Re: NULL pointer arithmetic issues

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> Unless I remember wrong, older C standards explicitly say that the
> integer 0 can be converted to a pointer, and that will be the NULL
> pointer, and a NULL pointer cast as an integer shall give the value
> 0.

The only one I have anything close to a copy of is C99, for which I
have a very late draft.

Based on that:

You are not quite correct.  Any integer may be converted to a pointer,
and any pointer may be converted to an integer - but the mapping is
entirely implementation-dependent, except in the integer->pointer
direction when the integer is a "null pointer constant", defined as
"[a]n integer constant expression with the value 0" (or such an
expression cast to void *, though not if we're talking specifically
about integers), in which case "the resulting pointer, called a null
pointer, is guaranteed to compare unequal to a pointer to any object or
function".  You could have meant that, but what you wrote could also be
taken as applying to the _run-time_ integer value 0, which C99's
promise does not apply to.  (Quotes are from 6.3.2.3.)

I don't think there is any promise that converting a null pointer of
any type back to an integer will necessarily produce a zero integer.

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