本文实现mqtt发送端发送cjson数据，接受端接收数据并使用cjson解析。
主要功能是发送端发送位姿，接收端接收位姿话题并解析使用

1.发送端程序

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)

project(mqttdemopub)
add_executable(${PROJECT_NAME} main.cpp cJSON.c cJSON.h)
target_link_libraries(${PROJECT_NAME} mosquitto)

main.cpp

#include <stdio.h>
#include <stdlib.h>
#include <mosquitto.h>
#include <string.h>

#include "cJSON.h"

#include <unistd.h>
#define HOST "localhost"
#define PORT  1883
#define KEEP_ALIVE 60
#define MSG_MAX_SIZE  512
#define TOPIC_NUM 3

bool session = true;

int main()
{
    struct mosquitto *mosq = NULL;
    //libmosquitto 库初始化
    mosquitto_lib_init();
    //创建mosquitto客户端
    mosq = mosquitto_new(NULL,session,NULL);
    if(!mosq){
        printf("create client failed..\n");
        mosquitto_lib_cleanup();
        return 1;
    }

    //连接服务器
    if(mosquitto_connect(mosq, HOST, PORT, KEEP_ALIVE)){
        fprintf(stderr, "Unable to connect.\n");
        return 1;
    }
    //开启一个线程，在线程里不停的调用 mosquitto_loop() 来处理网络信息
    int loop = mosquitto_loop_start(mosq);
    if(loop != MOSQ_ERR_SUCCESS)
    {
        printf("mosquitto loop error\n");
        return 1;
    }

    while(1)
    {
        /*发布消息*/
        //1.fill cjson struct

        cJSON *pose;
        pose=cJSON_CreateObject();
        //pose x=1,y=2,yaw=30
        cJSON_AddNumberToObject(pose,"x",1.0);
        cJSON_AddNumberToObject(pose,"y",2.0);
        cJSON_AddNumberToObject(pose,"yaw",30.0);
        //2.cjson to char*
        char* buff = cJSON_Print(pose);
        if(NULL == buff)
        {
            continue;
        }
        mosquitto_publish(mosq,NULL,"topic_pose ",strlen(buff)+1,buff,0,0);
        printf("%s\n",buff);

        cJSON_Delete(pose);
        free(buff);
        sleep(1);
    }

    mosquitto_destroy(mosq);
    mosquitto_lib_cleanup();

    return 0;
}

2.接收端程序

cmake_minimum_required(VERSION 2.8)

project(mqttdemosub)
add_executable(${PROJECT_NAME} main.cpp cJSON.c cJSON.h)
target_link_libraries(${PROJECT_NAME} mosquitto)

main.cpp

#include <stdio.h>
#include <stdlib.h>
#include <mosquitto.h>
#include <string.h>

#include "cJSON.h"
#include <unistd.h>
#define HOST "localhost"
#define PORT  1883
#define KEEP_ALIVE 60
#define MSG_MAX_SIZE  512
#define TOPIC_NUM 3

bool session = true;


void my_message_callback(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *message)
{

    ///message->topic is topic name
    ///(char *)message->payload is the data
    if(message->payloadlen){
        //printf("%s %s", message->topic, (char *)message->payload);
        //parse json here
        char * data=(char *)message->payload;
        cJSON* pose=cJSON_Parse(data);
        cJSON *item_x = cJSON_GetObjectItem(pose,"x"); //获取这个对象成员
        double pose_x=item_x->valuedouble;
        printf("pose_x :%lf\n",pose_x);

        cJSON *item_y = cJSON_GetObjectItem(pose,"y"); //获取这个对象成员
        double pose_y=item_y->valuedouble;
        printf("pose_y :%lf\n",pose_y);

        cJSON *item_yaw = cJSON_GetObjectItem(pose,"yaw"); //获取这个对象成员
        double pose_yaw=item_yaw->valuedouble;
        printf("pose_yaw :%lf\n",pose_yaw);

    }else{
        printf("%s (null)\n", message->topic);
    }
    fflush(stdout);
}

void my_connect_callback(struct mosquitto *mosq, void *userdata, int result)
{
    int i;
    if(!result){
        /* Subscribe to broker information topics on successful connect. */
        mosquitto_subscribe(mosq, NULL, "topic_pose ", 2);
    }else{
        fprintf(stderr, "Connect failed\n");
    }
}

void my_subscribe_callback(struct mosquitto *mosq, void *userdata, int mid, int qos_count, const int *granted_qos)
{
    int i;
    printf("Subscribed (mid: %d): %d", mid, granted_qos[0]);
    for(i=1; i<qos_count; i++){
        printf(", %d", granted_qos[i]);
    }
    printf("\n");
}

void my_log_callback(struct mosquitto *mosq, void *userdata, int level, const char *str)
{
    /* Pring all log messages regardless of level. */
    printf("%s\n", str);
}

int main()
{
    struct mosquitto *mosq = NULL;
    //libmosquitto 库初始化
    mosquitto_lib_init();
    //创建mosquitto客户端
    mosq = mosquitto_new(NULL,session,NULL);
    if(!mosq){
        printf("create client failed..\n");
        mosquitto_lib_cleanup();
        return 1;
    }
    //设置回调函数，需要时可使用
    ///1.show log:Client mosq-UunBize2TyoN0P2ubI received
    ///PUBLISH (d0, q0, r0, m0, 'topic_pose ', ... (33 bytes))
    //mosquitto_log_callback_set(mosq, my_log_callback);

    ///2.set sub topic name
    mosquitto_connect_callback_set(mosq, my_connect_callback);
    ///3.get topic name and data
    mosquitto_message_callback_set(mosq, my_message_callback);
   //mosquitto_subscribe_callback_set(mosq, my_subscribe_callback);

    //连接服务器
    if(mosquitto_connect(mosq, HOST, PORT, KEEP_ALIVE)){
        fprintf(stderr, "Unable to connect.\n");
        return 1;
    }
    //开启一个线程，在线程里不停的调用 mosquitto_loop() 来处理网络信息
    int loop = mosquitto_loop_start(mosq);
    if(loop != MOSQ_ERR_SUCCESS)
    {
        printf("mosquitto loop error\n");
        return 1;
    }


    while(1)
    {
       sleep(1);
    }

    mosquitto_destroy(mosq);
    mosquitto_lib_cleanup();

    return 0;
}