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Date: Fri, 03 Sep 2010 10:24:00 -0000
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Subject: Re: printing array in function
From: Steffen Dettmer <steffen.dettmer@googlemail.com>
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On Fri, Sep 3, 2010, Nicolas Sabouret <Nicolas.Sabouret@lip6.fr> wrote:
> The problem we have is that arrays passed to functions are seen as
> pointers by gdb. Here is a simple example :
>
> 1: =A0void f(int tab[]) {
> 5: =A0 =A0int t[] =3D {-1,-1};
>
> (gdb) p tab =A0 =A0 -> (int *) 0xbffff440

I think this is absolutely correct.

tab is an int pointer, the storage size of a referenced "array" isn't known.

> The only solution we found to display tab as an array is to use "p
> *tab@2", but this requires knowing the exact size of the array (2 in
> this example).

Yes, absolutely, same as in C:
------------------------------------------------------------------->8=3D=3D=
=3D=3D=3D=3D=3D
   #include <stdio.h>

   void f(int tab[]) {
     tab[0] =3D 1;
     /* (%z is not known to all compilers) */
     printf("sizeof(tab) =3D %lu\n", (unsigned long)sizeof(tab));
   }
   int main() {
     int t[] =3D {-1,-1};
     printf("sizeof(t) =3D %lu\n", (unsigned long)sizeof(t));
     f(t);
     return 0;
   }
=3D=3D=3D=3D=3D=3D=3D8<----------------------------------------------------=
---------------

produces

   sizeof(t) =3D 8
   sizeof(tab) =3D 4

so function f() also needs to know the size or number of
elements, even when a "size" is given:

   void f(int tab[2]) {
       assert(sizeof(tab) =3D=3D sizeof(int*));
   }

to illustrate this, development rules may forbid array notation
for function notation instead of pointer, leading to:

   void f(int *tab) {
     tab[0] =3D 1;
   }

which, as far as I know, the same as above and should be portable.

> Our problem is that the gdb calls are integrated in a front-end for
> students (they do not type gdb commands directly) and that our frontend
> has no way of "guessing" what is the correct size for the array.

maybe by passing a size parameter (which is usually needed anyway
to be able to know the size of the array):

   void f(int *tab, size_t tab_size) {
     tab[0] =3D 1;
   }
   {
     int t[] =3D {-1,-1};
     f(t, sizeof(t)/sizeof(t[0]));
   }

and using tab_size in the frontend?

oki,

Steffen