struct accessing

[Jacob Gore]

/ comp.lang.c / mcdaniel at uicsrd.csrd.uiuc.edu (Tim McDaniel) / Jun 27, 1989 /
> In article <470001 at gore.com> jacob at gore.com (Jacob Gore) writes:
> > #define TOM	(&fred_proto.tom   - &fred_proto)
> > #define DICK	(&fred_proto.dick  - &fred_proto)
> > #define HARRY	(&fred_proto.harry - &fred_proto)
> 
> Close, but no cigar.  The pointer subtraction yields an error message,
> since &fred_proto is of type "struct fred *" and &fred.proto.tom is of
> type "int *".

I suppose casting one into the other is non-portable?

> Suppose, instead, that it was
>     #define TOM 	(&fred_proto.tom   - &fred_proto.tom)
>     #define DICK	(&fred_proto.dick  - &fred_proto.tom)
>     #define HARRY	(&fred_proto.harry - &fred_proto.tom)
> 
> Even if tom is the first int member of a struct fred, and all the int
> members are contiguous, it still isn't guaranteed to work.  Pointer
> subtraction is defined by pANS C only over pointers into the same
> array.  In particular, a compiler can put an arbitrary number of bytes
> of padding between struct members, and such padding wouldn't have to
> be a multiple of sizeof(int) bytes long.

Why is the padding relevant?  The only requirement is that a field's offset
is the same for all instances of the struct.

--
Jacob Gore	Jacob at Gore.Com		{nucsrl,boulder}!gore!jacob