Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mcvax!hp4nl!botter!star.cs.vu.nl!maart
From: maart@cs.vu.nl (Maarten Litmaath)
Newsgroups: comp.lang.c
Subject: A not so nice macro (was: A nice macro)
Keywords: macros, arrays, addresses, overflow
Message-ID: <2792@solo8.cs.vu.nl>
Date: 22 Jun 89 19:10:37 GMT
References: <2784@solo8.cs.vu.nl> <1330@garcon.cso.uiuc.edu>
Organization: V.U. Informatica, Amsterdam, the Netherlands
Lines: 108

mcdaniel@uicsrd.csrd.uiuc.edu (Tim McDaniel) writes:
\...
\(But, presumably, Maarten meant
\    "one is a Pascal dweeb => one tends to complain about C subscripts"
\rather than
\    "one tends to complain about C subscripts => one is a Pascal dweeb").

Right, but if you read between the lines... :-)
(Just some fuel for the flames, OK?)

\One application for non-0-based-subscripting is, in fact,
\> in the MINIX kernel's `proc' table user processes have positive
\> indices, while kernel tasks have negative. 

It's MINIX' very `proc_addr()' macro which `inspired' me to write the article.

\...
\Not an array "reference", whatever that means, but an expression
                           ^^^^^^^^^^^^^^^^^^^
			   Huh? I didn't invent the term, you know!

\...
\It might be disconcerting, as you note, to see
\        zork(5) = 20;
\in a C program.

VERY disconcerting...

\...  If you ever think that Chris Torek has
\made a mistake, it is prudent to think again.  The odds say that you
\made the mistake.

I know, I know. Hey! I just wanted to see if you all were paying attention!
>;->

\>	bar	_foo[HIGH - LOW + 1];                   /* #1 */
\>	#define foo         (_foo - LOW)                /* #2 */
\[with the example]
\>	foo[-4] == (_foo - -5)[-4] == *((_foo + 5) - 4) == /* #3 */
\>           *(_foo + 1) == _foo[1]
\...
\Objection 2 (minor): with this #define, constructs like
\        a = func foo;
\would be syntactically legal, and
\        foo = 10;
\would give a confusing error message.  I might prefer
\        #define foo     &_foo[-LOW]
\which is (more or less) equivalent.

Almost right: you fell into `your own' trap! `foo[-4]' would evaluate to
`&_foo[-LOW][-4]'. You need an extra pair of parentheses.

\Objection 3: the result of the computation is undefined in pANS C.
\>	foo[-4] == (_foo - -5)[-4] == *((_foo + 5) - 4)
\OK so far, in a syntactic sense at least.
\>           == *(_foo + 1)
\Wrong, at least in pANS C.  pANS C is not permitted to rearrange
\expressions so as to ignore parentheses.

Completely right! My brain needed a reboot when I wrote the article. :-(

\Under what conditions is Maarten's scheme guaranteed to work?  (From
\here on, assume there's no integer overflow.)

[Nice calculations leading to:]

\        HIGH >= LOW
\        LOW <= 0
\        HIGH >= -1
\...
\The first #define, attributed to Chris Torek,

I doubt again Chris was involved at all; perhaps he's saved the article I was
referring to.

\...
\>	#define		foo_addr(n)	(&(foo)[  ((n)-(LOW))  ])

\There's a general rule of thumb: in the right-hand side of a macro
\definition, parenthesize everything, even where you think parentheses
\are unnecessary.  Here's yet one more example of where the rule of
\thumb is useful.

Right.

\% Actually, there's the "as if" rule, which permits an implementation
\to do anything it wants as long as the result derived is correct for
\all cases in which pANS C defines an answer.  For example, if "a+b"
\would overflow, the value of "(a+b)+c" is not defined under pANS C,
\and an implementation may do whatever it likes.  It might produce, in
\this case, "a+(b+c)", which might happen to be the numerically correct
\answer.  Or it might send 110 volts at 100 amps AC through your chair.

That's a `Quality of Implementation' issue! :-)

\# Actually, I'm implicitly assuming another identity, that
\        &*x == x
\for any address x.  I don't know about this identity under pANS C,
\either.

Save the normal restrictions (`x' is a valid pointer, etc.) I don't know
the pANS wording. How about segmented architectures with `near' and `far'
pointers?
Anyway, thanks a lot for destroying my nice macro! >:-(
Really, thanks for getting things straight.
-- 
"I HATE arbitrary limits, especially when |Maarten Litmaath @ VU Amsterdam:
   they're small."  (Stephen Savitzky)    |maart@cs.vu.nl, mcvax!botter!maart