Pointer Comparison and Portability

[tps at sdchem.UUCP]

In article <454 at mipos3.UUCP> pinkas at mipos3.UUCP (Israel Pinkas) writes:
>In article <416 at vaxine.UUCP> nw at vaxine.UUCP (Neil Webber) writes:
>>Consider the following C function:
>>
>>        same_char (p, q)
>>        char *p;
>>        char *q;
>>        {
>>                return (p == q);
>>        }
>>
>>Does this function only return a non-zero value when p and q point
>>to the same physical character?  This may seem like a silly question,
>>but I haven't found an iron-clad answer in K & R yet.   I quote:
>...
>True.  Consider the 80x86 achitecture and many of the existing C ompilers
>for it.  Many of them try to optimize in memory models with large data
>segments by using as small an offset as possible for the first element of
>the array.  (This is a very small optimization, but it does save some
>comparisons if the array is known to be smaller than a certain size.)  For
>example:
>
>	char p[16], q[16];
>...
>Considering that K&R specically said that only pointers to the same array
>should be compared, and the fact that many C compilers take K&R to be the
>final word on everything, I would say that unless your compiler manual
>states otherwise (or it works), you should avoid comparing pointers to
>different arrays.
>-Israel

I would say that a function call which compared its two pointer arguments
would *have* to work, no matter how K&R are interpreted on this point, or
else strcpy wouldn't work.

Proof:
	Using your declaration as a basis, declare
		
		char p[16];
		char q[] =	"stRing q";

		strcpy( p, q );

	now (assuming strcpy() is a real function call) look at
	strcpy():
		
		char *
		strcpy( s1, s2 )
			char	*s1, *s2;
	
	If (s1 == s2) could be true even if s1 and s2 pointed to different
	areas, then strcpy would copy s2 on top of itself.

That is, even on a segmented architecture, when pointers get passed to a
subroutine, they MUST have distinct addresses -- how else can the subroutine
know what area of memory to access?

"<" and ">" are still another matter.

|| Tom Stockfisch, UCSD Chemistry	tps%chem at sdcsvax.UCSD