qt实现mqtt客户端

MQTT（Message Queue Telemetry Transport）是一种轻量级的消息传输协议，适用于物联网设备与服务器之间的数据传输，具有低带宽、低功耗、高可靠性等特点。Qt是一种跨平台的C++图形界面应用程序开发框架，具有简单易学、代码简洁、易于维护等优点，可以用来实现MQTT客户端。下面将介绍如何在Qt中实现MQTT客户端。

1.安装MQTT库

Qt没有自带MQTT库，需要手动安装第三方库。可以选择paho.mqtt.c或eclipse.mosquitto等开源MQTT库，这里以paho.mqtt.c为例。下载并解压paho.mqtt.c库，将其包含到Qt项目中，同时在.pro文件中添加如下语句：

INCLUDEPATH += $$PWD/paho.mqtt.c/include

LIBS += -L$$PWD/paho.mqtt.c/build/output -lpaho-mqtt3as-static

2.实现MQTT客户端

在Qt中实现MQTT客户端，需要用到MQTT库中的头文件和API函数。下面以订阅主题为例，展示如何在Qt中实现MQTT客户端：

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

#define ADDRESS "tcp://localhost:1883"
#define CLIENTID "ExampleClientSub"
#define TOPIC "sample/topic"
#define QOS 1
#define TIMEOUT 10000L

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);
exit(EXIT_FAILURE);
}

MQTTClient_subscribe(client, TOPIC, QOS);

printf("Subscribed to topic %s\n", TOPIC);

getchar();

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

3.编译运行MQTT客户端

将以上代码保存为main.c文件，运行以下命令编译生成可执行文件：

gcc -o main main.c -lpaho-mqtt3as

运行可执行文件即可实现MQTT客户端的订阅功能。

以上就是利用Qt实现MQTT客户端的步骤。通过第三方MQTT库和Qt的API函数，可以方便地实现MQTT客户端的开发，适用于物联网等领域的数据传输和处理。