c mqtt 示例代码

以下是一个基本的 C MQTT 示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "MQTTClient.h"

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "MQTT Examples"
#define QOS         1
#define TIMEOUT     10000L

volatile MQTTClient_deliveryToken deliveredtoken;

void delivered(void *context, MQTTClient_deliveryToken dt) {
    printf("Message with token value %d delivery confirmed\n", dt);
    deliveredtoken = dt;
}

int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    int i;
    char* payloadptr;

    printf("Message arrived\n");
    printf("     topic: %s\n", topicName);
    printf("   message: ");

    payloadptr = message->payload;
    for(i=0; i<message->payloadlen; i++) {
        putchar(*payloadptr++);
    }
    putchar('\n');
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

void connlost(void *context, char *cause) {
    printf("\nConnection lost\n");
    printf("     cause: %s\n", cause);
}

int main(int argc, char* argv[]) {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    int rc;

    MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
    printf("Connected to %s\n", ADDRESS);

    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to subscribe, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    pubmsg.payload = "Hello world!";
    pubmsg.payloadlen = strlen("Hello world!");
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Waiting for up to %d seconds for publication of %s\n"
            "on topic %s for client with ClientID: %s\n",
            (int)(TIMEOUT/1000), pubmsg.payload, TOPIC, CLIENTID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);

    if ((rc = MQTTClient_unsubscribe(client, TOPIC)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to unsubscribe, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

以上代码使用 Eclipse Paho C 客户端库实现了 MQTT 客户端，其中包含连接到代理，订阅主题，发布消息等基本操作。