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Path: utzoo!watmath!clyde!rutgers!dayton!meccts!viper!john
From: john@viper.UUCP
Newsgroups: comp.lang.c
Subject: Re: zero length array
Message-ID: <563@viper.UUCP>
Date: Sat, 21-Feb-87 14:57:32 EST
Article-I.D.: viper.563
Posted: Sat Feb 21 14:57:32 1987
Date-Received: Sun, 22-Feb-87 05:38:39 EST
References: <4498@brl-adm.ARPA> <1987Feb19.134433.737@sq.uucp>
Reply-To: john@viper.UUCP (John Stanley)
Organization: DynaSoft Systems
Lines: 37

In article <1987Feb19.134433.737@sq.uucp> msb@sq.UUCP (Mark Brader) writes:
 >
 >You need an actual object of the struct type, say tmp; and then, of course,
 >you can say:
 >
 >	(char *) &tmp.any_field - (char *) &tmp
 >
 >In the current ANSI draft, there is a predefined macro "offsetof" which
 >does something like this for you without needing you to specify an object.
 >
 >However, I've never met a case where this operation was really necessary.
 >Usually the job can be done more comprehensibly with unions.
 >
 >Mark Brader, utzoo!sq!msb			C unions never strike!

  Mark,
I agree with you that the other construct is poor, that the one you gave
is considerably better, but I disagree with you on the subject of unions.
  I've had experience with many (far -too- many) compilers that have
multiple bugs in the handleing of unions.  For portability, it's best
to -not- use unions in general, and in this instance especialy.  (Unions
are in many cases totaly non-portable across systems with different byte
order rules.  And there is more than one compiler that fails to poperly
handle increment/decrement if you have a union consisting of several
pointer types...)
 
  The best solution I've come up with is as follows:
 
#define ANOFFSET (int)(((char*)&tmp.any_field) - ((char*)&tmp))
 
  The construct you created will be typed as an int on some systems and
as a long on others.  This means you can't portably use it in a
parameter list.  Adding the (int) (or (long)) cast at the beginning
solves this portability problem.

----
John Stanley (john@viper.UUCP) - DynaSoft Systems