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From: throopw@xyzzy.UUCP (Wayne A. Throop)
Newsgroups: comp.lang.c
Subject: Re: Style [++i vs i++]
Message-ID: <189@xyzzy.UUCP>
Date: Sun, 9-Aug-87 18:10:45 EDT
Article-I.D.: xyzzy.189
Posted: Sun Aug  9 18:10:45 1987
Date-Received: Sun, 9-Aug-87 23:06:57 EDT
References: <8298@brl-adm.ARPA> <587@cblpe.ATT.COM>
Organization: Data General, RTP NC.
Lines: 55

> apc@cblpe.ATT.COM (Alan Curtis)
> Does this scare anyone (does any else's compiler  do the same thing?)
>   main(a)
>   char (*a)[];
>   { a = 0; printf("a=0x%x\n", a); a++; printf("a=0x%x\n", a);	}
> produces:
> 	a=0x0
> 	a=0x0

It doesn't scare me.  It perhaps causes me a momentary fright that
anybody thought that that should do something sensible, but then I'm
used to such things by now.

> I realize that is the logical extension of a++ somtimes
> adding 8,4,23.5, whathavyou, to a, but it still scares me.
> PS:  In my book a++ should add 1 (one, uno, etc).
> If I wanted to add sizeof(*a) Ida SAID
> 	a += sizeof(*a);

What scares me is that a++ *does* always add 1.  Never 8, 4, 23.5, or
anything else.  Always 1.  It adds one byte, or one word, or one
*whatever* to the address in question.  It would be illogical, immoral,
byte-chauvanist, and all kinds of perjorative things for it to add one
byte to the address of anything but a byte.

Granted, most people are under the delusion that all C address types are
byte granular offsets into per-process monolithic address segments.  But
then, C implementors are not bound to abide by the delusions of the
masses, thank goodness.  

But the scariest thing about all this is that *none* *of* *my* *tools*
*caught* *this* *bug*!!!!  Lint happily passed the program, as did other
typecheckers.  And the compiler didn't complain (though on our system
the output is

        a=0x0
        a=0x1

instead, (but then the behavior is undefined since the program is illegal
in the first place, so I can't blame the compiler too harshly... though
I *CAN* blame lint and other type-and-usage checkers for falling down on
the job).

(By the way, for those of you who missed it, the program is illegal for
the obvious reason that it increments a pointer to an object of unknown
size, but *also* because it performs arithmetic on a null pointer, and
of course, this is illegal.)

--
At once words appeared on the screen:
  "Hello.  My name is Hank Thoro II.  Please type your name."
  "My whole name?" he typed.
  "Good.  Do you like baseball?  Just type Y for Yes or N for No."
                                --- from "Roderick" by John Sladek
-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw