一、mqtt介绍

MQTT(消息队列遥测传输)是ISO 标准(ISO/IEC PRF 20922)下基于发布/订阅范式的消息协议。它工作在 TCP/IP协议族上，是为硬件性能低下的远程设备以及网络状况糟糕的情况下而设计的发布/订阅型消息协议，为此，它需要一个消息中间件。
MQTT是一个基于客户端-服务器的消息发布/订阅传输协议。MQTT协议是轻量、简单、开放和易于实现的，这些特点使它适用范围非常广泛。在很多情况下，包括受限的环境中，如：机器与机器（M2M）通信和物联网（IoT）。其在通过卫星链路通信传感器、偶尔拨号的医疗设备、智能家居、及一些小型化设备中已广泛使用。

二、paho mqtt介绍

eclipse paho项目以各种编程语言提供了MQTT和MQTT-SN的开源（主要是客户端）实现。
项目地址：https://github.com/eclipse/paho.mqtt.embedded-c

三、连接腾讯云的步骤

3.1 在腾讯云控制台的IOT HUB创建产品和设备（略）

3.2 根据产品信息填充MQTTClient的连接参数

关注文件开头的连接配置信息的宏，参考https://cloud.tencent.com/document/product/634/14630填冲相应的宏

MQTT_URI ：服务器uri，如tcp://E822xxxxxx.iotcloud.tencentdevices.com:1883
MQTT_USERNAME|MQTT_PASSWORD ：mqtt的用户名和密码，在设备列表找到设备，可以获取这两项。

/*
 * Copyright (c) 2006-2019, RT-Thread Development Team
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Change Logs:
 * Date           Author       Notes
 * 2018-06-03     ZeroFree     first implementation
 */

#include <rtthread.h>
#include <rtdevice.h>
#include <board.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include "paho_mqtt.h"
#include "wifi_config.h"

#define DBG_TAG "main"
#define DBG_LVL DBG_LOG
#include <rtdbg.h>

/**
 * MQTT URI farmat:
 * domain mode
 * tcp://iot.eclipse.org:1883
 *
 * ipv4 mode
 * tcp://192.168.10.1:1883
 * ssl://192.168.10.1:1884
 *
 * ipv6 mode
 * tcp://[fe80::20c:29ff:fe9a:a07e]:1883
 * ssl://[fe80::20c:29ff:fe9a:a07e]:1884
 */
#define MQTT_URI "tcp://E822xxxxxx.iotcloud.tencentdevices.com:1883"
#define MQTT_USERNAME "E822Zxxxxxxtest;12010126;ccxxx;168114xxxx"
#define MQTT_PASSWORD "xxxxxxxxxx"
#define MQTT_SUBTOPIC "E822xxxxxx/test/control"
#define MQTT_PUBTOPIC "E822xxxxxx/test/event"

/* define MQTT client context */
static MQTTClient client;
static void mq_start(void);
static void mq_publish(const char *send_str);

char pub_topic[48] = {0};
char sub_topic[48] = {0};


int main(void)
{
    /* 配置 wifi 工作模式 */
    rt_wlan_set_mode(RT_WLAN_DEVICE_STA_NAME, RT_WLAN_STATION);
    /* 注册 MQTT 启动函数为 WiFi 连接成功的回调函数 */
    rt_wlan_register_event_handler(RT_WLAN_EVT_READY, (void (*)(int, struct rt_wlan_buff *, void *))mq_start, RT_NULL);
    /* 初始化自动连接功能 */
    wlan_autoconnect_init();
    /* 使能 wlan 自动连接 */
    rt_wlan_config_autoreconnect(RT_TRUE);
}

static void mqtt_sub_callback(MQTTClient *c, MessageData *msg_data)
{
    *((char *)msg_data->message->payload + msg_data->message->payloadlen) = '\0';
    LOG_D("Topic: %.*s receive a message: %.*s",
          msg_data->topicName->lenstring.len,
          msg_data->topicName->lenstring.data,
          msg_data->message->payloadlen,
          (char *)msg_data->message->payload);

    return;
}

static void mqtt_sub_default_callback(MQTTClient *c, MessageData *msg_data)
{
    *((char *)msg_data->message->payload + msg_data->message->payloadlen) = '\0';
    LOG_D("mqtt sub default callback: %.*s %.*s",
          msg_data->topicName->lenstring.len,
          msg_data->topicName->lenstring.data,
          msg_data->message->payloadlen,
          (char *)msg_data->message->payload);
    return;
}

static void mqtt_connect_callback(MQTTClient *c)
{
    LOG_I("Start to connect mqtt server");
}

static void mqtt_online_callback(MQTTClient *c)
{
    LOG_D("Connect mqtt server success");
    LOG_D("Publish message: Hello,RT-Thread! to topic: %s", pub_topic);
    mq_publish("Hello,RT-Thread!");
}

static void mqtt_offline_callback(MQTTClient *c)
{
    LOG_I("Disconnect from mqtt server");
}

/* 创建与配置 mqtt 客户端 */
static void mq_start(void)
{
    /* 初始 condata 参数 */
    MQTTPacket_connectData condata = MQTTPacket_connectData_initializer;
    static char cid[20] = {0};

    static int is_started = 0;
    if (is_started)
    {
        return;
    }
    /* 配置 MQTT 文本参数 */
    {
        client.isconnected = 0;
        client.uri = MQTT_URI;

        /* 生成随机客户端 ID */
        rt_snprintf(cid, sizeof(cid), "rtthread%d", rt_tick_get());
        rt_snprintf(pub_topic, sizeof(pub_topic), "%s", MQTT_PUBTOPIC);
		rt_snprintf(sub_topic, sizeof(sub_topic), "%s", MQTT_SUBTOPIC);

		/* 配置连接参数 */
        memcpy(&client.condata, &condata, sizeof(condata));
        client.condata.clientID.cstring = cid;
        client.condata.keepAliveInterval = 60;
        client.condata.cleansession = 1;
        client.condata.username.cstring = MQTT_USERNAME;
        client.condata.password.cstring = MQTT_PASSWORD;

        /* 配置 mqtt 参数 */
        client.condata.willFlag = 0;
        client.condata.will.qos = 1;
        client.condata.will.retained = 0;
        client.condata.will.topicName.cstring = pub_topic;

        client.buf_size = client.readbuf_size = 1024;
        client.buf = malloc(client.buf_size);
        client.readbuf = malloc(client.readbuf_size);
        if (!(client.buf && client.readbuf))
        {
            LOG_E("no memory for MQTT client buffer!");
            goto _exit;
        }

        /* 设置事件回调 */
        client.connect_callback = mqtt_connect_callback;
        client.online_callback = mqtt_online_callback;
        client.offline_callback = mqtt_offline_callback;
        /* 设置要订阅的 topic 和 topic 对应的回调函数 */
        client.messageHandlers[0].topicFilter = sub_topic;
        client.messageHandlers[0].callback = mqtt_sub_callback;
        client.messageHandlers[0].qos = QOS1;

        /* 设置默认订阅回调函数 */
        client.defaultMessageHandler = mqtt_sub_default_callback;
    }

    /* 启动 MQTT 客户端 */
    LOG_D("Start mqtt client and subscribe topic:%s", pub_topic);
    paho_mqtt_start(&client);
    is_started = 1;

_exit:
    return;
}

/* MQTT 消息发布函数 */
static void mq_publish(const char *send_str)
{
    MQTTMessage message;
    const char *msg_str = send_str;
    const char *topic = pub_topic;
    message.qos = QOS1;
    message.retained = 0;
    message.payload = (void *)msg_str;
    message.payloadlen = strlen(message.payload);

    MQTTPublish(&client, topic, &message);

    return;
}