Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!wuarchive!udel!princeton!phoenix!pfalstad
From: pfalstad@phoenix.Princeton.EDU (Paul John Falstad)
Newsgroups: comp.lang.c
Subject: Re: How to get a byte offset
Message-ID: <16821@phoenix.Princeton.EDU>
Date: 29 May 90 06:11:28 GMT
References: <1990May28.034643.6962@cs.umn.edu> <16802@phoenix.Princeton.EDU>
Reply-To: pfalstad@phoenix.Princeton.EDU (Paul John Falstad)
Distribution: usa
Organization: Princeton University, NJ
Lines: 26

In article <16802@phoenix.Princeton.EDU> I write:
>In article <1990May28.034643.6962@cs.umn.edu> swie@cs.umn.edu (S. T. Tan) writes:
>>Is there an easy way to get the byte offset of a field in a structure without
>>counting it manually ?
>The first way is just to add up all the sizeof values of a, b, and c.

Oops #1:  This doesn't work, since there might be padding.  (It was
a stupid suggestion anyway, and unnecessary, as it turns out.)

Oops #2:

>  d_offset = (char *) &foo->d - (char *) foo;
>Of course, the result isn't a constant, and code is generated.  You

Well, I'm not sure about the above example, but even Manx got this right:

#define d_offset ((int) &(((struct foo *) 0)->d))

The result IS an integer constant.  You can even use it for an array size,
although it may not be portable because of the NULL pointer.

-- 
Paul Falstad  PLINK:Hypnos GEnie:P.FALSTAD net:pfalstad@phoenix.princeton.edu
Disclaimer: My opinions, which belong to me and which I own, are mine.
-Anne Elk (not AN elk!)  The sun never set on the British empire because
the British empire was in the East and the sun sets in the West.