Xref: utzoo comp.lang.c:13897 comp.lang.c++:1905
Path: utzoo!attcan!uunet!husc6!bloom-beacon!mit-eddie!rutgers!njin!princeton!njsmu!klg
From: klg@njsmu.UUCP (Kenneth Goodwin)
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Re: Something new for C?
Summary: offsetof()
Keywords: offset of vars within structures
Message-ID: <481@njsmu.UUCP>
Date: 8 Nov 88 15:33:26 GMT
References: <73@dsoft.UUCP>
Organization: NJ State Medical Underwriters, Lawrenceville
Lines: 33

In article <73@dsoft.UUCP>, root@dsoft.UUCP (Super user) writes:
>was a precompiler command to allow me to use
> the offset of an item into a structure. for example:
> 
> struct test {
>    long this;
>    int  that;
>    char those[8];
> };
> 
>    val = offset(test,that);

Until they actually stick something like this into the compiler , try this:

#define	offsetof(Struct, Member)	(&((struct Struct *)0)->Member)

I believe it;s parenthesized correctly.

	declares  0 to be a pointer of type  struct Struct and computes
	the address of Member within the structure.
	since 0 + anything = anything what you get (obviously)
	is the offset of the member within the structure.

	Since all kludges have an inherent risk of non-portability
	comments on the above method are welcome.


usage:

	value = offsetof(test, that);

Ken Goodwin
NJSMU.