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Re: va_args Conformance

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On 1/31/2012 12:19, Ben Bacarisse wrote:
> Shao Miller<sha0.miller@gmail.com>  writes:
>
>> On 1/31/2012 09:50, Ben Bacarisse wrote:
>>> Shao Miller<sha0.miller@gmail.com>   writes:
>>>
>>>> If the following program strictly conforming?
>>>>
>>>>     #include<stddef.h>
>>>>     #include<stdarg.h>
>>>>
>>>>     static void test(int, ...);
>>>>
>>>>     int main(void) {
>>>>         test(0, 42);
>>>>
>>>>         return 0;
>>>>       }
>>>>
>>>>     static void test(int x, ...) {
>>>>         typedef unsigned char one_char[1];
>>>>         /* Valid declaration on the next line */
>>>>         one_char * unused;
>>>>         va_list varargs;
>>>>         unsigned char * byte;
>>>>
>>>>         va_start(varargs, x);
>>>>         if (x)
>>>>             byte = va_arg(varargs, one_char);
>>>>         va_end(varargs);
>>>>
>>>>         return;
>>>>       }
>>>
>>> It looks fine to me.  Does some implementation reject it?
>>>
>>
>> Yes.  At least two of them, and in different ways that I didn't quite
>> expect.
>>
>> GCC 4.3.3 says:
>>
>>    test.c: In function 'test':
>>    test.c:21: error: invalid use of non-lvalue array
>>
>> Where 'va_arg' expands to '__builtin_va_arg'.
>
> gcc 4.6.1 is happy with it...
>

Aha.  Thanks a lot for checking that!

>> clang 2.9 says:
>>
>>    test.c:21:16: error: assigning to 'unsigned char *' from
>> incompatible type 'one_char' (aka 'unsigned char [1]')
>>              byte = va_arg(varargs, one_char);
>>                   ^ ~~~~~~~~~~~~~~~~~~~~~~~~~
>>    1 error generated.
>
> ... and my clang version 2.9 (tags/RELEASE_29/final) is also happy with
> it.
>

That's odd.  I was also using "tags/RELEASE_29/final".  I was using 
'-ansi -pedantic' as well, so maybe there's a C90 vs. C99 difference in 
treatment?

>> Where once again, 'va_arg' expands to '__builtin_va_arg'.
>>
>> My reading of N1256's 7.15.1.1 has me pondering these.
>
> It seems OK to me.  va_arg(varargs, one_char) must be an expression of
> type one_char.  An expression with type "array of T" gets converted to
> one of type "pointer to T" so the assignment should type-check.  (You
> can't execute the line without UB, but presumably that's why you put
> the if (x) check there.)
>

I had the same reasoning, so thank you very much for that.  Yes, I 
believe the undefined behaviour would be from the 'va_arg' result not 
being addressable, so the address of the first element would be 
undefined and hence the would-be-resulting pointer value.

Beyond all of that business, I don't know of a way to pass an array to 
any function, so the 'one_char' type could never be compatible with any 
parameter.  But that wasn't the point of the original question, of course.

> I was briefly concerned that maybe the array-to-pointer conversion might
> only apply to lvalue expressions, but it does not -- it applies to any
> expression with an array type (other than a string literal or one that
> is the operand of sizeof or unary&).
>

I had some moments where I shared this same concern and drew the same 
conclusion.  This seems to be related to something like:

   typedef unsigned char one_char[1];
   register one_char foo;

where there are now restrictions on the use of 'foo'; 'sizeof' and 
'_AlignOf' only, presumably.

-- 
"The stationery store has moved.  Aaargh!"

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va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 09:12 -0500
  Re: va_args Conformance Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 14:50 +0000
    Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 10:11 -0500
      Re: va_args Conformance Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 17:19 +0000
        Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 12:31 -0500
          Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 12:34 -0500
            Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 12:40 -0500
          Re: va_args Conformance Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 17:52 +0000
    Re: va_args Conformance James Kuyper <jameskuyper@verizon.net> - 2012-01-31 11:27 -0500
      Re: va_args Conformance Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 16:53 +0000
        Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 12:09 -0500
        Re: va_args Conformance James Kuyper <jameskuyper@verizon.net> - 2012-01-31 12:15 -0500
  Re: va_args Conformance Harald van Dijk <haraldvdijk@gmail.com> - 2012-01-31 09:46 -0800
    Re: va_args Conformance Shao Miller <sha0.miller@gmail.com> - 2012-01-31 13:45 -0500
      Re: va_args Conformance Harald van Dijk <haraldvdijk@gmail.com> - 2012-01-31 11:28 -0800
      Re: va_args Conformance Harald van Dijk <haraldvdijk@gmail.com> - 2012-01-31 11:33 -0800

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