MQTT客户端应用编程及接口分析

MQTT客户端应用编程及接口分析

MQTT协议简介

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。

客户端服务端安装

1.安装

sudo apt-add-repository ppa:mosquitto-dev/mosquitto-ppasudo 
sudo apt-get install mosquitto mosquitto-clients

1. 测试

mosquitto_sub -h test.mosquitto.org -t "#" -v

如果发现大量的消息滚动刷屏，说明该客户端链接成功

客户端开发包

SDK包下载地址：
eclipse/paho.mqtt.c

编译及生产lib和头文件
cmake -DMAKE_INSTALL_PREFIX=./output
make && make install

API接口介绍

接口类型	头文件	备注
Paho MQTT C Client Library	MQTTClient.h	非现程安全
Paho Asynchronous MQTT C Client Library	MQTTAsync.h	线程安全

数据结构

typedef void* MQTTClient;
typedef void* MQTTAsync;

MQTTClient 库接口

1. MQTTClient_create() : MQTTClient.h

int MQTTClient_create(MQTTClient *handle,
                      const char *serverURI,
                      const char *clientId,
                      int         persistence_type,
                      void *      persistence_context 
)

此函数创建一个 MQTT 客户端，以便连接到指定的服务器并使用指定的持久性存储（参考 MQTTClient_persistence 函数）。另外，与创建函数对应的是销毁函数 MQTTClient_destroy()。

1. MQTTClient_connect() : MQTTClient.h

int MQTTClient_connect(MQTTClient handle,
                       MQTTClient_connectOptions *options
)

此函数尝试使用指定的选项将以前创建的客户端（调用 MQTTClient_create() 函数）连接到 MQTT 服务器。如果要启用异步消息和状态通知，调用该函数之前必须先调用 MQTTClient_setCallbacks() 函数。

1. MQTTClient_destroy() : MQTTClient.h

void MQTTClient_destroy(MQTTClient *handle)

此函数用于释放分配给 MQTT 客户端的内存（调用 MQTTClient_create() 函数）。当不再需要客户端时，应该调用它。

4.MQTTClient_disconnect() : MQTTClient.h

int MQTTClient_disconnect(MQTTClient handle, int timeout)

此函数尝试断开客户端与 MQTT 服务器的连接。为了让客户端有时间完成对调用此函数时正在传递的消息的处理，需要指定一个超时期限。当超时期限过期时，即使仍有未完成的消息确认，客户端也会断开连接。下一次客户端连接到同一服务器时，任何未完成的 QoS 1 或 2 消息将根据前一个连接和新连接的清理设置重试（参考 MQTTClient_connectOptions.cleansession 和 MQTTClient_connect()）。

1. MQTTClient_free() : MQTTClient.h

void MQTTClient_free(void *ptr)

此函数释放 MQTT C Client 库分配的内存，特别是主题名称。当 client 库和应用程序使用不同版本的 C 编译器编译时，在 Windows 上需要这样做。因此，在释放任何 MQTT C Client 分配的内存时，始终使用此函数是一个很好的策略。

1. MQTTClient_freeMessage() : MQTTClient.h

void MQTTClient_freeMessage(MQTTClient_message ** msg)

此函数释放分配给 MQTT 消息的内存，包括分配给消息有效负载的附加内存。当完全处理消息时，客户端应用程序调用此函数。重要提示：此函数不释放分配给消息主题字符串的内存。客户端应用程序负责使用 MQTTClient_free() 库函数释放此内存。

1. MQTTClient_isConnected() : MQTTClient.h

int MQTTClient_isConnected(MQTTClient handle)

此函数允许客户端应用程序测试某个 client 与 MQTT 服务器连接情况，已连接返回 true，否则返回 false。

1. MQTTClient_publish() : MQTTClient.h

int MQTTClient_publish(MQTTClient                handle,
                       const char *              topicName,
                       int                       payloadlen,
                       const void *              payload,
                       int                       qos,
                       int                       retained,
                       MQTTClient_deliveryToken *dt
)

此函数试图将消息发布到给定主题（另请参见 MQTTClient_publishMessage 函数)。当此函数成功返回时，将发出 MQTTClient_deliveryToken。如果客户端应用程序需要测试 QoS1 和 QoS2 消息的成功传递，可以异步或同步地完成（参考 异步与同步客户端应用程序、 MQTTClient_waitForCompletion 和 MQTTClient_deliveryComplete 函数）。

1. MQTTClient_publishMessage() : MQTTClient.h

int MQTTClient_publishMessage(MQTTClient                handle,
                              const char *              topicName,
                              MQTTClient_message *      msg,
                              MQTTClient_deliveryToken *dt
)

