va_list and char* are ambiguous

Irfan Adilovic irfanadilovic@gmail.com
Sun Mar 2 21:55:00 GMT 2014


On Sun, Mar 2, 2014 at 10:50 PM, Václav Zeman wrote:
> On 03/02/2014 10:45 PM, Irfan Adilovic wrote:
>> On Sun, Mar 2, 2014 at 6:58 PM, Irfan Adilovic wrote:
>>> irfan@irfy:~$ cat x.cc
>>> #include <cstdarg>
>>> #include <iostream>
>>> using namespace std;
>>> void foo (...)        { cout << "varargs\n"; }
>>> void foo (va_list ap) { cout << "va_list\n"; }
>>> int main () {
>>>     foo ((const char *)NULL);
>>>     foo ((char *)NULL);
>>> }
>>> irfan@irfy:~$ make x
>>> g++     x.cc   -o x
>>> irfan@irfy:~$ ./x
>>> varargs
>>> va_list
>>> $ uname -a
>>> CYGWIN_NT-6.2 irfy 1.7.29(0.271/5/3) 2014-02-21 23:45 x86_64 Cygwin
>>>
>>> I would expect the varargs version of foo to be called both times --
>>> and it does on my linux machine -- but I get the above output under
>>> Cygwin. It looks like va_list is defined in terms of char*.
>>>
>>> Can anyone confirm this behavior on their Cygwin installations?
>>>
>>> Is this behavior legal? (in terms of whatever standards apply)
>>>
>>> Is there a way to "fix" this? (i.e. typedef va_list as a pointer to a
>>> struct defined just for the purpose of defining the va_list type)
>>>
>>> -- Irfan
>>
>> I forgot to mention that calling `foo ("");` will produce:
>>
>> x.cc:7:12: warning: deprecated conversion from string constant to
>> 'char*' [-Wwrite-strings]
>>      foo ("");
>>             ^
>>
>> at compile time and will end up calling va_list at runtime.
> I suspect this is due to compatibility with MS compiler on 32 bit
> windows, where va_list is also char*.
>
> Does it fail for AMD64 as well? I suspect it should not and it should
> work. IIRC the AMD64 MS compiler uses different definition than the 32
> bit one. (I cannot check right now.)
>
> I do not think there is much that can be done about it. You will simply
> have to rename one of the functions.
>
> --
> VZ

I am on a 64-bit Windows 8 running 64-bit Cygwin, so no, it doesn't work.

I'm not sure how stuff works behind the scenes when I compile my
program under Cygwin, but I do not use anything from Windows
explicitly -- in other words, I'm working with purely Linux code.
Would recompiling gcc be an option? Or is va_list somehow hard-wired
to the Windows version of it?

-- Irfan

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