c programming style - READ THIS

[Wayne Throop]

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
<the-known-world>!mcnc!rti-sel!rtp47!throopw