You can trivially get at the padding bytes, just cast to char pointer. Their contents are undefined and there is absolutely no guarantee that they will remain c

You can trivially get at the padding bytes, just cast to char pointer. Their contents are undefined and there is absolutely no guarantee that they will remain constant, but they're there. Generally you have several possibilities for how to escape this problem, but the simplest is to just add the padding and a static assert that the sizeof the struct is what you expect.