SDL_ConvertAudio takes one parameter, cvt, which was previously initilized. Initilizing a SDL_AudioCVT is a two step process. First of all, the structure must be passed to SDL_BuildAudioCVT along with source and destination format parameters. Secondly, the cvt->buf and cvt->len fields must be setup. cvt->buf should point to the audio data and cvt->len should be set to the length of the audio data in bytes. Remember, the length of the buffer pointed to by buf show be len*len_mult bytes in length.
Once the SDL_AudioCVTstructure is initilized then we can pass it to SDL_ConvertAudio, which will convert the audio data pointer to by cvt->buf. If SDL_ConvertAudio returned 0 then the conversion was completed successfully, otherwise -1 is returned.
If the conversion completed successfully then the converted audio data can be read from cvt->buf. The amount of valid, converted, audio data in the buffer is equal to cvt->len*cvt->len_ratio.
/* Converting some WAV data to hardware format */
+void my_audio_callback(void *userdata, Uint8 *stream, int len);
+
+SDL_AudioSpec *desired, *obtained;
+SDL_AudioSpec wav_spec;
+SDL_AudioCVT wav_cvt;
+Uint32 wav_len;
+Uint8 *wav_buf;
+int ret;
+
+/* Allocated audio specs */
+desired = malloc(sizeof(SDL_AudioSpec));
+obtained = malloc(sizeof(SDL_AudioSpec));
+
+/* Set desired format */
+desired->freq=22050;
+desired->format=AUDIO_S16LSB;
+desired->samples=8192;
+desired->callback=my_audio_callback;
+desired->userdata=NULL;
+
+/* Open the audio device */
+if ( SDL_OpenAudio(desired, obtained) < 0 ){
+ fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
+ exit(-1);
+}
+
+free(desired);
+
+/* Load the test.wav */
+if( SDL_LoadWAV("test.wav", &wav_spec, &wav_buf, &wav_len) == NULL ){
+ fprintf(stderr, "Could not open test.wav: %s\n", SDL_GetError());
+ SDL_CloseAudio();
+ free(obtained);
+ exit(-1);
+}
+
+/* Build AudioCVT */
+ret = SDL_BuildAudioCVT(&wav_cvt,
+ wav_spec.format, wav_spec.channels, wav_spec.freq,
+ obtained->format, obtained->channels, obtained->freq);
+
+/* Check that the convert was built */
+if(ret==-1){
+ fprintf(stderr, "Couldn't build converter!\n");
+ SDL_CloseAudio();
+ free(obtained);
+ SDL_FreeWAV(wav_buf);
+}
+
+/* Setup for conversion */
+wav_cvt.buf = malloc(wav_len * wav_cvt.len_mult);
+wav_cvt.len = wav_len;
+memcpy(wav_cvt.buf, wav_buf, wav_len);
+
+/* We can delete to original WAV data now */
+SDL_FreeWAV(wav_buf);
+
+/* And now we're ready to convert */
+SDL_ConvertAudio(&wav_cvt);
+
+/* do whatever */
+.
+.
+.
+.
+