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Path: utzoo!linus!decvax!minow
From: minow@decvax.UUCP (Martin Minow)
Newsgroups: net.lang.c
Subject: Re: ++ operator
Message-ID: <312@decvax.UUCP>
Date: Mon, 26-Dec-83 16:16:56 EST
Article-I.D.: decvax.312
Posted: Mon Dec 26 16:16:56 1983
Date-Received: Tue, 27-Dec-83 03:07:51 EST
References: <1092@mit-eddie.UUCP>
Organization: DEC UNIX Engineering Group
Lines: 36

Mark Plotnik asks for a "precise, consistent definition" of the
semantics of ++ and -- (among other things).  The definition
is "compiler dependent", but all side effect operators will
be evaluated when control passes to the next statement.
(It may be the case that side-effect operations will be evaluated
before passing over , && or ||, but I wouldn't bet on it.

Vax-11 C (on VMS) is very willing to evaluate auto-increment
operators at "unusual" times.  For example, consider the following:

	int	stack[123];		/* Evaluation stack	*/
	int	*stack_pointer = stack;
	#define push(x)	(*--stack = x)	/* Stuff into the stack	*/
	#define pop()	(*stack++)	/* Get from the stack	*/

	int
	eval(operator)
	{
		switch (operator) {
		...
		case OP_ADD:
		     push(pop() + pop());
	...

You may think that "*--stack = *stack++ + *stack++;" will "do the
right thing", but it is (by demonstration) compiler dependent.
(I found that out the hard way -- the original had the misleading
comment "don't need a temp variable since addition is commutative").
The correct expression of the above is:

		    temp = pop();
		    temp += pop();
		    push(temp);

Martin Minow
decvax!minow