Paho - MQTT C Cient的实现

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  在前几天，我大体了解了一下Paho C项目，并对其的一些内容进行了翻译。俗话说，光说不练假把戏，今天就给你们讲一下使用Paho的客户端库文件实现MQTT C Client的过程。

安装

  本文是在Linux下安装的，推荐直接进行克隆并安装便可。

git clone https://github.com/eclipse/paho.mqtt.c.git
cd paho.mqtt.c
make
sudo make install

  在make完以后，在paho.mqtt.c/build/output下能够找到以下的输出文件：

  而make install则是将生成的库文件移动到系统路径之下。在MQTT Client library for C这个翻译的文章中，Paho给出的建立一个客户端有以下相似的步骤：

  1.建立一个客户端对象；
  2.设置链接MQTT服务器的选项；
  3.若是多线程（异步模式）操做被使用则设置回调函数（详见 Asynchronous >vs synchronous client applications）；
  4.订阅客户端须要接收的任意话题；
  5.重复如下操做直到结束：
    a.发布客户端须要的任意信息；
    b.处理全部接收到的信息；
  6.断开客户端链接；
  7.释放客户端使用的全部内存。

  为了简单起见，咱们使用Paho自带的示例程序。打开paho.mqtt.c/src/samples下的MQTTClient_publish .c文件。将如下的代码更改：

#define ADDRESS "tcp://m2m.eclipse.org:1883"
#define CLIENTID "ExampleClientPub"
#define TOPIC "MQTT Examples"
#define PAYLOAD "Hello World!"

  若是你的MQTT服务器不容许匿名访问，则还须要添加姓名和密码：

char *username= "test_user"; //添加的用户名
  char *password = "aaa777"; //添加的密码

  并将用户名和密码写入链接选项中：

conn_opts.username = username; //将用户名写入链接选项中
    conn_opts.password = password; //将密码写入链接选项中

  添加的代码具体位置详细查看代码，更改后的代码以下所示：

/*******************************************************************************
 * Copyright (c) 2012, 2017 IBM Corp.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * and Eclipse Distribution License v1.0 which accompany this distribution. 
 *
 * The Eclipse Public License is available at 
 *   http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at 
 *   http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *    Ian Craggs - initial contribution
 *******************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "MQTTClient.h"
#if !defined(WIN32)
#include <unistd.h>
#else
#include <windows.h>
#endif

#define ADDRESS     "tcp://localhost:1883" //更改此处地址
#define CLIENTID    "aaabbbccc" //更改此处客户端ID
#define TOPIC       "topic01"  //更改发送的话题
#define PAYLOAD     "Hello Man, Can you see me ?!" //更改信息内容
#define QOS         1
#define TIMEOUT     10000L

int main(int argc, char* argv[])
{
    //声明一个MQTTClient
    MQTTClient client;
    char *username= "test_user"; //添加的用户名
    char *password = "aaa777"; //添加的密码
    //初始化MQTT Client选项
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    //#define MQTTClient_message_initializer { {'M', 'Q', 'T', 'M'}, 0, 0, NULL, 0, 0, 0, 0 }
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    //声明消息token
    MQTTClient_deliveryToken token;
    int rc;
    //使用参数建立一个client，并将其赋值给以前声明的client
    MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.username = username; //将用户名写入链接选项中
    conn_opts.password = password;//将密码写入链接选项中
     //使用MQTTClient_connect将client链接到服务器，使用指定的链接选项。成功则返回MQTTCLIENT_SUCCESS
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
    pubmsg.payload = PAYLOAD;
    pubmsg.payloadlen = strlen(PAYLOAD);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Waiting for up to %d seconds for publication of %s\n"
            "on topic %s for client with ClientID: %s\n",
            (int)(TIMEOUT/1000), PAYLOAD, TOPIC, CLIENTID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

  更改完成以后回到paho.mqtt.c目录，执行make,输出一下结果:

  打开paho.mqtt.c/build/output/samples目录，刚刚咱们修改的MQTTClient_publish .c文件生成了MQTTClient_publish ，执行如下命令：

./MQTTClient_publish

  输出如下结果：

  为了确认发送的信息已经到达客户端，我打开了mqtt.fx客户端，并链接到MQTT服务器，订阅了topic01的话题，此时能够看到发送过来的信息：

  至此，Paho - MQTT C 发送Cient已经实现了，后续我会详细的讲解一下这个过程。

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