Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!batcomputer!munnari.oz.au!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: finding offset of a member in C structures
Message-ID: <5859@goanna.cs.rmit.oz.au>
Date: 21 May 91 00:35:23 GMT
References: <1276@unisql.UUCP> <16194@smoke.brl.mil> <1991May20.201857.635@lynx.CS.ORST.EDU>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 52

In article <1276@unisql.UUCP> pckim@unisql.UUCP (Pyung-Chul Kim) writes:
> can I get the offset of member2 at *compiling time* and *portably* ?

In article <16194@smoke.brl.mil> gwyn@smoke.brl.mil (Doug Gwyn) writes:
> Yes, but you shouldn't very often need to do so.
> Standard C provides an offsetof() macro, but you don't really need one.

In article <1991May20.201857.635@lynx.CS.ORST.EDU>, osbornk@mist.CS.ORST.EDU (Kasey S. Osborn) writes:
> Really?  I can't find a reference to offsetof() in any of my C references.
> In fact, GNU CC does not preprocess offsetof() and instead treats it as an
> external reference.  Are you sure this is standard?

It sounds as though you need some up-to-date C references.
The little Plauger & Brodie handbook is cheap and excellent.

The GNU C compiler *does* understand offsetof, *provided* you #include
the right header file.  Doug Gwyn wrote "Standard C provides an
offsetof() MACRO" and he chose his words carefully.  *None* of the
standard C macros is available *unless* you #include an appropriate
header file.  In this case the file in question is <stddef.h>,
and our copy of GCC certainly has it.

> What compiler are you using?  What is the syntax of offsetof()?

	offsetof(TYPE, MEMBER)

where TYPE is a type (either a typedef id or 'struct Tag'),
not a structure tag.  Example:

    #include <stddef.h>

    struct a_struct
	{
	    short member1;
	    char  member2;
	    int   member3;
	};

    typedef struct a_struct a_type;

    main()
	{
	    printf("%d %d %d\n",
		offsetof(struct a_struct, member1),
		offsetof(struct a_struct, member2),
		offsetof(a_type, member3));
	    exit(0);
	}

On an Encore Multimax I get the output "0 2 4".
-- 
There is no such thing as a balanced ecology; ecosystems are chaotic.