.626;  // middle C
    static double ret[2];
    
    double value = sin(two_pi*frequency*fmod(time,1.))*exp(-3.0*fmod(time,1.));
    
    ret[0]=0.2*value;
    ret[1]=0.2*value;
    return ret;
}

//save to file
void write_word(FILE *f,uint32_t val, size_t sz){
    for (; sz; --sz, val >>= 8){
        fprintf(f,"%c",(char)(val & 0xFF));
    }
}

void gen_sound() {
    FILE *f;
    printf("Saving sound...\n");
    f = fopen("background.wav", "w");
    if (f==NULL) printf("Can not save sound to file.\n");
    else
    {
    fprintf(f,"%s","RIFF----WAVEfmt ");     // (chunk size to be filled in later)
    write_word( f,     16, 4 );  // no extension data
    write_word( f,      1, 2 );  // PCM - integer samples
    write_word( f,      2, 2 );  // two channels (stereo file)
    write_word( f,  44100, 4 );  // samples per second (Hz)
    write_word( f, 176400, 4 );  // (Sample Rate * BitsPerSample * Channels) / 8
    write_word( f,      4, 2 );  // data block size (size of two integer samples, one for each channel, in bytes)
    write_word( f,     16, 2 );  // number of bits per sample (use a multiple of 8)
    size_t data_chunk_pos = ftell(f);
    fprintf(f,"%s","data----");
    const double max_amplitude = 32760;  // "volume"
    double hz        = 44100;    // samples per second
    double seconds   = 6.5;      // time
    int N = hz * seconds;  // total number of samples
    for (int n = 0; n < N; n++)
    {
        double * val=Sound(n/hz);
        write_word( f, (int)((max_amplitude ) * val[0]), 2 ); //left ear
        write_word( f, (int)((max_amplitude ) * val[1]), 2 ); //right ear
    }
    size_t file_length = ftell(f);
    fseek(f, data_chunk_pos + 4 ,SEEK_SET);
    write_word( f, file_length - data_chunk_pos + 8, 4 );
    fseek(f,  0 + 4 ,SEEK_SET);
    write_word( f, file_length - 8, 4 ); 
    fclose(f);
    }
}

int main(void)
{
  gen_sound();
  printf("done\n");
}