Array bounds checking with C????

[Niels J|rgen Kruse]

steve at taumet.com (Stephen Clamage) writes:

>There is nothing to prevent the C compiler from carrying around enough
>information with arrays and pointers to detect those problems at runtime
>which cannot be found at compile time.  (...)
>if possible, at run time otherwise.  This approach is in fact implemented
>in some compilers.
>--

Oh.  Which ones?

Assume the following code:

        char *a,*c; double *b,d[17/sizeof(double)];

        if (a = malloc (17)) {
          b = (double *)a;
          c = (char *)b;
          /*  A  */
        }

At location A, a[16] is of course legal and a + 17 is
computable but not dereferenceable.  Also, it is obvious that
b[17/sizeof(double) - 1] is legal and b + 17/sizeof(double) is
computable but not dereferenceable.  But what about c?
Is c[16] legal?  Note that c[16] does not constitute part of
any double within bounds of b (unless sizeof(double) == 17 or 1).
What kind of object is b pointing to?  How does it differ from
the object pointed to by (d+0)?

What does your bounds-checking C compiler have to say?
What does the standard say?
-- 
Niels J|rgen Kruse 	DIKU Graduate 	njk at diku.dk