Declaration within a loop.

[Conor P. Cahill]

In article <30174 at news.Think.COM>, barmar at kulla (Barry Margolin) writes:
> In article <2085 at hydra.gatech.EDU> dvu at prism.gatech.EDU (Dinh Vu) writes:
> >     do {
> >		 int i;
> >		 ........ ;
> >     } while (1);
> >Is it true that every time through the loop, a new i variable
> >is declared (more memory allocated ??)?  
> 
> Yes, a new i variable is declared.  However, at the end of each time
> through the loop it is "undeclared", so it can be deallocated.  Most C

I disagree.  Using the following source code:

int main()
{
	int fd;
	char *p;
	char t;
	extern char *ttyname();

	int i;

	for(i=0; i < 10; i++)
	{
	    	char buffer[50];

		printf("buffer = 0x%x\n",buffer);
	}
}

The following assembly code is generated (using "cc" on 386/ix):

	.globl	main
main:
	jmp	.L54
.L53:
	movl	$0,-16(%ebp)			/* i = 0 */
	jmp	.L58
.L59:
	leal	-66(%ebp),%eax			/* get address of buffer */
	pushl	%eax				/* put on stack for printf */
	pushl	$.L60				/* put string on stack	*/
	call	printf				/* call printf		*/
	addl	$8,%esp				/* clean stack		*/
	incl	-16(%ebp)			/* i++			*/
.L58:
	movl	$10,%eax			/* get 10 for comparison*/
	cmpl	%eax,-16(%ebp)			/* compare i to 10	*/
	jl	.L59				/* I < 10 ?		*/
.L57:
.L52:
	leave
	ret
.L54:
	pushl	%ebp
	movl	%esp,%ebp
	subl	$68,%esp		
	jmp	.L53
	.def	main;	.val	.;	.scl	-1;	.endef
	.data
.L60:
	.byte	0x62,0x75,0x66,0x66,0x65,0x72,0x20,0x3d,0x20,0x30
	.byte	0x78,0x25,0x78,0x0a,0x00


Note that at .L54 the data is allocated ONCE and only once.  It is not 
allocated/deallocated for each iteration.  This is unoptimized assembler
source generated using the -S flag.


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