jeenglis@nunki.usc.edu (Joe English) (10/28/89)
bdm659@csc.anu.oz writes: > Section 3.2.2.3 (on the result of casting 0 to a pointer type): > > "Such a pointer, called a null pointer, is guaranteed not to > point to any object or function." > > Section 3.3.9 (on pointer equality): > > "If two pointers to object or incomplete types compare equal, > they point to the same object." > >From these two rules, it logically follows that (int*)0 != (int*)0 . Not really; 3.3.9 says (loosely translated) [(A and B point to an object) and (A == B)] ==> [it is the same object]. (1) (2) (3) If (1) is false it does not follow that (2) is false. --Joe English jeenglis@nunki.usc.edu
bdm659@csc.anu.oz (10/28/89)
In article <8952@goofy.megatest.UUCP>, djones@megatest.UUCP (Dave Jones) writes: > > 1. The sections concerning pointers are poorly written. > It will not surprise me at all if a literal reading of them leaves some > expected behavior unspecified. > > 2. It doesn't matter. Section 3.2.2.3 (on the result of casting 0 to a pointer type): "Such a pointer, called a null pointer, is guaranteed not to point to any object or function." Section 3.3.9 (on pointer equality): "If two pointers to object or incomplete types compare equal, they point to the same object." From these two rules, it logically follows that (int*)0 != (int*)0 . Choose between :-) and :-( yourself. ========================== Brendan McKay. bdm@anucsd.oz or bdm@anucsd.oz.au (via uunet.uu.net) The address in the header may be scrambled.
gwyn@smoke.BRL.MIL (Doug Gwyn) (10/28/89)
In article <872.254895b5@csc.anu.oz> bdm659@csc.anu.oz writes:
- Section 3.2.2.3 (on the result of casting 0 to a pointer type):
- "Such a pointer, called a null pointer, is guaranteed not to
- point to any object or function."
- Section 3.3.9 (on pointer equality):
- "If two pointers to object or incomplete types compare equal,
- they point to the same object."
-From these two rules, it logically follows that (int*)0 != (int*)0 .
Of course this is utter nonsense. The conclusion does not follow
from those rules. (int*)0 is not a pointer to an object or an
incomplete type.datanguay@watmath.waterloo.edu (David Adrien Tanguay) (10/28/89)
In article <872.254895b5@csc.anu.oz> bdm659@csc.anu.oz writes:
: Section 3.2.2.3 (on the result of casting 0 to a pointer type):
:
: "Such a pointer, called a null pointer, is guaranteed not to
: point to any object or function."
:
: Section 3.3.9 (on pointer equality):
:
: "If two pointers to object or incomplete types compare equal,
: they point to the same object."
:
:From these two rules, it logically follows that (int*)0 != (int*)0 .
(int *) 0 is not a pointer to an object, so section 3.3.9 doesn't come
into play.
:Brendan McKay. bdm@anucsd.oz or bdm@anucsd.oz.au (via uunet.uu.net)
David Tanguaybdm659@csc.anu.oz (10/28/89)
In article <11432@smoke.BRL.MIL>, gwyn@smoke.BRL.MIL (Doug Gwyn) writes: > In article <872.254895b5@csc.anu.oz> bdm659@csc.anu.oz writes: > - Section 3.2.2.3 (on the result of casting 0 to a pointer type): > - "Such a pointer, called a null pointer, is guaranteed not to > - point to any object or function." > - Section 3.3.9 (on pointer equality): > - "If two pointers to object or incomplete types compare equal, > - they point to the same object." > -From these two rules, it logically follows that (int*)0 != (int*)0 . > > Of course this is utter nonsense. The conclusion does not follow > from those rules. (int*)0 is not a pointer to an object or an > incomplete type. I didn't say it was. In fact, I agree it isn't either of those things. The trouble is that the sentence only requires "pointers to object or incomplete types", which can only be parsed as "pointers to object types or pointers to incomplete types". If it said "objects" instead of "object" you would have a strong case, but it doesn't, so you don't. In any case, I could use (void*)0 instead of (int*)0 to avoid this semantic point, as (void*)0 is certainly a "pointer to incomplete type". Thus, you are wrong in claiming that the two sentences (AS QUOTED - see my query at the end) don't imply my conclusion. However, it turns out that I'M WRONG TOO, for another reason. Merv Rubinstein informs me that in the draft of Dec. 7 1988 it says "If two pointers to object or incomplete types are both null, they compare equal." This proves (a) that the phrase "pointers to object or incomplete types" is NOT intended to exclude null pointers. [So much for the "utter nonsense", Doug.] Just as well, too, because it would be a pity if we had to give up applying 3.2.2.3 and 3.3.4 to them. (b) that the difficulty I found in my earlier draft has been already found and taken care of. (c) that I should wait for my copy of the latest draft to arrive before posting articles like that. (I have a few other "jokes?" up my sleeve, but I'll wait until I can argue them with complete knowledge of the facts.) Query: Has the sentence from 3.3.9 in the ">" section above been modified to avoid the contradiction? (I quoted it from a posting of Bill Wells. My copy of the draft is so old that it doesn't have it at all.) That isn't very important with the new sentence close by, of course, but any mathematician knows that practically anything can be proved or disproved if one contradiction is assumed. Brendan McKay. ============================================================= bdm@anucsd.oz or bdm@anucsd.oz.au (via uunet.uu.net)
bdm659@csc.anu.oz (10/28/89)
> Merv Rubinstein informs me that in the draft of Dec. 7 1988 it says
That should be "Marv". Sorry Marv.davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) (10/30/89)
In article <876.2549cbb7@csc.anu.oz>, bdm659@csc.anu.oz writes: | However, it turns out that I'M WRONG TOO, for another reason. | Merv Rubinstein informs me that in the draft of Dec. 7 1988 it says | | "If two pointers to object or incomplete | types are both null, they compare equal." Another case of "no substantive changes," right? -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) "The world is filled with fools. They blindly follow their so-called 'reason' in the face of the church and common sense. Any fool can see that the world is flat!" - anon
gwyn@smoke.BRL.MIL (Doug Gwyn) (10/31/89)
In article <1518@crdos1.crd.ge.COM> davidsen@crdos1.UUCP (bill davidsen) writes: >| "If two pointers to object or incomplete >| types are both null, they compare equal." > Another case of "no substantive changes," right? Right. Or do you want to argue that X3J11 ever intended that (int*)0 != (int*)0