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Re: multidimensional heap arrays

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On 04/27/11 11:38 AM, Ben Bacarisse wrote:
> Ian Collins<ian-news@hotmail.com>  writes:
>
>> On 04/27/11 09:49 AM, Ben Bacarisse wrote:
> <snip>
>>> In addition, the change is very sight.  A program that behaves
>>> differently were a VLA to become a plain arrays is already something of
>>> a monster.  I can come up with such programs, but they are all very
>>> contrived.  There may be a natural example, but it seems unlikely.
>>
>> I was trying to come up with one and the first example I conjured up
>> surprised me:
>>
>> typedef char C[16];
>>
>> void f( C* c ) {}
>>
>> int main(void)
>> {
>>    char c[16];
>>
>>    f(&c );
>>
>>    const unsigned n = 15;
>>
>>    char cc[n];
>>
>>    f(&cc );
>> }
>>
>> Compiles fine as c99 (which was the surprise),
>
> Hmm...  At first glance it looks like a diagnostic is required.  6.7.5.2
> p6 says:
>
>    "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."
>
> so are '&cc' and the parameter 'c' pointers to compatible types?  This
> paragraph does not say.  Maybe it is specified somewhere else, but I
> can't find it.

Me neither, that's what surprised me.

> I think the intent of the last sentence is to make the program's
> behaviour is undefined, but that would imply the need for a diagnostic.
>
>> but fails as C++ (type
>> mismatch).  What did surprise me was the VLA type matching even though
>> the size differed.  Changing the declaration of cc to cc[15] did
>> produce the expected compiler warnings.
>
> 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!

-- 
Ian Collins

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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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