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From: tulp@klipper.UUCP
Newsgroups: comp.sys.atari.st
Subject: Megamax bug or crappy C ?
Message-ID: <756@klipper.cs.vu.nl>
Date: Wed, 20-May-87 04:36:58 EDT
Article-I.D.: klipper.756
Posted: Wed May 20 04:36:58 1987
Date-Received: Thu, 21-May-87 07:10:27 EDT
Reply-To: tulp@cs.vu.nl (Tulp E)
Organization: VU Informatica, Amsterdam
Lines: 50

Try this piece of code with the Megamax C compiler.  I am not sure whether 
this  is a bug (and if so I don't remember if it has been reported yet  or 
if it was fixed in a newer release; I use Version 1.1), maybe this is just 
an  example of bad C programming.  I also tried this test with the  Alcyon 
(DRI)  C compiler and this compiler did translate it the way I thought  it 
should be.

---
#include "stdio.h"
#include "osbind.h"

main()

{ int array[7];
  register int i;

  array[3] = 10;
  i = 3;
  while (i > 0)
        array[i - 1] = array[i--] - 1;              /* Trouble here.  */
  i = 3;
  while (i < 6)
        array[i + 1] = array[i++] + 1;            /* And trouble here. */
  for (i = 0;i< 7;i++)
      printf("Array index %d is %d.\n",i,array[i]);
  Cconin();
}

---

I  looked  at the code the compiler brewed and it appeared that  first  it 
computes array[i--] - 1,  including the post decrement,  and then (with  i 
already decremented !) it computes array[i - 1].  So array[i - 1],  really 
becomes  array[i - 2] !  As far as I know the post decrement  should  take 
place  after the entire expression is evaluated (the Alcyon compiler  does 
that).  But I am not absolutely sure whether K & R is unambiguously  clear 
about  this  (I thought it said that the decrement takes place  after  the 
assignment is done, but I don't have K & R laying around here, sorry).
Anyone ?

Of course I know that the expression could much easier be written as a for 
loop.  But this phenomenon occurred in a different context and I tried  to 
make a simple and clear test.

Drive and Megamax carefully,
Eduard Tulp                   tulp@cs.vu.nl

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