Bare minimum C++ SDL tone generator

gen.cpp

/* Compilation:
Requires libsdl2-dev

$ g++ --std=c++14 -lSDL2 -o gen gen.cpp
*/

#include <SDL2/SDL.h>
#include <SDL2/SDL_audio.h>
#include <iostream>
#include <cmath>
using namespace std;

float state = 0;
const int rate = 44100;
float freq = 440;
void audio_callback(void *userdata, Uint8 *_stream, int len) {
  // Cast byte stream into 16-bit signed int stream
  Sint16 *stream = (Sint16*) _stream;
  len /= 2;
  for (int i = 0; i < len; ++i) {
    stream[i] = ((1<<15)-1) * sin(state);
    state += 1./rate*2*M_PI*freq;
    state = fmod(state, 2*M_PI);
  }
}


int main(int argc, char* argv[]) {
  SDL_Init(SDL_INIT_AUDIO);
  
  SDL_AudioSpec spec;
  spec.freq = rate;
  spec.format = AUDIO_S16SYS;
  spec.channels = 1;
  spec.samples = 1024;
  spec.callback = audio_callback;
  spec.userdata = nullptr;
  SDL_AudioSpec obtainedSpec;
  SDL_OpenAudio(&spec, &obtainedSpec);
  SDL_PauseAudio(0);
  
  float nfreq;
  while (cin >> nfreq) {
    // Lock the audio thread to safely subsitute the frequency
    SDL_LockAudio();
    freq = nfreq;
    SDL_UnlockAudio();
  }
  SDL_CloseAudio();
}