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Re: pointer = &membuff[-2];

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Noob <root@127.0.0.1> writes:
> Keith Thompson wrote:
>> Converting the offset value from unsigned int to int is probably
>> cleaner:
>> 
>>     int offset = n1 - n2;
>>     sp = (struct dummy *) &membuff[offset];
>
> Cleaner, perhaps, but it has implementation-defined behavior
> when n1 < n2
>
> "When [...] an unsigned integer is converted to its corresponding
> signed integer, if the value cannot be represented the result is
> implementation-defined."

D'oh!.  You're right -- and for reasons that I emphasized in the same
article.

I actually can't think of a good clean way to compute the difference
between two unsigned int values as a signed int.  You can convert
both values to int:

    int offset = (int)n1 - (int)n2;

but that potentially fails if either value exceeds INT_MAX.  Or you can
write:

    int offset = (n1 >= n2) ? (n1 - n2) : -(n2 - n1);

(the equivalent if statement might be more legible), but that's ugly.

You could convert to a wider type, but there's no guarantee that there
is a wider type.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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pointer = &membuff[-2]; "Bo S." <bo@boo.invalid> - 2011-06-09 21:07 +0000
  Re: pointer = &membuff[-2]; Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-06-09 16:47 -0600
    Re: pointer = &membuff[-2]; BGB <cr88192@hotmail.com> - 2011-06-09 22:59 -0700
    Re: pointer = &membuff[-2]; pete <pfiland@mindspring.com> - 2011-06-10 06:28 -0400
  Re: pointer = &membuff[-2]; Keith Thompson <kst-u@mib.org> - 2011-06-09 17:54 -0700
    Re: pointer = &membuff[-2]; Noob <root@127.0.0.1> - 2011-06-10 13:03 +0200
      Re: pointer = &membuff[-2]; Keith Thompson <kst-u@mib.org> - 2011-06-10 08:10 -0700
    Re: pointer = &membuff[-2]; Keith Thompson <kst-u@mib.org> - 2011-06-10 08:12 -0700
  Re: pointer = &membuff[-2]; Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-09 21:02 -0400
  Re: pointer = &membuff[-2]; James Kuyper <jameskuyper@verizon.net> - 2011-06-09 22:25 -0400
  Re: pointer = &membuff[-2]; Shao Miller <sha0.miller@gmail.com> - 2011-06-10 01:10 -0500
    Re: pointer = &membuff[-2]; Shao Miller <sha0.miller@gmail.com> - 2011-06-10 01:13 -0500
      Re: pointer = &membuff[-2]; Shao Miller <sha0.miller@gmail.com> - 2011-06-10 01:20 -0500
  Re: pointer = &membuff[-2]; Shao Miller <sha0.miller@gmail.com> - 2011-06-10 03:33 -0500
  Re: pointer = &membuff[-2]; James Kuyper <jameskuyper@verizon.net> - 2011-06-10 08:23 -0400
  Re: pointer = &membuff[-2]; Sherm Pendley <sherm.pendley@gmail.com> - 2011-06-10 09:30 -0400
  Re: pointer = &membuff[-2]; ImpalerCore <jadill33@gmail.com> - 2011-06-10 07:15 -0700

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