printing array in function

printing array in function

[Nicolas Sabouret]

Hi all,

In the frame of a 1st-year course in C (starting in next october), we
are using a sub-set of gdb features (embedded in a geany plugin) to help
our students understand what is going on in their code.

The problem we have is that arrays passed to functions are seen as
pointers by gdb. Here is a simple example :

1:  void f(int tab[]) {
2:    tab[0] = 1;
3:  }
4:  int main() {
5:    int t[] = {-1,-1};
6:    f(t);
7:    return 0;
8:  }


$ gcc -g -o test test.c
$ gdb test
(gdb) b 6
(gdb) run
(gdb) p t       -> {-1, -1}
(gdb) s
(gdb) p tab     -> (int *) 0xbffff440


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).

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.

My question is : is there a way to display an array in a function
(without knowing the size a priori).

Thanks in advance for any help on this subject.
-- 
Nicolas Sabouret, Assistant Professor
University Pierre & Marie Curie
http://www-poleia.lip6.fr/~sabouret

Re: printing array in function

[Jan Kratochvil]

On Fri, 03 Sep 2010 11:06:02 +0200, Nicolas Sabouret wrote:
> 1:  void f(int tab[]) {
> 2:    tab[0] = 1;
> 3:  }
> (gdb) p tab     -> (int *) 0xbffff440

I have only an idea by C++:

void f(int (&tab)[2]) {
(gdb) p tab
$1 = (int (&)[2]) @0x7fffffffde50: {-1, -1}


Regards,
Jan

Re: printing array in function

[Nicolas Sabouret]

Dear Jan,

Le 03/09/2010 11:15, Jan Kratochvil a écrit :
> On Fri, 03 Sep 2010 11:06:02 +0200, Nicolas Sabouret wrote:
>> 1:  void f(int tab[]) {
>> 2:    tab[0] = 1;
>> 3:  }
>> (gdb) p tab     -> (int *) 0xbffff440
> 
> I have only an idea by C++:
> 
> void f(int (&tab)[2]) {
> (gdb) p tab
> $1 = (int (&)[2]) @0x7fffffffde50: {-1, -1}

Thanks for the quick answer. However, your solution requires :
1) to fix the array size in the function signature (this is something we
would prefer to avoid but why not)
2) to use a non-regular syntax in the function signature, which is
someting we cannot do (we want our students to lear the proper C-syntax).

Any other idea ?

-- 
Nicolas Sabouret
LIP6, BC169, 4 place Jussieu, 75005 Paris
http://www-poleia.lip6.fr/~sabouret

Re: printing array in function

[Steffen Dettmer]

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:  void f(int tab[]) {
> 5:    int t[] = {-1,-1};
>
> (gdb) p tab     -> (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=======
   #include <stdio.h>

   void f(int tab[]) {
     tab[0] = 1;
     /* (%z is not known to all compilers) */
     printf("sizeof(tab) = %lu\n", (unsigned long)sizeof(tab));
   }
   int main() {
     int t[] = {-1,-1};
     printf("sizeof(t) = %lu\n", (unsigned long)sizeof(t));
     f(t);
     return 0;
   }
=======8<-------------------------------------------------------------------

produces

   sizeof(t) = 8
   sizeof(tab) = 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) == 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] = 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] = 1;
   }
   {
     int t[] = {-1,-1};
     f(t, sizeof(t)/sizeof(t[0]));
   }

and using tab_size in the frontend?

oki,

Steffen

Re: printing array in function

[Andreas Schwab]

Nicolas Sabouret <Nicolas.Sabouret@lip6.fr> writes:

> The problem we have is that arrays passed to functions are seen as

There is no such thing as an array parameter in C.

> pointers by gdb. Here is a simple example :
>
> 1:  void f(int tab[]) {

This is the same as f(int *), see §6.7.5.3#7.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: printing array in function

[Nicolas Sabouret]

Hi,

Le 03/09/2010 18:21, Andreas Schwab a écrit :
> There is no such thing as an array parameter in C.

I know this. I was just wondering if there was any workaround in gdb. I
can understand why there is not, though.

> This is the same as f(int *), see §6.7.5.3#7.

Thanks for your answer.
I think we will have to write a specific parser... or to use other
pedagogical artifacts.

Regards,
-- 
Nicolas Sabouret
LIP6, BC169, 4 place Jussieu, 75005 Paris
http://www-poleia.lip6.fr/~sabouret

Re: printing array in function

[Joachim Protze]

Hi,

Am 03.09.2010 21:18, schrieb Nicolas Sabouret:
> I think we will have to write a specific parser... or to use other
> pedagogical artifacts.

As a student in the final phase of the study of computer science i do
not see the benefit of feigning your students to wrong facts. Within the
function f there is no knowledge of the length of the array. Why do you
want to display it anyway? The only effect is to give the students a
wrong insight to C. The consequence is, that they will tend to produce
code with buffer overflows.

> (we want our students to lear the proper C-syntax)
Proper C-syntax, but wrong C-semantik ?!?

Joachim