mqtt客户端强行关闭连接，服务器如何检测该连接是否可用

当MQTT客户端强制关闭连接时，服务器不会立即知道该连接已被关闭。服务器需要通过发送心跳包来检测连接是否可用。MQTT协议中规定了心跳包的机制，客户端和服务器需要定期向对方发送心跳包以维持连接。如果服务器在一定时间内没有收到来自客户端的心跳包，就认为该连接已经关闭。这个时间间隔由keepalive参数决定，通常情况下设置为30秒或1分钟。

如果服务器检测到连接已经关闭，会将连接状态标记为“断开”，并在一定时间内等待客户端重新连接。如果客户端在这段时间内重新连接成功，服务器会将连接状态标记为“连接”，并继续处理该连接的消息。如果客户端未能在规定时间内重新连接，服务器会将该连接关闭。

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mqtt客户端连接服务器的C语言代码实现

下面是基于MQTT C语言库的MQTT客户端连接服务器的代码实现：

1. 引入MQTT C语言库

#include "MQTTClient.h"

2. 定义MQTT客户端连接参数

MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;

3. 初始化MQTT客户端连接参数

MQTTClient_create(&client, "tcp://localhost:1883", "MQTTClientC", MQTTCLIENT_PERSISTENCE_NONE, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;

4. 连接MQTT服务器

if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
    printf("Failed to connect, return code %d\n", rc);
    exit(-1);
}

5. 断开与MQTT服务器的连接

MQTTClient_disconnect(client, 10000);
MQTTClient_destroy(&client);

完整代码实现：

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

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

    MQTTClient_create(&client, "tcp://localhost:1883", "MQTTClientC", MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    }

    printf("MQTT client connected to server successfully!\n");

    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);

    return rc;
}

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。