john_bode@my-deja.com (John Bode) wrote in message news:<43618c0e.0410121005.1fdb06e7@posting.google. com>...[color=blue]
>
dati_remo@libero.it (dati_remo@libero.it) wrote in message news:<eddc78be.0410112337.57c012f0@posting.google. com>...[color=green]
> > Karthik Kumar <kaykaylance_nospamplz@yahoo.com> wrote in message news:<416adc48$1@darkstar>...[color=darkred]
> > >
dati_remo@libero.it wrote:
> > > > Hi, is it possible to find the dimension of an array using a pointer?
> > > >
> > > > main()
> > > > {
> > > > int a[10];
> > > >
> > > > f(a);
> > > >
> > > > return;
> > > > }
> > > >
> > > >
> > > > f(int *b)
> > > > {
> > > > /*how can I know here the size of b??*/
> > > > /*because sizeof(b) is always 4*/
> > > > /*and sizeof(b[0]) is always 4*/
> > >
> > > Both ought to be the same, since both are pointers.
> > > And you are trying to get the size of a pointer variable
> > > (which is implementation - dependent).
> > >
> > > As Artie had already suggested, pass the length
> > > explicitly as an argument to the function.[/color]
> >
> > I know, but I can't do it.
> >[/color]
>
> Why not?
>[color=green]
> > Do you thing it is possible to find in the stack this information?
> >[/color]
>
> No. Seriously, there is no (portable) way to find out the physical
> size of an array based on a pointer to the first element alone. You
> have two choices:
>
> 1. Save the size of the array and either pass it as an argument
> or save it in a file scope variable.
>
> 2. Write a special sentinel value to the last element of the
> array,
> like how C strings are arrays of char terminated with a 0.
> This
> method is far more error prone, though, and won't necessarily
> tell
> you the *physical* size of the array, just how many elements
> come
> before the sentinel.
>
> Actually, there's a third choice: pass a pointer to the array (which
> is not the same thing as passing a pointer to the first element):
>
> int f(int (*b)[10])
> {
> /* sizeof *b == sizeof (int) * 10 */
> }
>
> int main (void)
> {
> int a[10];
>
> f(&a);
> return 0;
> }
>
> The only problem is that this assumes you're only working with arrays
> with 10 elements, so it's not very flexible.
>[color=green]
> > I mean, I have the position of the first element of the array, then,
> > moving up and down in the stack, I find this number. Or is the stack a
> > simple storage for the program and only it knows this information.
> >[/color]
>
> Aside from the fact that not all machines even *use* a stack for
> passing arguments to functions, what you're passing is a *pointer*
> type, not an array type. As far as the called function is concerned,
> b is a pointer to a single int object, not an array. That object may
> be the first element of an array. It may be the last element of an
> array. It may be a scalar variable. There is simply no way for the
> called function to know based on the pointer alone.
>[color=green]
> > And then how does really operate sizeof? Has it an internal table with
> > all variables?
> >[/color]
>
> Possibly. Or it could do something completely different. It's up to
> the compiler writer to decide how sizeof actually gets implemented.
>[color=green]
> > Thank you
> >
> > Best Regards
> >
> > Dati Remo[/color][/color]
Thanks to all.
I'll try other solutions.
Dati Remo
