Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #76746

Re: sizeof on an uninitialized pointer to a VLA

Show all headers | View raw

Keith Thompson <kst-u@mib.org> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
<snip>
>> If we replace *arr with arr[0] but with the 0 being produced by a
>> function call:
>>
>>   sizeof arr[f()]
>>
>> then the wording says that f must be called.  gcc obliges but clang does
>> not.
<snip example>
> The output using every version of gcc and clang I've tried is:
>
>     zero()
>     sizeof vla[zero()] = 10

And I've just tried to see how I determined that clang did not evaluate
the argument to sizeof and I could not.  I don't know why I thought it
didn't but I agree that gcc clang to the same with your example (and
now, it seems, with mine!).

<snip>
>> As you say, the intent may well have been to limit evaluation to
>> expressions contained in type names of VLAs, but it certainly doesn't
>> say that, and at least one implementation has taken the literal meaning.
>>
>> Maybe rather than this:
>>
>>   "If the type of the operand is a variable length array type, the
>>   operand is evaluated; otherwise, the operand is not evaluated and the
>>   result is an integer constant."
>>
>> what was meant was this:
>>
>>   "If the operand is a variable length array type name, any expressions
>>   in the operand are evaluated; otherwise, the operand is not evaluated
>>   and the result is an integer constant."
>
> The result of `sizeof vla_object` is not an integer constant.  (Aside:
> "integer constant" is a poor choice of words, since that phrase has a
> different meaning.)

Agreed.

> The type of the operand to sizeof is known at compile time, and
> I believe that the size of that type can always be determined
> without evaluating anything other than the values of anonymous
> compiler-generated objects (or whatever mechanism the implementation
> uses to remember the size of a VLA type).
>
> I suggest something like:
>
>     If the operand is a parenthesized variable length array type name,
>     any expressions in the operand are evaluated.  Otherwise, the
>     operand is not evaluated.  The result is constant unless the
>     type of the operand is a variable length array type.

I find the idea that the result of an expression might be anything but
constant rather odd.  Would "The result is *a* constant" be better?  But
then does the last sentence add anything very useful?  I'd be inclined
to omit it.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web