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Path: utzoo!mnetor!seismo!lll-crg!rutgers!dayton!viper!dave
From: dave@viper.UUCP (David Messer)
Newsgroups: comp.lang.c
Subject: Re: structure element offsets
Message-ID: <386@viper.UUCP>
Date: Fri, 28-Nov-86 17:29:47 EST
Article-I.D.: viper.386
Posted: Fri Nov 28 17:29:47 1986
Date-Received: Fri, 28-Nov-86 21:16:25 EST
References: <1096@spice.cs.cmu.edu> <768@nike.UUCP> <3622@watmath.UUCP>
Reply-To: dave@viper.UUCP (David Messer)
Organization: Lynx Data Systems, Minneapolis, MN
Lines: 31
Keywords: structure, offset

In article <3622@watmath.UUCP> rbutterworth@watmath.UUCP (Ray Butterworth) writes:
 >In article <768@nike.UUCP>, hahn@fred (Jonathan Hahn) writes:
 >> > Is there any way of finding the offset of a structure element from
 >> > the beginning of a structure in a portable AND efficient way?
 >> Try:
 >> #define OFFSET(elem, type)    (&(((type *)0)->elem))
 >> This utilizes a pointer of address 0, for which the address of the
 >> element reference yeilds the offset of the element.
 >
 >typedef struct { int f1; int f2; } Str;
 >
 >I tried OFFSET(Str,f2) on my machine and got 262,144 (=01000000 =2^18).
 >That's a pretty big offset considering it only has to pass
 >over one int.
 >I won't mention what Lint had to say about it.
 
It could be because you used OFFSET(Str,f2) instead of the correct
OFFSET(f2,Str).  You got the parameters reversed.

A simpiler definition of the OFFSET macro is the following:

	#define OFFSET(mos)  ((long)(&(((char *)0)->mos)))

This will produce a proper offset on almost all machines.  (But
not all, some machines have different formats to pointers to
different types.  Also, this macro assumes that (long)((char *)0) == 0L.)
-- 
Disclaimer:                       | David Messer 
I'm always right and I never lie. | Software Consultant 
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