+If the underlying accept socket is non-blocking and BIO_do_accept() is
+called to await an incoming connection it is possible for
+BIO_should_io_special() with the reason BIO_RR_ACCEPT. If this happens
+then it is an indication that an accept attempt would block: the application
+should take appropriate action to wait until the underlying socket has
+accepted a connection and retry the call.
+
+BIO_set_accept_port(), BIO_get_accept_port(), BIO_set_nbio_accept(),
+BIO_set_accept_bios(), BIO_set_bind_mode(), BIO_get_bind_mode() and
+BIO_do_accept() are macros.
+