Re: pointer addition and arrays

[Andreas F. Borchert]

> ==================================== foo.c 
> ====================================
> #include <stddef.h>
> #include <stdlib.h>
> #ifdef __GNUC__
> # define unreachable() __builtin_unreachable ()
> #endif
> typedef struct { int a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t; } foo;
> int main ()
> {
>   char *m = malloc (1);
>   if (m == NULL)
>     unreachable ();
>   foo *p = (foo *) m;     /* CAST */
>   ptrdiff_t d = p - p;    /* DIFFERENCE */
>   foo *q = p + 0;         /* ADDITION */
>   free (m);
> }
> /* gcc 
> -fsanitize=address,undefined,signed-integer-overflow,shift,integer-divide-by-zero
>  foo.c */
> /* clang 
> -fsanitize=address,undefined,signed-integer-overflow,shift,integer-divide-by-zero
>  foo.c */
> ===============================================================================
> 
> gcc's and clang's sanitizers let this program pass at runtime.
> 
> The line /* CAST */ is probably OK because ISO C 23 § 6.5.4 specifies no
> undefined behaviour.

This line is not guaranteed to be ok due to § 6.3.2.3, (7):

   A pointer to an object type may be converted to a pointer to a different
   object type. If the resulting pointer is not correctly aligned for the
   referenced type, the behaviour is undefined.

alignof(char) will be 1 and the alignment of foo will probably be 4, hence
there is no guarantee that the pointer value of p is properly aligned.
You have just the exception that you are free to convert a (foo *) to (char *)
and to even convert a (char *) back to (foo *) if it was originally a (foo *).

See also § 6.2.5, (33) according to which pointers to structure types
(as foo in the example) can have possibly a different representation
than a pointer to char. To see the problem, assume a word addressed
machine where char pointers need extra representation for the byte
offset within a word. How could you convert a non-word-aligned (char *)
to a (foo *) on such a machine?

Andreas.

-- 
Dr. Andreas F. Borchert,  Institut für Numerische Mathematik, Universität Ulm
Helmholtzstr. 20, 89081 Ulm, +49 7315023572 https://mathematik.uni-ulm.de/afb

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