On 4/21/06, Andrew Haley <aph@xxxxxxxxxxx> wrote:> Thibaud GUERIN writes:> > On 4/21/06, Andrew Haley <aph@xxxxxxxxxxx> wrote:> > > Thibaud GUERIN writes:> > > > On 4/20/06, Andrew Haley <aph@xxxxxxxxxx> wrote:> > > > > Thibaud GUERIN writes:> > > > > >> > > > > >> > > > > > Not clear in the first message, (and maybe not in this one too..), sorry> > > > > >> > > > > >> > > > > > This asm inline was something like a "test/patch code".> > > > > >> > > > > > I try to have a simple :> > > > > >> > > > > > char **ap = (char **)(&fmt);> > > > >> > > > > > so i try by my self... to do :> > > > > >> > > > > > ap = &fmt;> > > > > > *ap = fmt;> > > > > >> > > > > > in asm inline.... (dirty i know...)> > > > > >> > > > > > problem is :> > > > > > With this asm code in the binary *s is equal to fmt> > > > > > Without this asm code in the binary *s isn't equal to fmt> > > > > >> > > > > >> > > > > > All the 's' variable stuffs are from my debug...> > > > > > Again :> > > > > > My only aim is to have an 'ap = &fmt' valid (->ap = &fmt AND *ap = fmt)> > > > >> > > > > So why not do the obvious> > > > >> > > > > const char **ap = &fmt ;> > > > >> > > > > ?> > > >> > > > because i need to do some :> > > > ap++;> > > > to get the next args in stack, as my end aim is to do re-write a printf...> > >> > > So why not use va_list? That's what va_list is for.> > >> > > > > > The resulting asm was here to help you to understand wath's wrong ....> > > > > > I'm looking for some days now without answer...> > > > >> > > > > You're still not explaining yourself. You have a const char* arg that> > > > > you are trying to alter, but instead of doing it the obvious way with> > > > > an assignment, you're taking the address of the arg, casting the> > > > > resulting pointer to a different pointer type, and then overwriting> > > > > the arg through the resulting pointer.> > > > >> > > > > What's the point of all this?> > > >> > > > i'm not trying to alter an (const char*) but to get the args in my> > > > stack by getting some pointers on it, as in all va_args fonctions....> > >> > > Trying to do all this stuff behind the compiler's back is likely to> > > break things. Use va_list.> >> > quote :> > I'm compiling with (to run on my VM):> > -std=gnu99 -Wall -Werror -nostdinc -Wstrict-aliasing=2 -fno-builtin> > -I../include -I../> >> > no-builtin ....> >> > I'm doing that because the va_list wasn't working too... (i come to> > that dirty code by simplifing the code to found the root of the> > problem)>> OK, so now I know what you're _really_ trying to do. It took some> work, but we're here now.>> > And my final aim is to have a printf without any deps form any parent> > system (no use of standard includes, ....)>> Fair enough. va_list and its friends can't be written in C, which is> why gcc provides builtin functions for them. The standard ones look> like this:>> #define va_start(v,l) __builtin_va_start(v,l)> #define va_end(v) __builtin_va_end(v)> #define va_arg(v,l) __builtin_va_arg(v,l)> #define va_copy(d,s) __builtin_va_copy(d,s)>> 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" ....
so it seems there is a real problem...for the memory :building with :gcc-3.4.6 -std=gnu99 -Wall -nostdinc-Wstrict-aliasing=2 -fno-builtinrunning on a multi segmented environnement (in a qemu VM).
i'm always trying to make it work on my own env (or crash in my linuxbox) without success...
--Thibaud
