Re: sizeof on an uninitialized pointer to a VLA

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: sizeof on an uninitialized pointer to a VLA
Date	2015-11-21 12:09 -0800
Organization	None to speak of
Message-ID	<lnd1v36rnf.fsf@kst-u.example.com> (permalink)
References	(3 earlier) <87mvu8rbor.fsf@bsb.me.uk> <lnsi40axv9.fsf@kst-u.example.com> <871tbjpr4m.fsf@bsb.me.uk> <lnbnan8uf7.fsf@kst-u.example.com> <81b2f3fc-c570-4daf-850b-237640f1fd22@googlegroups.com>

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supercat@casperkitty.com writes:
> On Saturday, November 21, 2015 at 5:27:02 AM UTC-6, Keith Thompson wrote:
>> 2. sizeof ( vla_type )
>>    Not a constant expression.  Any expressions that are part of the type
>>    name are evaluated
>
> What if one has a VLA of pointers to a VLA type?  Would expressions forming
> part of the name of the thing being pointed to be evaluated?

Good question.  A concrete example:

    #include <stdio.h>

    int five(void) {
        puts("five()");
        return 5;
    }

    int six(void) {
        puts("six()");
        return 6;
    }

    int main(void) {
        printf("%zu\n", sizeof (char(*[five()])[six()]));
    }

On my system (where pointers are 8 bytes), the result of the
sizeof expression is 40, and both five() and six() are called.
The call to six() is not necessary to determine the result of the
sizeof expression.

(I'm not sure whether it's possible for the size to vary depending
on the value returned by six().)

I suggest that all expressions that are part of the type name should
be evaluated, just for the sake of keeping the description simple.
The standard could, I suppose, specify which expressions are needed
to determine the size and which are not, but it would be complex,
potentially ambiguous, and not worth the effort.  I could live
with it being unspecified if that would make things easier for
some compilers.

If I wanted to do something similar to the above in real-world
code, I would almost certainly define a typedef, so the issue
wouldn't arise:

    typedef char(*funky_array[five()])[six()];
    printf("%zu\n", sizeof (funky_array));

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

sizeof on an uninitialized pointer to a VLA John Bode <jfbode1029@gmail.com> - 2015-11-19 11:39 -0800
  Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 12:09 -0800
  Re: sizeof on an uninitialized pointer to a VLA "A. Bolmarcich" <aggedor@earl-grey.cloud9.net> - 2015-11-19 21:40 +0000
    Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 13:50 -0800
  Re: sizeof on an uninitialized pointer to a VLA James Kuyper <jameskuyper@verizon.net> - 2015-11-19 17:50 -0500
    Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 15:31 -0800
    Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 01:09 +0000
      Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 19:10 -0800
    Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 03:52 -0800
      Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 14:24 +0000
        Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-20 10:25 -0800
        Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 18:29 -0800
          Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 16:35 +1300
            Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 20:24 -0800
              Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 17:38 +1300
                Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 21:08 -0800
          Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 10:46 +0000
            Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 03:26 -0800
              Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 12:46 +0000
                Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 14:01 +0000
              Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-21 09:02 -0800
                Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 12:09 -0800

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