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Re: What is the value after operator '!'?

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> supercat@casperkitty.com writes:
>> On Monday, October 26, 2015 at 8:43:14 AM UTC-5, Ben Bacarisse wrote:
>>> From the C standard:
>>> 
>>>   "The result of the logical negation operator ! is 0 if the value of
>>>   its operand compares unequal to 0, 1 if the value of its operand
>>>   compares equal to 0."
>>
>> It may be worth noting that 0 doesn't always mean "zero".  If p is a
>> pointer, then "p==0" will coerce the *literal* zero to a null
>> pointer;
>
> I would not put it quite like that.  0 always means zero, it's just that
> an integer constant expression with the value 0 is a null pointer
> constant as well as being an integer zero.  The 0 is not coerced to
> being a null pointer, it always is one.

A metaquibble: 0 is not a *null pointer*, it's a *null pointer
constant*.

A *null pointer* is a value of pointer type that can exist during
program execution.  A *null pointer constant* is a construct that can
appear in a C source file.  Evaluating a null pointer constant may or
may not yield a runtime null pointer value, depending on the context in
which it appears.

The expression 0 is simultaneously several things: an expression of type
int, an integer constant expression, an octal constant, a null pointer
constant.

>                                          There *is* a conversion
> implicit in writing p==0, but it's the conversion of the null pointer
> constant to a null pointer of the same type as p.  Your wording might be
> taken to suggest that there is some arithmetic type to pointer type
> conversion going on.

There is.  The expression 0 is of type int.  In p==0, if p is of pointer
type, the expression 0 is converted, via the special-case rule in N1570
6.3.2.3p3, from int to the type of p.  (This conversion is almost
certainly done at compile time, though the standard doesn't say so.)

[...]

>>     unsigned char *p = 0;   // A literal zero -- a null pointer
>>     unsigned char *q = (unsigned char*)(intptr_t)(1-1); // A zero value
>>
>>     // The following two expressions both return 1
>>
>>     int resp1 = !p;  // Since p is null)
>>     int resq1 = !(intptr_t)q; // Since (intptr_t)q is zero
>>
>>     // The following expressions could return 0 if a system uses a bit
>>     // pattern for null which, when cast to intptr_t does not yield zero.
>>
>>     int resp2 = !(intptr_t)p; // Comparing possibly-non-zero number vs zero
>>     int resq2 = !q; // Compare possibly non-null pointer against null
>
> As for the very last part, I don't think so.  Both p and q are null
> pointers of the same type.  Whilst I agree that !(intptr_t)p might be
> 0, I can't see how can !q be anything other than 1.

Agreed.  (1-1) is a null pointer constant, and so is (intprt_t)(1-1)
(since casts are permitted in integer constant expressions).  An integer
constant expression with the value 0 cast to any pointer type is a null
pointer constant, so (unsigned char*)(intptr_t)(1-1) is a null pointer
constant.

A better example would be:

    unsigned char *p = 0;
    const int not_a_constant zero = 0;
    unsigned char *q = (unsigned char*)zero;

The value of p is a null pointer.  The value of q may or may not be.

And converting a null pointer value to intptr_t may or may not yield 0.

> <snip>

-- 
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"

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Thread

What is the value after operator '!'? fl <rxjwg98@gmail.com> - 2015-10-26 06:21 -0700
  Re: What is the value after operator '!'? Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-26 09:29 -0400
  Re: What is the value after operator '!'? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 13:42 +0000
    Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 07:53 -0700
      Re: What is the value after operator '!'? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 17:37 +0000
        Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 11:05 -0700
          Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 11:38 -0700
            Re: What is the value after operator '!'? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-26 20:27 +0000
              Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 14:16 -0700
                Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 17:15 -0700
              Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 17:12 -0700
            Re: What is the value after operator '!'? Ian Collins <ian-news@hotmail.com> - 2015-10-27 10:27 +1300
            Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 15:15 -0700
              Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 16:16 -0700
                Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-27 08:39 -0700
  Re: What is the value after operator '!'? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-26 14:04 +0000
  Re: What is the value after operator '!'? Geoff <geoff@invalid.invalid> - 2015-10-26 09:50 -0700

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