在mosquitto中有:

int sock;
int sockpairR;
int sockpairW;

	int sv[2];

	if(socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1){
		return MOSQ_ERR_ERRNO;
	}
	if(_mosquitto_socket_nonblock(sv[0])){
		COMPAT_CLOSE(sv[0]);
		COMPAT_CLOSE(sv[1]);
		return MOSQ_ERR_ERRNO;
	}
	if(_mosquitto_socket_nonblock(sv[1])){
		COMPAT_CLOSE(sv[0]);
		COMPAT_CLOSE(sv[1]);
		return MOSQ_ERR_ERRNO;
	}
	*pairR = sv[0];
	*pairW = sv[1];
 

其中: sockpairR=sv[0]; sockpairW=sv[1];

當需要傳送資料時呼叫: write(mosq->sockpairW, &sockpair_data, 1)

這樣在: select -> (sock, sockpairR) 時就會返回

FD_SET(mosq->sock, &readfds);
FD_SET(mosq->sockpairR, &readfds);
if(mosq->out_packet)
FD_SET(mosq->sock, &writefds);

 pselect(maxfd+1, &readfds, &writefds, NULL, &local_timeout, NULL);

if( FD_ISSET(mosq->sockpairR, &readfds) ){
  read(sockpairR....);
  FD_SET(mosq->sock, &writefds);
}
if(FD_ISSET(mosq->sock, &writefds)){
   write...>> sock
}