PerlIO *
PerlIO_openn(pTHX_ const char *layers, const char *mode, int fd,
             int imode, int perm, PerlIO *old, int narg, SV **args)
{
    if (narg) {
        if (narg > 1) {
            Perl_croak(aTHX_ "More than one argument to open");
        }
        if (*args == &PL_sv_undef)
            return PerlIO_tmpfile();
        else {
            char *name = SvPV_nolen(*args);
            if (*mode == IoTYPE_NUMERIC) {
                fd = PerlLIO_open3(name, imode, perm);
                if (fd >= 0)
                    return PerlIO_fdopen(fd, (char *) mode + 1);
            }
            else if (old) {
                return PerlIO_reopen(name, mode, old);
            }
            else {
                return PerlIO_open(name, mode);
            }
        }
    }
    else {
        return PerlIO_fdopen(fd, (char *) mode);
    }
    return NULL;
}