mqtt C客户端

Paho MQTT C客户端是一个使用标准C编写的流畅的MQTT。 实际上包含了两个C API，同步的和异步的，分别为MQTTClient和MQTTAsync。同步的API可以更简单更使用，执行完操作后，一些调用会被阻拦，编程也更容易；而在异步API没有调用的阻拦操作，所有API的调用都是通过会调来实现的，这使得API更适合有窗口环境的操作系统，许多应用没有主线程控制。

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linux环境：
在linux环境下建立MQTT C 客户端：

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

安装MQTT

sudo make install

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在windows环境下

在Windows下使用Visual Studio 或者Visual C++创建

git clone https://github.com/eclipse/paho.mqtt.c.git
cd org.eclipse.paho.mqtt.c.git
msbuild "Windows Build\Paho C MQTT APIs.sln" /p:Configuration=Release

为了设置msbuild的路径，可以运行 vcvars32.bat，它的创建如下：

C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin

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以下是一个简单的同步类型的MQTT C客户端：

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "MQTT Examples"
#define PAYLOAD     "Hello World!"
#define QOS         1
#define TIMEOUT     10000L

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

    MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);
    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(-1);
    }
    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;
}

来源：https://www.eclipse.org/paho/clients/c/