if (tab->fsize != sizeof(PerlIO_funcs)) {
        Perl_croak( aTHX_
            "%s (%d) does not match %s (%d)",
            "PerlIO layer function table size", tab->fsize,
            "size expected by this perl", sizeof(PerlIO_funcs) );
    }
    if (tab->size) {
        PerlIOl *l;
        if (tab->size < sizeof(PerlIOl)) {
            Perl_croak( aTHX_
                "%s (%d) smaller than %s (%d)",
                "PerlIO layer instance size", tab->size,
                "size expected by this perl", sizeof(PerlIOl) );
        }