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

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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.  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.

> likewise !p
> will compare p against a null pointer.  It would be perfectly legitimate for
> a system to define a mapping between integers and pointers such that:
>
>     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.

<snip>
-- 
Ben.

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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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