Function callback_c 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn callback_c(
    camera_handle: VmbHandle_t,
    _stream_handle: VmbHandle_t,
    frame: *mut VmbFrame_t,
)

Safety

This function will not propagate panics that happen in the callback, but it should print an error to stderr and then soon stop further image-ready callbacks.