mqtt客户端连接服务器的C语言代码实现

以下是一个简单的MQTT客户端连接服务器的C语言代码实现示例：

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

void on_connect(struct mosquitto *mosq, void *userdata, int rc)
{
    if(rc == 0) {
        printf("Connected to MQTT broker successfully\n");
        mosquitto_subscribe(mosq, NULL, "test", 0);
    } else {
        printf("Failed to connect to MQTT broker\n");
    }
}

void on_message(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *msg)
{
    printf("Received message: %s\n", (char *)msg->payload);
}

int main()
{
    struct mosquitto *mosq = NULL;
    int rc;

    mosquitto_lib_init();

    mosq = mosquitto_new(NULL, true, NULL);
    if(!mosq) {
        printf("Failed to create MQTT client instance\n");
        return 1;
    }

    mosquitto_connect_callback_set(mosq, on_connect);
    mosquitto_message_callback_set(mosq, on_message);

    rc = mosquitto_connect(mosq, "localhost", 1883, 60);
    if(rc != MOSQ_ERR_SUCCESS) {
        printf("Failed to connect to MQTT broker: %s\n", mosquitto_strerror(rc));
        mosquitto_destroy(mosq);
        return 1;
    }

    mosquitto_loop_forever(mosq, -1, 1);

    mosquitto_destroy(mosq);
    mosquitto_lib_cleanup();

    return 0;
}

这个例子使用了Mosquitto库来实现MQTT客户端连接，包括连接回调函数(on_connect)和消息回调函数(on_message)。在主函数中，首先初始化Mosquitto库，然后创建MQTT客户端实例(mosq)，然后设置连接回调函数和消息回调函数，然后连接到MQTT服务器并循环等待消息。当接收到来自MQTT服务器的消息时，消息回调函数(on_message)将被调用，打印消息的内容。最后，清除Mosquitto库并退出主函数。