On 4/21/06, Andrew Haley <aph@xxxxxxxxxxx> 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, @functionmy_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
