Getting started with SDL2 sound generation: Simple sine wave synth with moving the freq up and down

soundwaves-ogg.cpp

#include <SDL.h>
#include "stb_vorbis.h"

short audiodata[4096];
volatile bool needsData = true; // poor mans semaphore

void callback(void*  userdata, Uint8* stream, int len)
{
	printf("%p,%d\n", stream, len);
	while(needsData); // yay
	short * snd = reinterpret_cast<short*>(stream);
	len /= sizeof(*snd);
	for(int i = 0; i < len; i++)
	{
		snd[i] = audiodata[i];
	}
	needsData = true;
}

int main(int argc, char ** argv)
{
	SDL_Init(SDL_INIT_AUDIO);
	
  // the specs of our piece of music
  // this MUST match the ogg file we're playing,
  // we don't do bitrate conversions
	SDL_AudioSpec spec, aspec;
	SDL_zero(spec);
	spec.freq = 44100;
	spec.format = AUDIO_S16SYS;
	spec.channels = 1;
	spec.samples = 4096;
	spec.callback = callback;
	spec.userdata = NULL;
	
	int id;
	if ((id = SDL_OpenAudioDevice(nullptr, 0, &spec, &aspec, SDL_AUDIO_ALLOW_ANY_CHANGE)) <= 0 )
	{
	  fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
	  exit(-1);
	}
	
	printf("freq = %d\n", aspec.freq);
	printf("chan = %d\n", aspec.channels);
	printf("samp = %d\n", aspec.samples);
	printf("fmt  = %d\n", aspec.format);
	
	printf("dev = %d\n", id);
	
	stb_vorbis * ogg = stb_vorbis_open_filename(
		"music.ogg", //const char *filename,
		nullptr, // int *error, 
		nullptr  // const stb_vorbis_alloc *alloc_buffer
		);
	if(!ogg)
		return 1;
	
	stb_vorbis_info info = stb_vorbis_get_info(ogg);
	
	printf("freq = %d\n", info.sample_rate);
	printf("chan = %d\n", info.channels);
	
	stb_vorbis_seek(ogg, 0);
	
	unsigned int len = stb_vorbis_stream_length_in_samples(ogg);
	
	/* Start playing */
	SDL_PauseAudioDevice(id, 0);
	
	while(true)
	{
		if(needsData)
		{
			short * data[] = { audiodata };
			memset(audiodata, 0, sizeof(audiodata));
			len = stb_vorbis_get_samples_short(
				ogg, // stb_vorbis *f, 
				1, // int channels, 
				data, 
				4096);
			needsData = false;
			if(len == 0)
				break;
		}
	}
	
	stb_vorbis_close(ogg);
	
	SDL_Quit();
}

soundwaves.cpp

#include <SDL.h>

#include "stb_vorbis.h"

#define PI2 6.28318530718

float time = 0;
float freq = 440;

void callback(void*  userdata, Uint8* stream, int len)
{
	printf("%p,%d\n", stream, len);
	short * snd = reinterpret_cast<short*>(stream);
	len /= sizeof(*snd);
	for(int i = 0; i < len; i++)
	{
		snd[i] = 32000 * sin(time);
		
		time += freq * PI2 / 48000.0;
		if(time >= PI2)
			time -= PI2;
	}
}

int main(int argc, char ** argv)
{
	SDL_Init(SDL_INIT_AUDIO);
	
	for (int i = 0; i < SDL_GetNumAudioDrivers(); ++i) {
			const char* driver_name = SDL_GetAudioDriver(i);
			printf("[%d] %s\n", i, driver_name);
	}
	
	int devs = SDL_GetNumAudioDevices(0);
	for(int i = 0; i < devs; i++)
	{
		printf("[%d] %s\n", i, SDL_GetAudioDeviceName(i,0));
	}
	
	SDL_AudioSpec spec, aspec; // the specs of our piece of "music"
	SDL_zero(spec);
	spec.freq = 48000;
	spec.format = AUDIO_S16SYS;
	spec.channels = 1;
	spec.samples = 4096;
	spec.callback = callback;
	spec.userdata = NULL;
	
	int id;
	if ((id = SDL_OpenAudioDevice(nullptr, 0, &spec, &aspec, SDL_AUDIO_ALLOW_ANY_CHANGE)) <= 0 )
	{
	  fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
	  exit(-1);
	}
	
	printf("freq = %d\n", aspec.freq);
	printf("chan = %d\n", aspec.channels);
	printf("samp = %d\n", aspec.samples);
	printf("fmt  = %d\n", aspec.format);
	
	printf("dev = %d\n", id);
	
	/* Start playing */
	SDL_PauseAudioDevice(id, 0);
	
	while(true)
	{
		for(freq = 440; freq < 880; freq += 10)
			SDL_Delay(10);
		for(freq = 870; freq > 450; freq -= 10)
			SDL_Delay(10);
	}
	
	SDL_Quit();
}