Where is PerlIO_isutf8()?

[Steve Hay]

Hi,

I'm trying to reproduce in an XSUB what Perl 5.8.0's built-in print() 
function does with regard to being given "flagged" UTF-8 data.  (Namely, 
when the filehandle has a ":utf8" layer, it upgrades the data to UTF-8 
before output; otherwise, if the "bytes" pragma is not in effect, it 
downgrades any UTF-8 data to the machine's native 8-bit encoding before 
output.  (I think.))

I think I've found where Perl does this -- Perl_do_print() in "doio.c" 
-- and wanted to copy the chunk of code concerned:

   if (PerlIO_isutf8(fp)) {
       if (!SvUTF8(sv))
           sv_utf8_upgrade(sv = sv_mortalcopy(sv));
   }
   else if (DO_UTF8(sv)) {
       if (!sv_utf8_downgrade((sv = sv_mortalcopy(sv)), TRUE)
           && ckWARN_d(WARN_UTF8))
       {
           Perl_warner(aTHX_ packWARN(WARN_UTF8), "Wide character in 
print");
       }
   }

However, when I try to link this I get an "unresolved external symbol" 
error regarding PerlIO_isutf8().  This is declared in "perlio.h" (hence 
the compiler was happy), but listing the symbols exported by my 
"perl58.lib" I don't see it (hence the linker is unhappy).

Where is PerlIO_isutf8() to be found, and can it be accessed at all from XS?

If not, then how do I do what I'm trying to do here?

Thanks,

Steve