使用mosquitto实现MQTT通信
1 MQTT 编程的前期准备
MQTT通信时，使用发布/订阅消息模式，其通信模型如下：
在编写客户端程序时，需要准备好服务器(broker)的ip地址和端口号。
获取 ip地址:
1 如果是本地服务器，ip地址设为localhost即可。
2 通过ipconfig指令查看ip地址
$ ipconfig
ens33: flags=4163 mtu 1500
inet 192.168.0.109 netmask 255.255.255.0 broadcast 192.168.0.255
..........
io地址为：192.168.0.109
端口号：默认服务器监听的端口是1883
2 MQTT 发布者（客户端）：
2.1 libmosquitto 库初始化
int res = 0;
res = mosquitto_lib_init();
if(res < 0)
{
std::cout<<"mosquitto_lib_init()失败"<<std::endl;
return;
}
2.2 创建发布者客户端实例
Mosq *pub_mosq = NULL;
pub_mosq = mosquitto_new(NULL,session,NULL);
if(pub_mosq == NULL)
{
std::cout<<"mosquitto_new()失败"<<std::endl;
mosquitto_lib_cleanup();
return;
}
2.3 连接服务器：ip地址、端口号、保持连接的时间间隔
char HOST[] = "localhost";
int PORT = 1883 ;
int KEPP_LIVE = 10;
if(mosquitto_connect(pub_mosq,HOST,PORT,KEPP_LIVE)
{
std::cout<<"mosquitto_connect()失败"<<std::endl;
mosquitto_destroy(pub_mosq);
mosquitto_lib_cleanup();
return;
}
2.4 开启线程，处理网络信息
int res;
res = mosquitto_loop_start(pub_mosq);
if(res != MOSQ_ERR_SUCCESS)
{
std::cout<<"mosquitto_loop_start()失败"<<std::endl;
return;
}
2.5 发布消息：话题名、负载、Qos
int *mid =NULL; //消息的id
char topic[] = "temp";
char buffer[MSG_MAX_SIZE];
int len = strlen(buffer)+1;
int qos = 0;
bool retain = false;//是否保留消息
mosquitto_publish(pub_mosq,mid,topic,len,buffer,qos,retain);
2.6 销毁发布者客户端，清除libmosquitto库
mosquitto_destroy(mosq);
mosquitto_lib_cleanup();
mqtt_pub.cpp代码：
#include 3 MQTT 订阅者（客户端）：
订阅者客户端和发布者客户端在编写步骤上差不多，订阅者客户端在回调函数对话题消息进行处理。
3.1 libmosquitto 库初始化
int res = 0;
res = mosquitto_lib_init();
if(res < 0)
{
std::cout<<"mosquitto_lib_init()失败"<<std::endl;
return ;
}
3.2 创建客户端实例
Mosq *mosq = NULL;
mosq = mosquitto_new(NULL,session,NULL);
if(mosq == NULL)
{
std::cout<<"mosquitto_new()失败"<<std::endl;
mosquitto_lib_cleanup();
return ;
}
3 设置回调函数
//建立连接后,订阅话题的回调函数
void mqtt_connect_calllback(Mosq *mosq,void *userdata, int result)
{
int ret;
if(!result)
{
ret = mosquitto_subscribe(mosq, NULL ,"temp",2);
if(ret <0)
{
std::cout<<"订阅失败"<<std::endl;
}
else
{
std::cout<<"订阅成功"<<std::endl;
}
}
else
{
std::cout<<"订阅者连接失败"<<std::endl;
}
}
//订阅成功后的回调函数
void mqtt_subscriber_callback(Mosq *mosq, void *userdata,int mid,int qos_count,const int *granted_qos)
{
int i;
std::cout<<"订阅者(消息的id是: "<<mid<<"}:"<<granted_qos[0];
for(int i=0; i< qos_count;i++)
{
std::cout<<granted_qos[0]<<std::endl;
}
}
//消息送达时的回调函数
void mqtt_message_callback(Mosq *mosq,void *userdata,const struct mosquitto_message *message)
{
if(message->payloadlen)
{
printf("订阅的消息的话题：%s \n", message->topic, (char *)message->payload);
}
else
{
std::cout<<"话题的负载（内容）：%s \n"<<message->payload<<std::endl;
}
}
//断开连接时的回调函数
void my_disconnect_callback(Mosq *mosq,void *obj,int rc)
{
std::cout<<"断开连接"<<std::endl;
running = 0;
}
//日志回调函数
void mqtt_log_callback(Mosq *mosq ,void *userdata,int level,const char*str)
{
std::cout<<"日志级别"<<level<<":"<<str<<std::endl;
}
// 设置回调函数
mosquitto_connect_callback_set(sub_mosq,mqtt_connect_calllback);
mosquitto_subscribe_callback_set(sub_mosq,mqtt_subscriber_callback);
mosquitto_message_callback_set(sub_mosq,mqtt_message_callback);
mosquitto_disconnect_callback_set(sub_mosq,my_disconnect_callback);
mosquitto_log_callback_set(sub_mosq,mqtt_log_callback);
4 连接服务器：ip地址，端口号，保持连接的时间间隔
if(mosquitto_connect(mosq,HOST,PORT,KEEP_LIVE))
{
std::cout<<"mosquitto_connect()失败"<<std::endl;
return 1;
}
5 开启线程，处理网络信息
static int running = 1;
while(running)
{
mosquitto_loop(mosq,-1,1);
}
6 销毁订阅者客户端，清除libmosquitto 库
mosquitto_destroy(mosq);
mosquitto_lib_cleanup();
mqtt_sub.cpp代码：
#include 4 Makefile编译
Makefile文件
all:Subscriber Publiser
@echo ""
@echo "Start compiling"
@echo ""
Subscriber:
@g++ -o Subscriber mqtt_sub.cpp -lmosquitto -lpthread -ldl -lssl -lcrypto
Publiser:
@g++ -o Publisher mqtt_pub.cpp -lmosquitto -lpthread -ldl -lssl -lcrypto
clean:
-rm Publiser Subscriber
5运行
打开一终端，启动mosquitto服务器
$ mosquitto -v
另起一终端，运行订阅者
$ ./Subscriber
再起一终端，运行发布者
$./Publisher
当前文件结构：
