bufferevent_setcb
bufferevent_setcb(bufev, readcb, writecb, errorcb, cbarg);
void bufferevent_setcb(struct bufferevent *bufev,
bufferevent_setcb(bufev, tcp_readcb, NULL, tcp_closecb, p);
bufferevent_setcb(bufev, tcp_dropcb, tcp_writecb, tcp_errorcb, f);