Re: valid_pointer_p

[Eli Zaretskii <eliz@gnu.org>]

> From: Andreas Schwab <schwab@suse.de>
> Cc: storm@cua.dk (Kim F. Storm), emacs-devel@gnu.org
> Date: Sat, 12 Aug 2006 14:36:22 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > ??? Don't we have macros, like INTEGERP, SUBRP, etc. to do that
> > without dereferencing?  The length of the primitive Lisp types is
> > known, right?
> 
> Try to find out the size of a vector object.
> 
>     /* Vector of Lisp objects, or something resembling it.
>        XVECTOR (object) points to a struct Lisp_Vector, which contains
>        the size and contents.  The size field also contains the type
>        information, if it's not a real vector object.  */
>     Lisp_Vectorlike,

I think something like this should work for such cases:

      struct Lisp_Vector *p = XVECTOR (obj);

      if (valid_pointer_p (p, offsetof (struct Lisp_Vector, size)
      	 		      + sizeof (EMACS_INT))
      	  && valid_pointer_p (p, p->size))
	return 1;  /* valid */

(Modulo the complications of storing other information in the leading
bits of the `size' field of struct Lisp_Vector.)  This is what I meant
when I wrote:

    Even if you are right, dereferencing a pointer accesses a region in
    memory whose length is known in advance, so at most we will need to
    call valid_pointer_p twice.

Am I missing something?