paho mqtt c waitfor

函数中的timeout参数是以毫秒为单位的等待时间，函数会在等待timeout毫秒之后超时并返回MQTTASYNC_FAILURE。如果timeout为0，则函数将立即返回，如果timeout为-1，则函数将一直等待直到接收到消息。该函数的返回值为MQTTASYNC_SUCCESS或MQTTASYNC_FAILURE，表示是否成功等待消息。

相关问题

eclipse Paho MQTT Utility连接mysql数据库

要在 Eclipse Paho MQTT Utility 中连接 MySQL 数据库，需要进行以下步骤：

1.下载并安装 JDBC 驱动程序，比如 MySQL Connector/J。

2.在 Eclipse Paho MQTT Utility 中编写 Java 代码，使用 JDBC 连接 MySQL 数据库。可以使用以下代码示例：

import java.sql.*;

public class MqttToMySQL {
    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    static final String DB_URL = "jdbc:mysql://localhost/mydatabase";

    static final String USER = "username";
    static final String PASS = "password";

    public static void main(String[] args) {
        Connection conn = null;
        Statement stmt = null;
        try{
            Class.forName("com.mysql.jdbc.Driver");

            System.out.println("Connecting to database...");
            conn = DriverManager.getConnection(DB_URL,USER,PASS);

            System.out.println("Creating statement...");
            stmt = conn.createStatement();
            String sql;
            sql = "SELECT id, name, age FROM employees";
            ResultSet rs = stmt.executeQuery(sql);

            while(rs.next()){
                int id  = rs.getInt("id");
                String name = rs.getString("name");
                int age = rs.getInt("age");

                System.out.println("ID: " + id);
                System.out.println("Name: " + name);
                System.out.println("Age: " + age);
            }
            rs.close();
            stmt.close();
            conn.close();
        }catch(SQLException se){
            se.printStackTrace();
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            try{
                if(stmt!=null) stmt.close();
            }catch(SQLException se2){
            }
            try{
                if(conn!=null) conn.close();
            }catch(SQLException se){
                se.printStackTrace();
            }
        }
        System.out.println("Goodbye!");
    }
}

这段代码连接了名为“mydatabase”的 MySQL 数据库，并从中读取了“employees”表中的数据。

3.将 MQTT 客户端代码与上面的数据库连接代码结合起来。可以在 MQTT 客户端接收到消息时，将消息写入 MySQL 数据库中。可以使用以下代码示例：

import org.eclipse.paho.client.mqttv3.*;
import java.sql.*;

public class MqttToMySQL implements MqttCallback {
    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    static final String DB_URL = "jdbc:mysql://localhost/mydatabase";

    static final String USER = "username";
    static final String PASS = "password";

    public static void main(String[] args) {
        String topic        = "MQTT Examples";
        int qos             = 2;
        String broker       = "tcp://localhost:1883";
        String clientId     = "JavaSample";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            Class.forName("com.mysql.jdbc.Driver");

            System.out.println("Connecting to database...");
            Connection conn = DriverManager.getConnection(DB_URL,USER,PASS);

            MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker: "+broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
            sampleClient.setCallback(new MqttToMySQL());

            sampleClient.subscribe(topic, qos);

            // Wait for messages
            while(true) {
                Thread.sleep(1000);
            }

            //sampleClient.disconnect();
            //System.out.println("Disconnected");
            //System.exit(0);
        } catch(MqttException me) {
            System.out.println("reason "+me.getReasonCode());
            System.out.println("msg "+me.getMessage());
            System.out.println("loc "+me.getLocalizedMessage());
            System.out.println("cause "+me.getCause());
            System.out.println("excep "+me);
            me.printStackTrace();
        } catch(SQLException se) {
            se.printStackTrace();
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public void connectionLost(Throwable cause) {
        System.out.println("Connection lost: "+cause);
    }

    public void messageArrived(String topic, MqttMessage message) throws Exception {
        String content = new String(message.getPayload());
        System.out.println("Message arrived: "+content);

        Connection conn = DriverManager.getConnection(DB_URL,USER,PASS);

        String sql = "INSERT INTO messages (content) VALUES (?)";
        PreparedStatement pstmt = conn.prepareStatement(sql);
        pstmt.setString(1, content);
        pstmt.executeUpdate();

        pstmt.close();
        conn.close();
    }

    public void deliveryComplete(IMqttDeliveryToken token) {
        System.out.println("Delivery complete");
    }
}

这段代码连接到名为“mydatabase”的 MySQL 数据库，并在接收到 MQTT 消息时，将消息写入名为“messages”的表中。

需要注意的是，在将 MQTT 客户端代码与数据库连接代码结合时，需要考虑线程安全和数据库连接池等问题，以提高应用程序的性能和稳定性。

c++ paho mqtt库 mqtt::topic 发布 json对象

以下是使用C++ paho mqtt库中的mqtt::topic发布json对象的示例代码：

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <json/json.h>
#include "mqtt/async_client.h"

using namespace std;

const string ADDRESS("tcp://localhost:1883");
const string CLIENTID("AsyncPublisher");
const string TOPIC("test");
const int QOS = 1;const int TIMEOUT = 10000L;

class callback : public virtual mqtt::callback
{
public:
    virtual void connection_lost(const std::string& cause)
    {
        cout << "\nConnection lost" << endl;
        if (!cause.empty())
            cout << "\tcause: " << cause << endl;
    }

    virtual void delivery_complete(mqtt::delivery_token_ptr token)
    {
        cout << "\n\tDelivery complete for token: "
             << (token ? token->get_message_id() : -1) << endl;
    }
};

int main(int argc, char* argv[])
{
    mqtt::async_client client(ADDRESS, CLIENTID);

    mqtt::connect_options connOpts;
    connOpts.set_keep_alive_interval(20);
    connOpts.set_clean_session(true);

    callback cb;
    client.set_callback(cb);

    try {
        mqtt::token_ptr conntok = client.connect(connOpts);
        conntok->wait_for_completion();

        Json::Value root;
        root["name"] = "John";
        root["age"] = 25;
        root["email"] = "john@example.com";

        string json_str = root.toStyledString();

        mqtt::message_ptr pubmsg = mqtt::make_message(TOPIC, json_str);
        pubmsg->set_qos(QOS);

        mqtt::delivery_token_ptr pubtok = client.publish(pubmsg);
        pubtok->wait_for_completion(TIMEOUT);

        cout << "Message '" << json_str << "' published to topic '"
             << TOPIC << "'" << endl;

        client.disconnect()->wait_for_completion();
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
        return 1;
    }

    return 0;
}

在这个示例中，我们使用了Jsoncpp库来创建一个Json对象，并将其转换为字符串。然后，我们使用mqtt::make_message()函数创建一个mqtt::message_ptr对象，该对象包含要发布的主题和消息。最后，我们使用mqtt::async_client类的publish()函数将消息发布到主题上。