用MQTTclient库来mqtt客户端连接服务器的C语言代码实现

以下是使用MQTTclient库来连接MQTT服务器的C语言代码示例：

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

#define ADDRESS     "tcp://mqtt.eclipse.org:1883" // MQTT服务器地址
#define CLIENTID    "ExampleClientSub" // 客户端ID
#define TOPIC       "test/topic" // 需要订阅的主题
#define QOS         1 // 服务质量等级
#define TIMEOUT     10000L // 超时时间

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) {
    printf("Message arrived\n");
    printf("  topic: %s\n", topicName);
    printf("  message: %.*s\n", message->payloadlen, (char*)message->payload);
    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);
        return -1;
    }

    printf("Subscribing to topic %s\nfor client %s using QoS%d\n\n", TOPIC, CLIENTID, QOS);
    MQTTClient_subscribe(client, TOPIC, QOS);

    printf("Press any key to exit\n");
    getchar();

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

在这个示例中，我们使用了MQTTClient库来创建一个MQTT客户端并连接到指定的MQTT服务器。我们需要指定MQTT服务器地址、客户端ID、需要订阅的主题、服务质量等级和超时时间等参数。在连接MQTT服务器之后，我们可以订阅指定的主题，并在消息到达时处理它们。最后，我们可以使用MQTTClient_disconnect函数关闭连接并销毁客户端。