#define Perl_PerlIO_or_Base(f, callback, base, failure, args)   \
        if (PerlIOValid(f)) {                                   \
                const PerlIO_funcs * const tab = PerlIOBase(f)->tab;\
                if (tab && tab->callback)                       \
                        return (*tab->callback) args;           \
                else                                            \
                        return PerlIOBase_ ## base args;        \
        }                                                       \
        else                                                    \
                SETERRNO(EBADF, SS_IVCHAN);                     \
        return failure

...

SSize_t
Perl_PerlIO_read(pTHX_ PerlIO *f, void *vbuf, Size_t count)
{
     Perl_PerlIO_or_Base(f, Read, read, -1, (aTHX_ f, vbuf, count));
}