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Path: utzoo!linus!decvax!mcnc!rti-sel!rtp47!throopw
From: throopw@rtp47.UUCP (Wayne Throop)
Newsgroups: net.lang.c
Subject: Re: c programming style - READ THIS
Message-ID: <90@rtp47.UUCP>
Date: Fri, 19-Jul-85 17:23:46 EDT
Article-I.D.: rtp47.90
Posted: Fri Jul 19 17:23:46 1985
Date-Received: Sun, 21-Jul-85 21:34:43 EDT
References: <11570@brl-tgr.ARPA> <935@teddy.UUCP> <1286@uwmacc.UUCP> <2439@sun.uucp>
Organization: Data General, RTP, NC
Lines: 26

In an otherwise OK article, Guy Harris says:

>        1) The only operation in C that adds an integer to the integer
>           with the same bit pattern as a given pointer and produces a
>           pointer value with the same bit pattern as the result is
>
>           (pointer_type) ((int)pointer + integer_value)
>
>        Guy Harris

This turns out not to be the case.  The expression "(int)p" where p is a
pointer does not in any way, shape, or form guarantee to yeild an
integer with the "same bits" as the pointer p.

The "only way" to treat the bits of a pointer as an integer, do
arithmetic on this integer, and yeild a pointer with the resulting
integer expression's bits is this:

    union { some_type *p; int i; } u;
           (u.p=pointer, (u.i += integer_value), u.p);

Note that even this code is non-portable, since it assumes the types
"int" and "some_type *" are the same size in bits.  (There IS no
portable way to do it.)
-- 
Wayne Throop at Data General, RTP, NC
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