Re: Null pointer constant

From	Manfred <noname@invalid.add>
Newsgroups	comp.lang.c
Subject	Re: Null pointer constant
Date	2017-06-20 13:16 +0200
Organization	Aioe.org NNTP Server
Message-ID	<oib06v$1jba$1@gioia.aioe.org> (permalink)
References	(6 earlier) <U5P1B.148289$GY6.107654@fx30.am4> <oi8geb$a5m$1@dont-email.me> <87shiwc4il.fsf@bsb.me.uk> <oi9hi4$1jep$1@gioia.aioe.org> <lnd19zehps.fsf@kst-u.example.com>

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On 6/20/2017 12:27 AM, Keith Thompson wrote:
> Manfred <invalid@invalid.add> writes:
> [...]
>> This may be true as long as the standard is strictly read (I take your
>> word for it), but I think there is a big difference in terms of the
>> rationale behind it:
>> C is a strongly typed language (less than C++, but more than many other
>> languages), which means that it has well-defined rules for what is the
>> type of a constant or expression, and it also has well-defined rules for
>> how types are converted (cast, either implicitly or explicitly) from one
>> to another.
> 
> A terminology quibble: A conversion may be either implicit or explicit.
> A cast is an explicit conversion.  There is no such thing as an implicit
> cast.
Fair enough.

> 
>> Now, 0 is an exception to this rule, in that it can be /either/ a
>> integer constant, /or/ a null pointer constant.
> 
> 0 *is* an integer constant, and it *is* a null pointer constant.
That's my point: 0 has (is of) two types (at a basic view, see below why 
considering it of a single type looks weird to me)

> 
>> Or, in terms more strictly close to the standard's wording, an integer
>> constant (expression) /in the special case/ of having value 0, can be
>> /also/ used as a null pointer constant.
>> In any case, there is for 0 an exception to the general well-defined
>> type rules in the language, and, like most exceptions, it raises
>> problems when such rules must (or are wanted to) be enforced, like e.g.
>> for compiler writers, but also for a programming style that would rely
>> heavily on type checking.
> 
> No, there's no exception to the type rules.  The constant 0 is always of
> type int.  The key think to remember is that a *null pointer constant*
> is not necessarily of pointer type.
Well, this may formally work (indeed it does work in standardese), but 
to me it is a clear source for potential confusion: if "a null /pointer/ 
constant is not necessarily of /pointer/ type" then my basic view on 
type rules would see something weird.

   It is an expression that can be
> *converted* (either implicitly, or explicitly via a cast) to a pointer
> type, with the guarantee that the result of the conversion is the null
> pointer value for that type.
> 
>> IIRC, I had even read in some book back in the early '90s something like
>> that the formal way to define a null pointer would be
>> void* p = (void*)0;
>> but "most compilers would allow" for the more compact
>> void* p = 0;
>> in the special case of 0, because it was long commonly used (and
>> obviously still is).
> 
> Both are guaranteed to store a null pointer value in p.  Neither is more
> or less formal than the other.  Perhaps the "most compilers would allow"
> phrasing was intended to allow for pre-ANSI compilers.
Probably so, it was old times.

> 
>> This is to say that, even if the standard has managed to specify all of
>> this in a formally correct way, and in this respect it is well written,
>> there is a deviation here from the general type rules of the language,
>> which explains why the examples indicated by the OP gave surprising results.
> 
> I don't think so.  I believe the standard's discussion of null
> pointers and null pointer constants is consistent and correct,
> and is consistent with the general type rules of the language.
> The null pointer constant rules in particular merely add some cases
> where pointer conversions may be done, and what value they yield.
Here too, that's my point: the fact that in the /specific/ case of 0 
there is a specific case for implicit conversion, which is an exception 
with respect to all other integer constants.

Don't understand me wrong, I am not complaining about 0 for null 
pointers. I know how this works and I am fine with it.
I see it as an exception to general type rules, and to me exceptions to 
rules carry some cost. It may well be the case that in this case such a 
cost is paid off by other benefits, like the one you are referring to below.

