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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: How are local vars allocated?
Message-ID: <9367@mimsy.UUCP>
Date: Sat, 14-Nov-87 19:46:37 EST
Article-I.D.: mimsy.9367
Posted: Sat Nov 14 19:46:37 1987
Date-Received: Sun, 15-Nov-87 20:30:55 EST
References: <189@dalcsug.UUCP> <7401@prls.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 61

In article <7401@prls.UUCP> gardner@prls.UUCP (Robert Gardner) writes:
>Is there any danger of stack overflow with the following construct:
>for(;;) {
>    int k;
>    etc. etc.
>}

No.

>In other words, is k allocated only once at the beginning of the
>procedure containing this construct, or is it allocated each time
>the {} block is entered? Is the answer implementation dependent?

The answer is implementation dependent, but if a loop like

	for (;;) {
		int k;
	}

runs for several minutes, then produces a stack overflow, the
implementation is horrible.  For example, a very straightforward
compiler might generate this:

	proc()
	{
		int a, b;

		for (;;) {
			int k;
			if (g())
				break;
		}
		a = 1;
	}
	--------
	_proc:
		sub	$8,sp		| create local variables a, b
	L1:
		sub	$4,sp		| create local variable k
		call	_g		| run g
		tst	r0		| what was g's return value?
		bne	L3		| nonzero, break loop
	L2:
		add	$4,sp		| destroy local variable k
		bra	L1		| back around the loop
	L3:
		add	$4,sp		| destroy local variable k
		mov	$1,-4(sp)	| a = 1
	L0:
		add	$8,sp		| destroy locals a, b
		ret

Label `L2' above is for a `continue' that might appear inside
the for loop.  It is not used, but so what?  Likewise, L0 is
allocated at the beginning of code generation for proc(), in
case there are any `return' statements in it.  I optimised away
the sequence a truly dumb compiler would probably emit for
`if (g()) break'.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris