uint8_t *f3s_page (f3s_socket_t *socket,
                   uint32_t flags,
                   uint32_t offset,
                   int32_t *size)

This function is called to access a window_size sized window at address offset from the start of the device; it must be provided for both bank-switched and linearly mapped flash devices. If the size parameter is non-NULL, you should set it to the size of the window. The function must return a pointer suitable for accessing the device at address offset. On error, it should return NULL and set errno to ERANGE.