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Re: *&aptr , &*aptr question

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: *&aptr , &*aptr question
Date 2016-06-15 00:14 +0100
Organization A noiseless patient Spider
Message-ID <87twgvxt19.fsf@bsb.me.uk> (permalink)
References (1 earlier) <12ba4330-5009-077d-83d8-3e41a7221f7f@verizon.net> <njpul4$eiv$1@dont-email.me> <81f2561c-bea5-4846-9d10-9862764110ef@googlegroups.com> <lvalue-20160614232823@ram.dialup.fu-berlin.de> <njq1ud$sam$1@dont-email.me>

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BartC <bc@freeuk.com> writes:

> On 14/06/2016 23:28, Stefan Ram wrote:
>> jameskuyper@verizon.net writes:
>>> I can confirm that gcc gives the message "error: lvalue
>>> required as unary & operand" for & &*Ptr. This
>>> seems to me to be clearly in conflict with the specification:
>>> "the result [of &*] is as if both were omitted". I'd consider
>>> this a defect in gcc.
>>
>>   »& &*Ptr« is not allowed because »the result is not an
>>   lvalue« (6.5.3.1p3). »&*Ptr« is not an lvalue, but the
>>   first »&« in »&*Ptr« requires an lvalue.
>>
>
> What's going on here then:
>
> int* Ptr;
>
> Ptr = Ptr;
> Ptr = *Ptr;

That's a constraint violation (assigning an int to a pointer).

> Ptr = &Ptr;

Ditto (assigning a pointer of the wrong type).

> Ptr = &*Ptr;
> Ptr = *&Ptr;
>
> Ptr = &*&Ptr;

Ditto (assigning a pointer of the wrong type).

> Ptr = *&*Ptr;

Ditto (assigning an int to a pointer).

> Ptr = &*&*Ptr;
> Ptr = *&*&Ptr;

The others are OK (though not necessarily reasonable code).

> These are all fine according to gcc (and TCC and clang; other
> compilers aren't quite so happy).

For me, gcc gives a diagnostic (as it should) for all of those that are
constraint violations, even with no options.  But that's quite a modern
gcc.  You might need to ask gcc to compile actual C rather that it's
almost C-like default language.

-- 
Ben.

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*&aptr , &*aptr question G G <gdotone@gmail.com> - 2016-06-14 13:43 -0700
  Re: *&aptr , &*aptr question Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 22:01 +0100
    Re: *&aptr , &*aptr question Ike Naar <ike@iceland.freeshell.org> - 2016-06-14 21:20 +0000
      Re: *&aptr , &*aptr question G G <gdotone@gmail.com> - 2016-06-14 14:38 -0700
    Re: *&aptr , &*aptr question Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 22:42 +0100
    Re: *&aptr , &*aptr question Ken Brody <kenbrody@spamcop.net> - 2016-06-15 14:22 -0400
      Re: *&aptr , &*aptr question Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 19:24 +0100
  Re: *&aptr , &*aptr question "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 17:17 -0400
  Re: *&aptr , &*aptr question Keith Thompson <kst-u@mib.org> - 2016-06-14 14:37 -0700
  Re: *&aptr , &*aptr question "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 17:40 -0400
    Re: *&aptr , &*aptr question "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 17:49 -0400
    Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-14 22:57 +0100
      Re: *&aptr , &*aptr question jameskuyper@verizon.net - 2016-06-14 15:22 -0700
        Re: *&aptr , &*aptr question "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 18:40 -0400
        Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-14 23:44 +0100
        Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-14 23:54 +0100
          Re: *&aptr , &*aptr question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 00:14 +0100
          Re: *&aptr , &*aptr question Keith Thompson <kst-u@mib.org> - 2016-06-14 16:51 -0700
            Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-15 01:15 +0100
              Re: *&aptr , &*aptr question Keith Thompson <kst-u@mib.org> - 2016-06-14 17:44 -0700
              Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-15 09:12 +0200
                Re: *&aptr , &*aptr question Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 08:28 +0100
                Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-15 10:26 +0200
                Re: *&aptr , &*aptr question Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 09:48 +0100
                Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-15 11:05 +0100
                Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-15 12:22 +0200
                Re: *&aptr , &*aptr question Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-15 03:55 -0700
                Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-15 13:15 +0200
                Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-15 13:08 +0100
                Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-15 14:15 +0200
                Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-15 14:02 +0100
                Re: *&aptr , &*aptr question Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-15 13:38 -0700
                Re: *&aptr , &*aptr question David Brown <david.brown@hesbynett.no> - 2016-06-16 00:01 +0200
                Re: *&aptr , &*aptr question Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-16 00:58 -0700
                Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-15 09:06 -0700
                Re: *&aptr , &*aptr question BartC <bc@freeuk.com> - 2016-06-15 17:54 +0100
                Re: *&aptr , &*aptr question raltbos@xs4all.nl (Richard Bos) - 2016-06-15 20:10 +0000
    Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-14 16:28 -0700
      Re: *&aptr , &*aptr question Keith Thompson <kst-u@mib.org> - 2016-06-14 16:54 -0700
        Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-14 19:13 -0700
          Re: *&aptr , &*aptr question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 13:19 +0100
            Re: *&aptr , &*aptr question Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 10:39 -0700
              Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-16 12:06 -0700
                Re: *&aptr , &*aptr question Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 12:21 -0700
              Re: *&aptr , &*aptr question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-16 21:06 +0100
                Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-16 14:35 -0700
                Re: *&aptr , &*aptr question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-17 01:22 +0100
                Re: *&aptr , &*aptr question Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 22:21 -0700
                Re: *&aptr , &*aptr question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-18 00:55 +0100
                Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-17 17:26 -0700
                Re: *&aptr , &*aptr question Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 12:19 -0700
                Re: *&aptr , &*aptr question supercat@casperkitty.com - 2016-06-21 13:09 -0700
  Re: *&aptr , &*aptr question Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-14 22:30 -0400
    Re: *&aptr , &*aptr question Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 14:20 -0700

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