> 
>> It might be worth recalling, at the risk of triggering some passionate
>> feedback, that it is exactly to provide an answer to this kind of
>> trouble that C++ introduced the keyword nullptr.
> 
> I certainly wouldn't have minded if C had defined a nullptr keyword
> from the beginning, and made it the only valid null pointer constant.
> (Except that there's an advantage in treating the null pointer as
> "zero", so that for example { 0 } works as an initializer for any
> type that can be initialized.)
>

Thread

Null pointer constant bartc <bc@freeuk.com> - 2017-06-18 17:15 +0100
  Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-18 11:20 -0700
  Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-18 19:23 +0100
    Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-18 20:57 +0100
      Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-18 13:38 -0700
        Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 04:35 -0700
          Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-19 18:08 +0000
            Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 21:05 -0700
          Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:12 -0700
            Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 21:08 -0700
      Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 00:34 +0100
        Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 04:45 -0700
          Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 13:55 +0200
        Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 13:16 +0100
          Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 16:58 +0100
  Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-18 20:58 +0000
    Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 00:11 +0100
      Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 03:21 +0200
        Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 10:11 +0100
          Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 12:38 +0200
            Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 13:05 +0100
              Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 14:39 +0200
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-19 11:06 -0400
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 15:51 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-19 12:22 -0400
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 17:29 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-19 13:45 -0400
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:31 -0700
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 15:10 +0000
                Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 16:53 +0100
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-19 13:06 -0400
                Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 20:03 +0100
                Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 22:37 +0200
                Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 22:10 +0100
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 14:25 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 22:46 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 08:43 -0700
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-20 09:17 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-21 00:56 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-21 10:07 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-21 23:52 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-22 10:12 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-22 17:33 -0700
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-23 08:54 -0700
                Re: Null pointer constant Manfred <invalid@invalid.add> - 2017-06-20 00:00 +0200
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 15:27 -0700
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-19 16:38 -0700
                Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-20 01:14 +0100
                Re: Null pointer constant Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-20 12:03 +0100
                Re: Null pointer constant Manfred <noname@invalid.add> - 2017-06-20 13:16 +0200
                Re: Null pointer constant jameskuyper@verizon.net - 2017-06-20 05:34 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 08:49 -0700
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-20 16:35 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-20 13:07 -0400
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 17:29 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-20 13:55 -0400
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 18:06 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-20 14:42 -0400
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 19:10 +0000
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-20 18:31 +0000
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 18:36 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-20 14:45 -0400
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 17:14 +0000
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 11:55 -0700
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 19:23 +0000
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-20 15:39 -0400
                Re: Null pointer constant scott@slp53.sl.home (Scott Lurndal) - 2017-06-21 13:44 +0000
                Re: Null pointer constant scott@slp53.sl.home (Scott Lurndal) - 2017-06-21 13:49 +0000
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-21 23:57 -0700
                Re: Null pointer constant Philip Lantz <prl@canterey.us> - 2017-06-28 20:04 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-07-10 08:38 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 12:39 -0700
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-20 20:53 +0100
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 13:07 -0700
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-20 20:50 +0000
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-20 15:35 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-21 00:26 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-21 09:54 -0700
                Re: Null pointer constant "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-21 13:09 -0400
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-22 00:53 -0700
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-21 18:09 +0100
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-22 00:50 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 10:24 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 22:32 -0700
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 19:40 +0100
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:08 -0700
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 20:32 +0100
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:48 -0700
                Re: Null pointer constant jameskuyper@verizon.net - 2017-06-19 13:08 -0700
                Re: Null pointer constant Nick Bowler <nbowler@draconx.ca> - 2017-06-19 20:40 +0000
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 22:01 +0100
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-19 14:15 -0700
                Re: Null pointer constant jameskuyper@verizon.net - 2017-06-19 14:25 -0700
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 21:06 +0000
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 14:28 -0700
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 21:35 +0000
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-19 14:48 -0700
                Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-20 15:38 +0000
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 15:16 -0700
                Re: Null pointer constant supercat@casperkitty.com - 2017-06-19 14:29 -0700
                Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 15:18 -0700
                Re: Null pointer constant Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 22:48 -0700
                Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-20 15:09 +0100
              Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 10:19 -0700
            Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 10:15 -0700
              Re: Null pointer constant David Brown <david.brown@hesbynett.no> - 2017-06-19 22:44 +0200
            Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:24 -0700
      Re: Null pointer constant Ike Naar <ike@iceland.freeshell.org> - 2017-06-19 08:21 +0000
        Re: Null pointer constant bartc <bc@freeuk.com> - 2017-06-19 10:17 +0100
          Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 10:04 -0700
  Re: Null pointer constant Keith Thompson <kst-u@mib.org> - 2017-06-19 12:34 -0700

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