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Re: multidimensional heap arrays

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Ian Collins <ian-news@hotmail.com> writes:
> On 04/27/11 12:38 PM, Keith Thompson wrote:
>> Ian Collins<ian-news@hotmail.com>  writes:
>>> On 04/27/11 11:38 AM, Ben Bacarisse wrote:
>>>>
>>>> The standard can't mandate a type compatibility test that requires an
>>>> arbitrary non-constant expression to be evaluated, so in one way it is
>>>> not surprising that there is no error message.  Unfortunately I can't
>>>> square that with the requirement to issue a diagnostic when a pointer to
>>>> an incompatible type is passed to a function.
>>>
>>> I agree.  I tried the code with both gcc 4.5.1 (-ansi -Wall -pedantic
>>> -std=c99) and Sun c99.
>>>
>>> I thought a VLA would always have an incompatible type, given its size
>>> isn't known at compile time.  In cases like this, I miss C++'s typeid!
>>
>> C99 6.7.5.2p6:
>>
>>      For two array types to be compatible, both shall have compatible
>>      element types, and if both size specifiers are present, and are
>>      integer constant expressions, then both size specifiers shall
>>      have the same constant value. If the two array types are used in
>>      a context which requires them to be compatible, it is undefined
>>      behavior if the two size specifiers evaluate to unequal values.
>
> Ah, so they are incompatible, but no diagnostic was issued by either
> compiler.  I'd be interested in seeing if another C99 compiler issues
> one.

It's some rather odd wording.  The types are compatible, but using them
in a context that requires compatibility is undefined behavior.

It makes sense, or at least the consequences do.  Type compatibility is
purely a compile-time issue; VLAs introduce cases where compatible types
can't be used together, but in a way that the error can't (generally) be
detected at compile time.

-- 
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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multidimensional heap arrays Jack Boot <jackboot@mailinator.com> - 2011-04-25 19:36 +0000
  Re: multidimensional heap arrays Francois Grieu <fgrieu@gmail.com> - 2011-04-25 23:16 +0200
  Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-25 22:35 +0100
    Re: multidimensional heap arrays Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2011-04-25 21:19 -0700
      Re: multidimensional heap arrays Francois Grieu <fgrieu@gmail.com> - 2011-04-26 08:19 +0200
        Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 12:19 +0100
          Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-26 23:25 +1200
            Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 21:06 +0100
              Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 08:59 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 14:39 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 10:03 +1200
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 23:41 +0100
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 16:33 -0700
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 04:05 +0100
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 22:49 +0100
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 10:19 +1200
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 00:38 +0100
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 11:48 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 17:38 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 13:08 +1200
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-26 18:31 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 14:01 +1200
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-27 00:04 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 19:58 +1200
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 20:03 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-27 00:03 -0700
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-26 18:20 -0700
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 03:59 +0100
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-27 00:24 -0700
    Re: multidimensional heap arrays "io_x" <a@b.c.invalid> - 2011-04-26 09:42 +0200
  Re: multidimensional heap arrays Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-25 20:50 -0400
  Re: multidimensional heap arrays James Kuyper <jameskuyper@verizon.net> - 2011-04-25 22:57 -0400

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