paho.mqtt.embedded-c-master\MQTTPacket\samples，这个目录里面封装了发布消息、订阅消息的示例。运行pub0sub1，这个示例里面会去订阅主题消息、发布主题消息。并且订阅和发布的消息是同一个主题，所以在运行过程中会看到循环打印同一份消息。代码如下：
#include <stdio.h>
#include <string.h>
#include <stdlib.h>#include "MQTTPacket.h"
#include "transport.h"/* This is in order to get an asynchronous signal to stop the sample, as the code loops waiting for msgs on the subscribed topic. Your actual code will depend on your hw and approach*/
#include <signal.h>int toStop = 0;void cfinish(int sig)
{signal(SIGINT, NULL);toStop = 1;
}void stop_init(void)
{signal(SIGINT, cfinish);signal(SIGTERM, cfinish);
}
/* */int main(int argc, char *argv[])
{MQTTPacket_connectData data = MQTTPacket_connectData_initializer;int rc = 0;int mysock = 0;unsigned char buf[200];int buflen = sizeof(buf);int msgid = 1;MQTTString topicString = MQTTString_initializer;int req_qos = 0;char* payload = "mypayload";int payloadlen = strlen(payload);int len = 0;char *host = "m2m.eclipse.org";int port = 1883;stop_init();if (argc > 1)host = argv[1];if (argc > 2)port = atoi(argv[2]);mysock = transport_open(host, port);if(mysock < 0)return mysock;printf("Sending to hostname %s port %d\n", host, port);data.clientID.cstring = "me";data.keepAliveInterval = 20;data.cleansession = 1;data.username.cstring = "testuser";data.password.cstring = "testpassword";//连接服务器len = MQTTSerialize_connect(buf, buflen, &data);rc = transport_sendPacketBuffer(mysock, buf, len);/* wait for connack */if (MQTTPacket_read(buf, buflen, transport_getdata) == CONNACK){unsigned char sessionPresent, connack_rc;if (MQTTDeserialize_connack(&sessionPresent, &connack_rc, buf, buflen) != 1 || connack_rc != 0){printf("Unable to connect, return code %d\n", connack_rc);goto exit;}}elsegoto exit;/* subscribe 订阅主题消息*/topicString.cstring = "substopic";len = MQTTSerialize_subscribe(buf, buflen, 0, msgid, 1, &topicString, &req_qos);rc = transport_sendPacketBuffer(mysock, buf, len);if (MQTTPacket_read(buf, buflen, transport_getdata) == SUBACK) /* wait for suback */{unsigned short submsgid;int subcount;int granted_qos; rc = MQTTDeserialize_suback(&submsgid, 1, &subcount, &granted_qos, buf, buflen);if (granted_qos != 0){printf("granted qos != 0, %d\n", granted_qos);goto exit;}}elsegoto exit;/* loop getting msgs on subscribed topic循环读取消息 */topicString.cstring = "pubtopic";while (!toStop){/* transport_getdata() has a built-in 1 second timeout,your mileage will vary */if (MQTTPacket_read(buf, buflen, transport_getdata) == PUBLISH){unsigned char dup;int qos;unsigned char retained;unsigned short msgid;int payloadlen_in;unsigned char* payload_in;int rc;MQTTString receivedTopic;rc = MQTTDeserialize_publish(&dup, &qos, &retained, &msgid, &receivedTopic,&payload_in, &payloadlen_in, buf, buflen);printf("message arrived %.*s\n", payloadlen_in, payload_in);}printf("publishing reading\n");//下面两行是用来发布消息。这里发布，上面订阅，就形成了一个循环。len = MQTTSerialize_publish(buf, buflen, 0, 0, 0, 0, topicString, (unsigned char*)payload, payloadlen);rc = transport_sendPacketBuffer(mysock, buf, len);}printf("disconnecting\n");len = MQTTSerialize_disconnect(buf, buflen);rc = transport_sendPacketBuffer(mysock, buf, len);exit:transport_close(mysock);return 0;
}
在这个示例中，改造一下，把发布消息的代码给注释掉。我们发现，过了一会，循环读消息的地方就再也接收不到服务器发送过来的消息了。推测是此时客户端与服务器之间的长连接已经断了。回到代码，
//设置心跳包间隔时间
data.keepAliveInterval = 20;Keep Alive timer
The Keep Alive timer is present in the variable header of a MQTT CONNECT message.The Keep Alive timer, measured in seconds, defines the maximum time interval between >messages received from a client. It enables the server to detect that the network connection to a client has dropped, without having to wait for the long TCP/IP timeout. The client has a responsibility to send a message within each Keep Alive time period. In the absence of a data-related message during the time period, the client sends a PINGREQ message, which the server acknowledges with a PINGRESP message.
