PerlIO *
PerlIO_push(pTHX_ PerlIO *f, PERLIO_FUNCS_DECL(*tab), const char *mode, SV *arg)
{
    if (tab->fsize != sizeof(PerlIO_funcs)) {
      mismatch:
        Perl_croak(aTHX_ "Layer does not match this perl");
    }
    if (tab->size) {
        PerlIOl *l;
        if (tab->size < sizeof(PerlIOl)) {
            goto mismatch;
        }