Re: Unexpected error on x[0] != y[0]

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Unexpected error on x[0] != y[0]
Date	2017-06-14 11:24 +0100
Organization	A noiseless patient Spider
Message-ID	<87wp8e9895.fsf@bsb.me.uk> (permalink)
References	<8ac725fb-f9d8-4332-b7d5-b29e13a1e0f3@googlegroups.com> <9f667a41-7a36-4cf4-9286-f1df3781dff4@googlegroups.com>

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joel.rees@gmail.com writes:
<snip>
> C does allow you to say x[0] about a scalar in some cases (which I had 
> forgotten until recently), but what it will want to do if you say that 
> is something like this, IIRC:
>
>    ((unsigned long) x)[0]
>
> In other words, it would attempt to treat x as a pointer. So, if x has 
> the ASCII value of 'A' (decimal 65, or hexadecimal 0x41), it would try 
> to add 0 to 65 to get a memory address.

Something has gone wrong here.  Given the description, I think you meant
to write

  ((unsigned long *)x)[0]

whether that can (or will) access an unsigned long at address 65 is not
something C has much to say about.  It may work, it may not.  It may
even terminate your program!

<snip>
>    'A'[0]
>
> is not the same thing as 
>
>    'A'
>
> and you have told the compiler that you want the 'A' or something like 
> it that is stored at x, not the data that 'A' or something like it 
> stored there would point at interpreted as a pointer.

If we are still talking about C, the most significant distinction is
that 'A'[0] is a contrast violation -- it must be diagnosed as an
error and the implementation is then free to do whatever it likes.

<snip>
> BTW, I spent a long time trying to figure out, given 
>
>    char xarray[ SIZE ];
>
> the difference between
>
>    char * ptr = xarray;
>
> and 
>
>    char ** pptr = &xarray;
>
> and such, which are not easy to understand, relative to the address 
> operator.

The latter is, again, a constraint violation.  The type of pptr is

  char **

and the type of &xarray is

  char (*)[SIZE]

These are not compatible types and an error must be reported.  I'm not
sure what you want to illustrate with this example so I can't say what
you intended to write.

<snip>
-- 
Ben.

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Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-12 06:57 -0700
  Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-12 15:11 +0100
    Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-12 07:18 -0700
      Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-12 15:29 +0100
        Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-12 07:41 -0700
          Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-12 18:32 +0100
            Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-12 10:56 -0700
              Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 08:14 -0700
                Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-13 17:52 +0100
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 10:00 -0700
                Re: Unexpected error on x[0] != y[0] scott@slp53.sl.home (Scott Lurndal) - 2017-06-13 17:19 +0000
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 10:33 -0700
                Re: Unexpected error on x[0] != y[0] scott@slp53.sl.home (Scott Lurndal) - 2017-06-13 17:56 +0000
                Re: Unexpected error on x[0] != y[0] Gareth Owen <gwowen@gmail.com> - 2017-06-13 19:45 +0100
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 17:54 -0700
                Re: Unexpected error on x[0] != y[0] joel.rees@gmail.com - 2017-06-13 20:57 -0700
                Re: Unexpected error on x[0] != y[0] Gareth Owen <gwowen@gmail.com> - 2017-06-14 06:41 +0100
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 03:37 -0700
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 03:54 -0700
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 17:45 -0700
                Re: Unexpected error on x[0] != y[0] David Brown <david.brown@hesbynett.no> - 2017-06-14 08:49 +0200
                Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-13 19:07 +0100
                Re: Unexpected error on x[0] != y[0] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 19:23 +0100
                Re: Unexpected error on x[0] != y[0] bartc <bc@freeuk.com> - 2017-06-13 22:30 +0100
                Re: Unexpected error on x[0] != y[0] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 23:29 +0100
                Re: Unexpected error on x[0] != y[0] supercat@casperkitty.com - 2017-06-13 11:51 -0700
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-13 17:53 -0700
                Re: Unexpected error on x[0] != y[0] Reinhardt Behm <rbehm@hushmail.com> - 2017-06-14 13:34 +0800
                Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 03:24 -0700
                Re: Unexpected error on x[0] != y[0] David Brown <david.brown@hesbynett.no> - 2017-06-14 09:10 +0200
  Re: Unexpected error on x[0] != y[0] John Bode <jfbode1029@gmail.com> - 2017-06-12 07:26 -0700
    Re: Unexpected error on x[0] != y[0] "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 10:54 -0400
      Re: Unexpected error on x[0] != y[0] Keith Thompson <kst-u@mib.org> - 2017-06-12 09:23 -0700
    Re: Unexpected error on x[0] != y[0] Gareth Owen <gwowen@gmail.com> - 2017-06-12 19:05 +0100
  Re: Unexpected error on x[0] != y[0] Anton Shepelev <anton.txt@gmail.com> - 2017-06-12 17:29 +0300
  Re: Unexpected error on x[0] != y[0] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-12 15:52 +0100
    Re: Unexpected error on x[0] != y[0] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-12 08:31 -0700
  Re: Unexpected error on x[0] != y[0] joel.rees@gmail.com - 2017-06-13 22:09 -0700
    Re: Unexpected error on x[0] != y[0] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-14 11:24 +0100

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