Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!ncar!noao!asuvax!stjhmc!p6.f18.n114.z1.fidonet.org!will.summers
From: will.summers@p6.f18.n114.z1.fidonet.org (will summers)
Newsgroups: comp.lang.c
Subject: Re: Union type conversions
Message-ID: <224.22F29448@stjhmc.fidonet.org>
Date: 29 Jul 88 22:21:27 GMT
Sender: ufgate@stjhmc.fidonet.org (newsout1.17)
Organization: FidoNet node 1:114/18.6 - Iasd Eng Bbs, Phoenix Az
Lines: 35


 > I am not sure.  In particular, if there is no testable assertion that
 > makes a union different from a structure, then a compiler that implements
 > a union as a structure will not break any (testable) rules and will
 > thus be correct.
 >
 > Write some correct code that produces a wrong answer if a union of a
 > set of elements were implemented as a structure containing all the
 > elements, and you will have a proof.
 
How about:
  union { int a; int b; } x;
 
  int offset_of_b = (int) (&x.b - &x);
    ...
 
    if  (offset_of_b != 0)
        printf("compiler is broke\n");
 
"A union may be thought of as a structure all of whose members begin at offset 
0" (K&R A.8.5, pg 197).
 
If I may "think of" a union as an "all 0 offset structure", then I may write 
code that fails when any aspect of that metaphor is violated.
 
So what's wrong with the above code?
 
   \/\/ill
 


--  
St. Joseph's Hospital/Medical Center - Usenet <=> FidoNet Gateway
Uucp: ...ncar!noao!asuvax!stjhmc!18.6!will.summers
Internet: will.summers@p6.f18.n114.z1.fidonet.org