SDL_RenderReadPixels() needs to flush asynchronous operations before it reads. The semantics are that it reads the back buffer, and those pixels may not be available once SDL_RenderPresent() has happened.

/*
 *  jumphack.h
 *  SDLiPhoneOS
 *
 */

#ifndef _jumphack_h
#define _jumphack_h

#include "setjmp.h"

/* see SDL_uikitevents.m for more info */

extern jmp_buf *jump_env();

#endif