Re: Fwd: error in variable dereferencing

["Thibaud GUERIN" <thibaud at ploposor dot com>]

On 4/21/06, Andrew Haley <aph@gcc.gnu.org> wrote:
> Thibaud GUERIN writes:
>  > >
>  > > Now, if you know absolutely for sure that your args are pushed onto
>  > > the stack in order without any holes, you might be able to get around
>  > > this.
>  > >
>  > > void pkludge (char *s, ...) __attribute__((noinline));
>  > > void pkludge (char *s, ...)
>  > > {
>  > >   void **p = &s;
>  > >   printf ("%d\n", *(int *)++p);
>  > >   printf ("%d\n", *(int *)++p);
>  > > }
>  > >
>  > > This isn't legal C, though, and trying to do things like this behind
>  > > gcc's back is asking for trouble.  The _real_ solution is to find out
>  > > why gcc's builtins are not working.
>  > >
>  >
>  > Thanks for all the tips (maybe REALLY usefull later).
>  >
>  > But my problem right now is that :
>  >
>  > int                  my_own_printf(char *fmt, ...)
>  > {
>  >   char               **s = (char **)(&fmt);
>  >
>  >  if (*s == fmt)
>  >    write(1, "they are the same\n", 18);
>  >  else
>  >    write(1, "they aren't the same\n", 21);
>  >   return (0);
>  > }
>  >
>  > prints "they aren't the same" ....
>
> OK, so we've got rid of the asms and casting away const, and the code
> still doesn't work.
>
> So, your code is now, at last, legal C.  Let's have a look at the
> assembly code generated for my_own_printf (use gcc -S).  Be sure to
> include the exact command line you used.
>

(my_own_printf is in the test_suites.c) :

gcc -c -o test_suites.o test_suites.c -std=gnu99 -Wall -nostdinc
-Wstrict-aliasing=2 -S -fno-builtin  -I../include -I../
give me :

(snip)

.LC0:
	.string	"they are the same\n"
.LC1:
	.string	"they aren't the same\n"
	.text
.globl my_own_printf
	.type	my_own_printf, @function
my_own_printf:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$8, %esp
	leal	8(%ebp), %eax
	movl	%eax, -4(%ebp)
	movl	-4(%ebp), %eax
	movl	(%eax), %eax
	cmpl	8(%ebp), %eax
	jne	.L33
	subl	$4, %esp
	pushl	$18
	pushl	$.LC0
	pushl	$1
	call	write
	addl	$16, %esp
	jmp	.L34
.L33:
	subl	$4, %esp
	pushl	$21
	pushl	$.LC1
	pushl	$1
	call	write
	addl	$16, %esp
.L34:
	movl	$0, %eax
	leave
	ret
	.size	my_own_printf, .-my_own_printf
	.section	.rodata
.LC2:
	.string	"string2"
.LC3:
	.string	"string1"
	.text

(snip)


--
Thibaud