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From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.atari.st
Subject: Re: Megamax bug or crappy C ?
Message-ID: <8705201814.AA22619@cory.Berkeley.EDU>
Date: Wed, 20-May-87 14:14:08 EDT
Article-I.D.: cory.8705201814.AA22619
Posted: Wed May 20 14:14:08 1987
Date-Received: Sat, 23-May-87 07:22:33 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 49

>        array[i - 1] = array[i--] - 1;              /* Trouble here.  */
>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 ?

	Totally and Completely Wrong.  You can make no assumptions as to the
order of evaluation for most C statements.   Except for &&, ||, the comma 
operator, and a very few other statements/operations, the order of evaluation
is totally dependant on the C compiler used.  The definition for post 
increment or decrement is that the particular variable's value is used in that
instance and then decremented or incremented by the end of the expression
(segment).  You can't even assume that the order of evaluation for an 
assignment is right-side-first:

	*ptr = *ptr++

	Could either evaluate the right side first, or evaluate the effective
address of the left side, then the right side, then do the assignment.

	a && b && c

	a is evaluated if TRUE b is evaluated, and if that is TRUE c is
	evaluated.  Thus,  char *a=NULL; if (a && *a) ...  is perfectly 
	valid C.... the *a is not evaluated if a is NULL.

	a || b || c

	a is evaluated if FALSE b is evaluated, etc...

	(a,b)

	a is evaluated.  b is evaluated, the result of the expression is b.

	So the only time you can use postincrement/postdecrement and *know*
	the evaluation order would be in something like this (example):

	if (a++ || b++ || c++) ..
	for (i++, j = i; j < 10; j++) ..

					-Matt

P.S.  somebody care to check if the Alcyon compiler does the right thing for
||,&&, and the comma operator??