size_t SDL_RWwrite(struct SDL_RWops* context,
                   const void*       ptr,
                   size_t            size,
                   size_t            num)

SDL_RWops *rw = SDL_RWFromFile("hello.txt", "w");
if(rw != NULL)
{
    const char *str = "Hello World";
    size_t len = SDL_strlen(str);
    if (SDL_RWwrite(rw, str, 1, len) != len)
    {
        printf("Couldn't fully write string\n");
    }
    else
    {
        printf("Wrote %d 1-byte blocks\n", len);
    }
    SDL_RWclose(rw);
}