Re: [13278] sizeof and allignment on 32bit target

[Segher Boessenkool <segher at koffie dot nl>]

John Yates wrote:
> Long ago I internalized the point that sizeof() does
> not return the minimum number of bytes that need to
> be copied in an aggregate assignment but rather the
> spacing between elements in an array:
>
> struct foo { ... };
> foo a[2];
>
> ((char*)&a[0]) + sizeof(foo) equals ((char*)&a[1])
>
> This is a bit of a hole in the language definition
> since there really is no way to obtain that minimum
> number.

struct foo {
	...
	some_type last_member;
};

offsetof(struct foo, last_member) + sizeof a[0].last_member;

> Hence using sizeof() as the length of a
> memcpy() to set a freestanding risks trashing some
> other object if the compiler has practiced any kind
> storage packing.

Overwriting padding bytes is guaranteed to be safe.


Segher