Use this function to get the size of the data stream in an SDL_RWops.
Sint64 SDL_RWsize(SDL_RWops *context);
the SDL_RWops to get the size of the data stream from
Returns the size of the data stream in the SDL_RWops on success, -1 if unknown or a negative error code on failure; call SDL_GetError() for more information.
Prior to SDL 2.0.10, this function was a macro.
This function is available since SDL 2.0.10.