Linux C语言实现mqtt客户端带用户名密码的连接

要实现带用户名和密码的MQTT客户端连接，可以使用MQTT C语言库进行开发。以下是一个使用MQTT C语言库的示例代码：

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define USERNAME    "your_username"
#define PASSWORD    "your_password"
#define TOPIC       "MQTT Examples"
#define QOS         1
#define TIMEOUT     10000L

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

    MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);

    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;
    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(EXIT_FAILURE);
    }

    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to subscribe, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    printf("Subscribed to topic: %s\n", TOPIC);
    printf("Waiting for messages...\n");

    while (1)
    {
        MQTTClient_receive(client, &msg, TIMEOUT);

        if (msg.payloadlen > 0)
        {
            printf("Received message: %s\n", (char*)msg.payload);
        }

        MQTTClient_freeMessage(&msg);
        MQTTClient_deliveryToken(&token);
    }

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

    return rc;
}

在上述代码中，我们使用了MQTT C语言库提供的函数来创建MQTT客户端、连接到MQTT服务器、订阅主题、接收消息等。在连接MQTT服务器时，我们设置了连接选项，包括用户名、密码、保持连接时间等。这些选项可以通过MQTTClient_connectOptions结构体来设置。

请注意，这只是一个简单的示例代码，实际应用中需要根据实际情况进行修改和完善。