Testing Equal Pointers (was: detecting invalid pointers)

[Chip Salzenberg]

Unfortunately, I apparently did not make myself clear when writing about
pointer comparison under Microsoft C.  Prescott Turner misunderstood me,
posting his expressions of disbelief.  For reference, I wrote in part:

> For totally correct comparisons of all pointers, it's necessary
> to normalize them by hand, or be sure that they are cast to "huge *" when
> any pointer arithmetic is done.  Otherwise, the different combinations of
> segment+offset that actually refer to the same address do not compare equal.

In the context of the quoted article, "all pointers" includes pointers
constructed *by hand*, i.e. non-portably.  If all pointers in a Microsoft C
program are generated in the normal C fashion, that is, by use of the "&"
operator and pointer arithmetic, the "==" and "!=" operators work as
expected.  No need to worry, Prescott.

Incidentally, these comments apply equally to Turbo C.

On the other hand, Doug Gwyn comments:

>(Except that certain operations not permitted of conforming applications
>might produce pointers that accidentally refer to the same location; but
>since that would be totally erroneous code, you shouldn't worry about it.)

I beg to disagree.  Non-portable is not the same as erroneous, at least in
the real world of applications programming.  (I'm sure Doug knows what I
mean here, even if he avoids such situations wherever possible, as I do.)

After all, if a given algorithm is non-portable and is therefore surrounded
by "#ifdef MSDOS"/"#endif", why bother trying to make its implementation
portable?

For an example:  Name a portable way to get the complete shift state of a
keyboard.  No, wait, that's too hard.  Name a portable way to *express* the
complete shift state of a keyboard.  Code involving such an entity is non-
portable from the get-go; so there's no reason to avoid non-portable coding
in its implementation.
-- 
Chip Salzenberg             <chip at ateng.com> or <uunet!ateng!chip>
A T Engineering             Me?  Speak for my company?  Surely you jest!
	  "It's no good.  They're tapping the lines."