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From: poser@csli.Stanford.EDU (Bill Poser)
Newsgroups: comp.lang.c
Subject: Re: How to get a byte offset
Message-ID: <13824@csli.Stanford.EDU>
Date: 28 May 90 18:51:51 GMT
References: <1990May28.034643.6962@cs.umn.edu> <16802@phoenix.Princeton.EDU>
Sender: poser@csli.Stanford.EDU (Bill Poser)
Reply-To: poser@csli.stanford.edu (Bill Poser)
Distribution: usa
Organization: Center for the Study of Language and Information, Stanford U.
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In article <16802@phoenix.Princeton.EDU> pfalstad@phoenix.Princeton.EDU (Paul John Falstad) writes:
>
>The first way is just to add up all the sizeof values of a, b, and c.
>
>  #define D_OFFSET (sizeof(int)+sizeof(char)*3+sizeof(int)*10)

This won't work in general since, in order to satisfy alignment requirements,
a struct may contain padding bytes. For example, on some machines the
structure:

	struct foo{
		char b;
		short a;
	};

will contain 4 bytes because it will be laid out in memory like this:

	0000	b
	0001	PADDING
	0002	low byte of a
	0003	high byte of a

Without the padding a would not begin on an even address, which
is not permitted in some architectures.
							Bill