If the server does not receive a message from the client within one and a half times the Keep Alive time period (the client is allowed “grace” of half a time period), it disconnects the client as if the client had sent a DISCONNECT message. This action does not impact any of the client’s subscriptions. See DISCONNECT for more details.
If a client does not receive a PINGRESP message within a Keep Alive time period after sending a PINGREQ, it should close the TCP/IP socket connection.
The Keep Alive timer is a 16-bit value that represents the number of seconds for the time period. The actual value is application-specific, but a typical value is a few minutes. The maximum value is approximately 18 hours. A value of zero (0) means the client is not disconnected.
The format of the Keep Alive timer is shown in the table below. The ordering of the 2 bytes of the Keep Alive Timer is MSB, then LSB (big-endian).
pub0sub1示例代码中，删除发布消息代码块只保留订阅消息功能以后，客户端在20秒之内都可以接收到服务器推送过来的消息。如果20秒内客户端没有向服务器发送PINGREQ消息，那么服务器会关闭掉TCP/IP端口连接。
因此，如果希望开启一个可以永远保持订阅消息的客户端，需要在设置的心跳间隔时间内向服务器发送PINGREQ消息。具体做法可以这样，代码如下：
while (!toStop){/****循环订阅消息*************/topicString.cstring = "pubtopic";len = MQTTSerialize_subscribe(buf, buflen, 0, msgid, 1, &topicString, &req_qos);rc = transport_sendPacketBuffer(mysock, buf, len);printf("publishing reading000\n");if (MQTTPacket_read(buf, buflen, transport_getdata) == SUBACK) /* wait for suback */{unsigned short submsgid;int subcount;int granted_qos;rc = MQTTDeserialize_suback(&submsgid, 1, &subcount, &granted_qos, buf, buflen);if (granted_qos != 0){printf("granted qos != 0, %d\n", granted_qos);goto exit;}}elsegoto exit;/*****************//* transport_getdata() has a built-in 1 second timeout,your mileage will vary */int state = MQTTPacket_read(buf, buflen, transport_getdata);//printf("state is = %d\n", state);//printf("PUBLISH is = %d\n", PUBLISH);if (state != -1){printf("state2 is = %d\n", state);}if (state == PUBLISH){unsigned char dup;int qos;unsigned char retained;unsigned short msgid;int payloadlen_in;unsigned char* payload_in;int rc;MQTTString receivedTopic;rc = MQTTDeserialize_publish(&dup, &qos, &retained, &msgid, &receivedTopic,&payload_in, &payloadlen_in, buf, buflen);printf("message arrived %.*s\n", payloadlen_in, payload_in); }//printf("publishing reading\n");//len = MQTTSerialize_publish(buf, buflen, 0, 0, 0, 0, topicString, (unsigned char*)payload, payloadlen);//rc = transport_sendPacketBuffer(mysock, buf, len);}我们在循环读取消息的代码块中加入了订阅消息，这样可以保持客户端与服务器之间长连接不会断开。当然，这么做效果并不好，因为循环发送订阅消息会对服务器产生比较多的负载。可以对代码做个优化，比如每间隔10秒钟发送一次订阅消息的请求。百度云官方提供的客户端代码就是这么实现的，下一次我们拿出来做对比。文档代码点这里