Re: Null pointer constant

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Null pointer constant
Date	Mon, 19 Jun 2017 15:27:27 -0700
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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.

> 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.

> 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.  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.

> 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.

> 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.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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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