va_list structure passing as argument

[Sean McNeil]

I'm looking at a problem I have on the amd64 with bsdtar.  Essentially,
you get a core dump if you try to run the following:

tar zxvvf nonexistent.tar.gz

I've tracked it down to an issue where the ap is getting changed as a
side-effect of calling __vfprintf.  It looks like this is happening
because the va_list structure is being passed by reference.  The va_list
structure on amd64 is 24 bytes.  I'm guessing that it is 16 bytes or
less for i386.  It has been a while since I've looked at the macro that
determines when a structure is passed by reference or value.  Does
anyone know what that is?  I'm guessing that 24 passes that cutoff but
16 does not and that is why I see this bug on amd64 and not i386.

Sean