Minimal code for callback function on SDL audio

synthesis.cpp

#include <SDL/SDL.h>
#include <stdio.h>

using namespace std;

void func_callback(void *unused, Uint8 *stream, int len) {

    for (int i=0;i<len;i++) {
        stream[i] = i;
    }
}

int main(int argc, char *argv[])
{
    if( SDL_Init(SDL_INIT_TIMER | SDL_INIT_AUDIO ) <0 ) {
        SDL_WM_SetCaption( "Something busy..", NULL );
        return 1;
    }

    SDL_AudioSpec as;

    as.freq = 22000;
    as.format = AUDIO_U8;
    as.samples = 1024;
    as.callback=func_callback;
    as.userdata=NULL;
    as.channels = 1;
    
    if ( SDL_OpenAudio(&as, NULL) < 0 ) {
        SDL_WM_SetCaption( "Unable to open audio..", NULL );
        return 1;
    }

    SDL_Surface *screen = SDL_SetVideoMode(200,200, 16, SDL_SWSURFACE);

    SDL_PauseAudio(0);
    SDL_Delay(3000);
    
    fclose(file);
    

    SDL_Quit();
    return 0;
}

hello, do you know how to change the parameters of the callback function?
I want to change the parameter "len" smaller.