Re: Null pointer constant

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Null pointer constant
Date	2017-06-19 22:44 +0200
Organization	A noiseless patient Spider
Message-ID	<oi9ctg$ro6$1@dont-email.me> (permalink)
References	(2 earlier) <jMD1B.215949$gM7.179726@fx44.am4> <oi78on$hu7$1@dont-email.me> <9zM1B.193976$KK7.183000@fx12.am4> <oi89cl$t3d$1@dont-email.me> <ln4lvbhpaf.fsf@kst-u.example.com>

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On 19/06/17 19:15, Keith Thompson wrote:
> David Brown <david.brown@hesbynett.no> writes:
>> On 19/06/17 11:11, bartc wrote:
>>> On 19/06/2017 02:21, David Brown wrote:
>>>> On 19/06/17 01:11, bartc wrote:
>>>
>>>>> (a?b:c) can be a constant expression, provided a, b and c are constants.
> 
> Correction: It can be a constant expression, provided a, b, and c are
> constant expressions.  (I'm assuming a, b, and c are intended to
> represent arbitrary subexpressions and needn't be just identifiers.)

Yes.

> Apart from that quibble, bartc's statement is correct.
> 
>>>> I presume you haven't bothered looking at the standard to see what a
>>>> "constant expression" actually /is/, in the C standards?  And in
>>>> particular, you haven't looked at what an "integer constant
>>>> expression" is?
>>>
>>> The standard says this:
>>>
>>>    "constant-expression:  conditional-expression"
>>>
>>> A condition-expression can be a a?b:c operation.
>>>
>>> But it was hardly necessary to check that, as plenty of programs rely on
>>> using ?: in constant expressions, including that assert macro of a few
>>> weeks back which declared an array parameter bound as either 1 or 0.
>>
>> Yes, and did you also look up "/integer/ constant expression" - the
>> particular type of constant expression Ike referred to?
> 
> David, your previous followup seemed to imply that a conditional
> expression cannot be an integer constant expression.  In fact it can;
> for example `1?0:0` is both an integer constant expression and a null
> pointer constant.

I don't /think/ I implied that, but it is possible - and if so, I was 
mistaken.  I have learned a few more details about integer constant 
expressions during this discussion.  But as Bart pointed out, 
conditional expressions form the heart of common pre-C11 static 
assertion macros, so I have certainly /used/ conditionals (with constant 
expression parts) as integer constant expressions!

> 
> But a conditional expression can be a constant expression only if all
> three of its operands are constant expressions.
> 
<snip>
>> There is a big difference between (a, b, 0) and (a ? 0 : 0).  In the
>> first case, you have the comma operator, so the type of the expression
>> comes entirely from the right hand side.
> 
> Note that a constant expression cannot contain a comma operator.

I see that now.  That is a far simpler explanation than I had been 
thinking of (with the added benefit of being the /correct/ explanation).

> 
>> In the second case, you have a conditional operator.  Look at the rules
>> in 6.5.15.  There is no option for both the "b" and "c" parts of a?b:c
>> to be null pointer constants (perhaps because that would be a useless
>> expression).  The only way to interpret "a ? 0 : 0" to fit the
>> constraints is to consider both zeros to be ints.  So you are trying to
>> assign the /int/ value 0 to a pointer, not the null pointer constant.
> 
> David, I think you're missing something.  A *null pointer constant*
> is a kind of expression, but it's not necessarily of pointer type,
> and it's not necessarily a constant.  (Remember that a constant, as
> opposed to a constant expression, is a single token.)  0 is a null
> pointer constant of type int.  It can be explicitly or implicitly
> *converted* to a pointer type, yielding a null pointer.
> 

OK, that is a distinction that I was a bit vague about.

> Given that `a` is the name of an object, the expression `a ? 0 : 0` is
> not a constant expression, and therefore not an integer constant
> expression, and therefore not a null pointer constant.  But the second
> and third operands, both of which are simply `0`, are:
>      constants
>      integer constants
>      octal constants
>      constant expressions
>      integer constant expressions
>      null pointer constants
>      expressions of type int
> 
> `0` is of type int, and is a null pointer constant.
> `1 ? 0 : 0` is of type int, and is a null pointer constant.
> `a ? 0 : 0` is of type int, and is *not* a null pointer constant.
> 

Thanks for the details and corrections.

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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