Re: is it possible to point to a slice of an array without malloc or VLAs?

Mark Summerfield <mark@qtrac.eu> writes:

On Wed, 28 Aug 2024 09:13:39 +0100, Phil Ashby wrote:
[snip]

To answer the specific question, I would use pointer arithmetic,
provided there is no intention to modify values, ie:
 
char **slice = argv + config->optind;
 
thus slice now points at the appropriate part of argv and can be indexed
or dereferenced / incremented to access elements.

[snip]
>
Thank you, that works great. I now do that plus store the slice's size using:
argc - optind
>
I tried to get the size using
>
#define ARRAYSIZE(a) (&(a)[1] - (a))

The difference between a pointer to the second element of an array and a
pointer to the first will always be 1.

Another way to look at this is to expand the expression.

  a[1] means *(a+1)
  &a[1] means &*(a+1)
  &*(a+1) is the same as a+1 (barring some details; see 6.5.3.2 p3)
  &a[1] - a means a+1 - a or just 1

char **folders = argv + optind;
int num_folders = argc - optind;
int size = ARRAYSIZE(folders);
printf("%d %d\n", num_folders, size);
>
but size was always 1.

Another fact that might help you is that the argv array will have a null
pointer as it's last element so you may not need to remember the length
of the slice since it is the tail slice of a null-terminated array.

--
Ben.