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From: dmj@cunixa.cc.columbia.edu (Douglas M Jaffe)
Newsgroups: comp.lang.c
Subject: Re: finding offset of a member in C structures
Message-ID: <1991May25.054608.6647@cunixf.cc.columbia.edu>
Date: 25 May 91 05:46:08 GMT
References: <1276@unisql.UUCP> <16194@smoke.brl.mil> <1991May20.201857.635@lynx.CS.ORST.EDU>
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In article <1991May20.201857.635@lynx.CS.ORST.EDU> osbornk@mist.CS.ORST.EDU (Kasey S. Osborn) writes:
>In article <16194@smoke.brl.mil> gwyn@smoke.brl.mil (Doug Gwyn) writes:
>|In article <1276@unisql.UUCP> pckim@unisql.UUCP (Pyung-Chul Kim) writes:
>|-struct a_struct {
>|-short    member1;
>|-char  member2;
>|-int   member3;
>|-};
>|-can I get the offset of member2 at *compiling time* and *portably* ?
>|
>|Yes, but you shouldn't very often need to do so.
>|
>|-It is possible to provide a compiler operator:
>|- offsetof(member_name,structure_type_name)
>|-as it provides 'sizeof' operator.
>|-Do you know if there is a compiler operator like above, or is there any
>|-alternative solution.
>|
>|Standard C provides an offsetof() macro, but you don't really need one.
>|  struct a_struct foo;
>|  int offset2 = (char *)&foo.member2 - (char *)&foo; /* for example */
>
>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?
>
>What compiler are you using?  What is the syntax of offsetof()?  I've
>been trying to find a palatable solution to this problem with no luck.


i have to check my ansi reference for offsetof().  however, i wouldn't
rely on the assumption that offsetof() will exist in every 
__STDC__ (stupid c) compiler that you encounter.  you might want 
to consider adding the following macros to your bag of tricks.
these offset macros should work in almost _every_ c  compiler/interpreter that 
exists:  from msc, turboc, SunOS cc, rs/6000 xlc, to tandem (yes, tandem) c.

NOTES
-----
1.  i wrote these many years ago, on a buggy ibm-pc (yes, p. c. !) compiler.
2.  because of #1, i had to code like a prude, with parens all over.
3.  these macros have been working ever since.
4.  don't flame me on the parameter names that i used!


======================================================================

#define FAKE_ADDR  ((char *) 256)

/* offset to start of field */
#define OFFSET_TO(tttt,mmmm)    \
    (((unsigned long)  (&(((tttt *)(FAKE_ADDR))->mmmm) )) - \
     ((unsigned long)  ((tttt *)(FAKE_ADDR))))

/* size from start of struct to end of mentioned member */
#define SIZE_TOEND(t,m) \
    (OFFSET_TO(t,m) + sizeof(((t *)(FAKE_ADDR))->m))

/* offset of last byte of mentioned member from start of struct */
#define OFFSET_TOEND(t,m) (SIZE_TOEND(t,m) - (unsigned long)1)


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