Re: [parisc-linux] Re: Looking at vfprintf.c and alloca.

["Michael S. Zick" <mszick@morethan.org>]

On Tue July 18 2006 10:51, Randolph Chung wrote:
> Carlos O'Donell wrote:
> > I was looking at our tst-printfsz failures in glibc, and I noticed
> > some very cute code in vfprintf.c. The code uses alloca to create a
> > specs structure, and then using certain know addresses decides if the
> > stack grows down or up.
> > 
> > Does this look right? Anyone care to review if this actually works
> > with a newer GCC on hppa?
> 
> I tried this with both old (gcc-3.[34]) and new gcc (gcc-4.[01]) and i 
> don't think this works....
> 
> tausq@riot:~$ cat alloca.c
> #include <alloca.h>
> #include <stdio.h>
> 
> int main(int argc, char **argv)
> {
>          int *old;
>          int *ptr = alloca(100 * sizeof(int));
> 
>          old = ptr;
> 
>          ptr = alloca(100 * sizeof(int));
> 
>          printf("ptr = %p\nold = %p\n&old[100] = %p\n", ptr, old, 
> &old[100]);
>          return 0;
> }
> tausq@riot:~$ gcc-4.1 -Wall -o alloca alloca.c; ./alloca
> ptr = 0xc04ca590
> old = 0xc04ca3d0
> &old[100] = 0xc04ca560
> 

For the case of gcc-4.2, stack-grows down...

ptr = 0xbfea1fc0
old = 0xbfea2160
&old[100] = 0xbfea22f0

In both the cases (up/down) &old[100] has an offset of
400 bytes (correct).

There is some compiler generated, machine dependent,
optimization dependent, space between "ptr" and "old[100]"
Most likely the "guaranteed stack alignment" thingy.

Mike
> randolph
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> 
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