Re: [boost] [test] va_copy

Subject: Re: [boost] [test] va_copy
From: Bjørn Roald (bjorn_at_[hidden])
Date: 2010-01-03 20:32:55

On Monday 04 January 2010 01:36:45 am Gennadiy Rozental wrote:
> Bjørn Roald wrote:
> > On Friday 01 January 2010 10:15:29 pm Steven Watanabe wrote:
> >> AMDG
> >>
> >> Boost.Test is still failing on some compilers that don't support
> >> va_copy.  Is va_copy always a macro?  Would it be possible
> >> to use #ifdef va_copy?
> >
> > If it helps, that is what is done here:
> > http://svn.apache.org/repos/asf/stdcxx/branches/4.1.3/src/exception.cpp
> 
> I'd rather stay away from ifdef va_copy. Does it required by standard
> for the va_copy to be a macro? 

Latest C++0x drafts say:

        Other runtime support [support.runtime]

        Table 21 — Header <cstdarg> synopsis
        Type		Name(s)
        Macros: 	va_arg va_end va_start va_copy
        Type: 	va_list

And state that <cstdarg> provide same features as C99 stdarg.h with some C++ 
adjustments. 

> >  The commit in trunk adding va_copy is also missing the va_end call and
> > possibly one more call of va_copy to be correct according to the manpage.
> 
> You mean I need va_end after va_copy? 

Yes - the Linux manual page says.

   va_copy()
       An obvious implementation would have a va_list be a pointer to the 
stack frame of the  variadic  func‐
       tion.  In such a setup (by far the most common) there seems nothing 
against an assignment

           va_list aq = ap;

       Unfortunately,  there  are also systems that make it an array of 
pointers (of length 1), and there one
       needs

           va_list aq;
           *aq = *ap;

       Finally, on systems where arguments are passed in registers, it may be  
necessary  for  va_start()  to
       allocate  memory, store the arguments there, and also an indication of 
which argument is next, so that
       va_arg() can step through the list.  Now va_end() can free the 
allocated memory again.  To accommodate
       this situation, C99 adds a macro va_copy(), so that the above 
assignment can be replaced by

           va_list aq;
           va_copy(aq, ap);
           ...
           va_end(aq);

       Each  invocation  of  va_copy()  must be matched by a corresponding 
invocation of va_end() in the same
       function.  Some systems that do not supply va_copy() have __va_copy 
instead, since that was  the  name
       used in the draft proposal.

=== end manual quote

> Where is another va_copy?

A bit further down the same function:

        if( tl != PASS ) {
            args = args_copy;   <=====

Looks a lot like a va_list assignment. But I have no clear understanding of 
the code, or va_list HOWTOs for that sake. 

-- 
Bjørn