Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!dkuug!freja.diku.dk!njk
From: njk@diku.dk (Niels J|rgen Kruse)
Newsgroups: comp.lang.misc
Subject: Re: Pointers as 3-tuples (Re: JLG's flogging of horses (was Re: Relationship between C and C++))
Message-ID: <1990Apr10.215213.18124@diku.dk>
Date: 10 Apr 90 21:52:13 GMT
References: <19844@megaron.cs.arizona.edu> <14305@lambda.UUCP> <5.Q2H42xds13@ficc.uu.net>
Organization: Department Of Computer Science, University Of Copenhagen
Lines: 58

peter@ficc.uu.net (Peter da Silva) writes:
->Using the syntax #A as the address of A (since &a now refers to a 3-tuple
->(#a, #a, #a)...
->>    t A [N] [M];    /* 2d array of type t */
->A looks like a tuple (#A[0][0], #A[0][0], #A[N][0]) of type (t (*)[M]).

Agreed.

->>    t *pA;
->>    ...
->>    pA = A;
->pA is the same tuple, but now looks like (#(t*)A[0], #(t*)A[0], #(t*)A[M*N])
->and has the type (t *).
------------------------------------------------------------------------^^^^^
Should be N, otherwise agreed.

->> Now, what bounds are associated with pA under your scheme?  N clearly
->> doesn't make sense.  But should the bound be M or N*M?  Whichever you
->> pick, I'll (sooner or later) want it to be the other.
->It's N*M. If you want it to be M, then you would have said:
->       pA = A[0];
->Since that's a tuple (#A[0][0], #A[0][0], #A[0][M]) with type (t[]).

I disagree.  It is my impression (i may be wrong), that the bounds
of a pointer to a subobject derive from the enclosing object,
not from the subobject itself.

To test your model of bounds-checking in C, what are the bounds of
        foo->buf
after
        struct v { int bar; char buf[1]; } *foo;
        foo = malloc (sizeof (struct v) + 2);
?
(ignoring possible allocation failure)

Is foo->buf[2] within bounds according to your model?

For your convenience, i quote the relevant passage of a summary
of X3J11's first meeting in the interpretations phase, which
Doug Gwyn was so considerate to post to comp.std.c.
        (various disclaimers in all caps (not official etc.) deleted)

        + There are no technical problems with using malloc()ed objects
        I.e. my dirent implementation is portable: bounds checking is NOT
        allowed in cases like the common struct record_header {...buffer[1];
        /*more than 1 actually allocated*/} technique where the right size
        is allocated for the buffer in the object via malloc() (as per
        Karl Heuer's argument).

Considering the disclaimers, you might choose to ignore this.

->--
-> _--_|\  `-_-' Peter da Silva. +1 713 274 5180. <peter@ficc.uu.net>.
->/      \  'U`
->\_.--._/
->      v

BTW, why to you have a map of Australia in your .sig? (Just curious).
-- 
Niels J|rgen Kruse 	DIKU Graduate 	njk@diku.dk