C/C++

本文主要介绍如何在 C 项目中使用 Eclipse Paho C ，实现 ST200TH 设备与 MQTT 服务器的连接、订阅、收发消息等功能。 Eclipse Paho C (opens in a new tab) 与 Eclipse Paho Embedded C 均为 Eclipse Paho 项目下的 C 语言客户端库（MQTT C Client），均为使用 ANSI C 编写的功能齐全的 MQTT 客户端。

前提条件

安装依赖包

sudo apt-get update
sudo apt-get -y install build-essential git cmake openssl libssl-dev

编译 Paho MQTT 库

Linux/Mac

git clone https://github.com/eclipse/paho.mqtt.c.git
cd paho.mqtt.c
make
sudo make install

Windows

mkdir build.paho
cd build.paho
call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64
cmake -G "NMake Makefiles" -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=FALSE -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=TRUE ..
nmake

连接使用

连接设置

本文将使用自定义的接入认证方式，服务器接入信息如下：

Broker: mqtt.geek-smart.cn
TCP Port: 1883
WebSocket Port: 8083

导入依赖包

#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

定义连接地址、认证信息以及消息发布主题

#define ADDRESS     "tcp://mqtt.geek-smart.cn:1883"
#define USERNAME    "************"  // 用户名
#define PASSWORD    "************"  // 密码
#define CLIENTID    "c-client"
#define QOS         0
#define SUB_TOPIC       "/HYUGHV/lVtAcHuor***/4cebd60bf***/subscribe" // 订阅主题
#define PUB_TOPIC       "/HYUGHV/lVtAcHuor***/4cebd60bf***/publish"   // 发布主题
#define TIMEOUT     10000L

定义消息发布函数

void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, PUB_TOPIC);
}

定义 on_message 回调函数，用于打印订阅主题接收的消息内容

int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

初始化 MQTT 客户端并订阅主题

int rc;
MQTTClient client;
 
MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
conn_opts.username = USERNAME;
conn_opts.password = PASSWORD;
MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
    printf("Failed to connect, return code %d\n", rc);
    exit(-1);
} else {
    printf("Connected to MQTT Broker!\n");
}
// subscribe topic
MQTTClient_subscribe(client, SUB_TOPIC, QOS);

循环调用 publish 函数，完成消息发布

char payload[64];
for (int i = 0; i < 100; i += 1) {
    // 获取设备基本信息 每隔两秒获取一次
    snprintf(payload, 64, "\{\"type\":\"info\"\}");
    publish(client, PUB_TOPIC, payload);
    sleep(2);
}

完整代码

#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"
 
#define ADDRESS     "tcp://mqtt.geek-smart.cn:1883"
#define USERNAME    "************"  // username
#define PASSWORD    "************" // password
#define CLIENTID    "c-client"
#define QOS         0
#define SUB_TOPIC       "/HYUGHV/lVtAcHuor***/4cebd60bf***/subscribe" // subscribe
#define PUB_TOPIC       "/HYUGHV/lVtAcHuor***/4cebd60bf***/publish"   // publish
#define TIMEOUT     10000L
 
 
void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, PUB_TOPIC);
}
 
int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}
 
int main(int argc, char *argv[]) {
    int rc;
    MQTTClient client;
 
    MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;
    MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    } else {
        printf("Connected to MQTT Broker!\n");
    }
    // 订阅获取到的内容
    MQTTClient_subscribe(client, SUB_TOPIC, QOS);
    char payload[64];
    for (int i = 0; i < 100; i += 1) {
        // 获取设备基本信息 每隔两秒获取一次
        snprintf(payload, 64, "\{\"type\":\"info\"\}");
        publish(client, PUB_TOPIC, payload);
        sleep(2);
    }
    MQTTClient_disconnect(client, TIMEOUT);
    MQTTClient_destroy(&client);
    return rc;
}

测试验证

编写 CMakeLists.txt

cmake_minimum_required(VERSION 3.17)
project(mqtt_c C)
include_directories(/usr/local/include)
link_directories(/usr/local/lib)
set(CMAKE_C_STANDARD 99)
add_executable(mqtt_c main.c)
target_link_libraries(mqtt_c paho-mqtt3c)

编译

cmake .
make

运行

./mqtt_c

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