argument is described below:
=over
+
+=item B<b>
+
The BIO the callback is attached to is passed in B<b>.
+=item B<oper>
+
B<oper> is set to the operation being performed. For some operations
the callback is called twice, once before and once after the actual
operation, the latter case has B<oper> or'ed with BIO_CB_RETURN.
+=item B<argp> B<argi> B<argl>
+
The meaning of the arguments B<argp>, B<argi> and B<argl> depends on
the value of B<oper>, that is the operation being performed.
+=item B<ret>
+
B<ret> is the return value that would be returned to the
application if no callback were present. The actual value returned
is the return value of the callback itself. In the case of callbacks