mqtt客户端c代码接收

以下是一个简单的MQTT客户端C代码示例，用于接收MQTT消息。

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "test"
#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;
    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("Subscribing to topic %s\nfor client %s using QoS%d\n\n", TOPIC, CLIENTID, QOS);
    MQTTClient_subscribe(client, TOPIC, QOS);

    while(1) {
        usleep(1000000L);
    }

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

这段代码使用Eclipse Paho MQTT C客户端库连接到MQTT代理，订阅名为“test”的主题，并在接收到消息时打印到控制台上。在连接到代理时，需要指定代理的地址和客户端ID，并设置连接选项（例如，保持连接的时间间隔和清除会话标志）。在订阅主题时，需要指定订阅的主题名称和所需的QoS级别。在接收到消息时，将调用msgarrvd函数，并将消息传递给该函数。在连接丢失时，将调用connlost函数。在退出程序之前，需要断开客户端与代理的连接并销毁客户端。