Out of range pointers

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In article <33547 at XAIT.XEROX.COM> g-rh at XAIT.Xerox.COM (Richard Harter) writes:
>
>As a side note, one argument for making x[-1] legal is that it permits
>you to use sentinels in both directions.  I don't see that this is a
>problem, regardless of architecture.  All that is required is that nothing
>be allocated on a segment boundary.  However, as the man says, they way
>it is is the way it is.  There never was a machine, a language, or an
>operating system without arcane restrictions.  [Except lisp :-)]
>-- 

Consider:

   some_big_type x[2];

If sizeof(some_big_type) is half the size of a segement, computing &x[-1] is
no harder (or easier) than computing &x[2].  The standard mandates that
&x[2] be computable but it does not mandate that &x[-1] be computable.  I
suspect that a C implementation that allows arrays as large as large as a
segment is able to compute both addresses.

Note, mandating that &x[-1] be computable does not mean that x[-1] is
referencable.  So evenif &x[-1] were computable we still could not have a
sentinel at the low end of the array.  A sentinal at the low end is easy.
just start the data with x[1].

   Marv Rubinstein