此函数试图将消息发布到给定主题（另请参见 MQTTClient_publish 函数）。当此函数成功返回时，将发出一个 MQTTClient_deliveryToken 令牌。如果客户端应用程序需要测试 QoS1 和 QoS2 消息的成功传递，可以异步或同步地完成（参考 异步与同步客户端应用程序、 MQTTClient_waitForCompletion 和 MQTTClient_deliveryComplete 函数）。

1. MQTTClient_receive() : MQTTClient.h

int MQTTClient_receive(MQTTClient            handle,
                       char **               topicName,
                       int *                 topicLen,
                       MQTTClient_message ** message,
                       unsigned long         timeout
)

此函数执行传入消息的同步接收。只有当客户端应用程序没有设置回调方法以支持消息的异步接收时，才应该使用它（请参见 异步与同步客户端应用程序 以及 MQTTClient_setCallbacks 函数）。使用此函数可以编写单线程客户端订阅者应用程序。当调用时，此函数将阻塞，直到下一条消息到达或指定的超时过期为止（另请参见 MQTTClient_yield 函数）。

重要提示：应用程序在处理完成后必须释放分配给主题和消息的内存（请参见 MQTTClient_freeMessage 函数）。

1. MQTTClient_setCallbacks() : MQTTClient.h

int MQTTClient_setCallbacks(MQTTClient                    handle,
                            void *                        context,
                            MQTTClient_connectionLost *   cl,
                            MQTTClient_messageArrived *   ma,
                            MQTTClient_deliveryComplete * dc
)

此函数设置特定客户端的回调函数。如果客户端应用程序不使用特定的回调，请将相关参数设置为 NULL。调用 MQTTClient_setCallbacks() 将客户端置于多线程模式。任何必要的消息确认和状态通信都在后台处理，不需要客户端应用程序的任何干预。更多信息请参见 异步与同步客户端应用程序。

注意：在调用此函数时，MQTT 客户端必须断开连接。

1. MQTTClient_setDisconnected() : MQTTClient.h

int MQTTClient_setDisconnected(MQTTClient handle,
                               void *     context,
                               MQTTClient_disconnected *co 
)

设置客户端的 MQTTClient_disconnected() 回调函数。如果从服务器接收到断开连接的数据包，将调用这个函数。该函数只对 MQTT V5 及以上版本有效。

1. MQTTClient_setPublished() : MQTTClient.h

int MQTTClient_setPublished(MQTTClient             handle,
                            void *                 context,
                            MQTTClient_published * co
)

MQTTAsync 库接口

• MQTTAsync_connect() : MQTTAsync.h
• MQTTAsync_create() : MQTTAsync.h
• MQTTAsync_createWithOptions() : MQTTAsync.h
• MQTTAsync_destroy() : MQTTAsync.h
• MQTTAsync_disconnect() : MQTTAsync.h
• MQTTAsync_free() : MQTTAsync.h
• MQTTAsync_freeMessage() : MQTTAsync.h
• MQTTAsync_getPendingTokens() : MQTTAsync.h
• MQTTAsync_getVersionInfo() : MQTTAsync.h
• MQTTAsync_global_init() : MQTTAsync.h
• MQTTAsync_isComplete() : MQTTAsync.h
• MQTTAsync_isConnected() : MQTTAsync.h
• MQTTAsync_malloc() : MQTTAsync.h
• MQTTAsync_reconnect() : MQTTAsync.h
• MQTTAsync_send() : MQTTAsync.h
• MQTTAsync_sendMessage() : MQTTAsync.h
• MQTTAsync_setAfterPersistenceRead() : MQTTAsync.h
• MQTTAsync_setBeforePersistenceWrite() : MQTTAsync.h
• MQTTAsync_setCallbacks() : MQTTAsync.h
• MQTTAsync_setConnected() : MQTTAsync.h
• MQTTAsync_setConnectionLostCallback() : MQTTAsync.h
• MQTTAsync_setDeliveryCompleteCallback() : MQTTAsync.h
• MQTTAsync_setDisconnected() : MQTTAsync.h
• MQTTAsync_setMessageArrivedCallback() : MQTTAsync.h
• MQTTAsync_setTraceCallback() : MQTTAsync.h
• MQTTAsync_setTraceLevel() : MQTTAsync.h
• MQTTAsync_setUpdateConnectOptions() : MQTTAsync.h
• MQTTAsync_strerror() : MQTTAsync.h
• MQTTAsync_subscribe() : MQTTAsync.h
• MQTTAsync_subscribeMany() : MQTTAsync.h
• MQTTAsync_unsubscribe() : MQTTAsync.h
• MQTTAsync_unsubscribeMany() : MQTTAsync.h
• MQTTAsync_waitForCompletion() : MQTTAsync.h
• MQTTProperties_add() : MQTTProperties.h
• MQTTProperties_copy() : MQTTProperties.h
• MQTTProperties_free() : MQTTProperties.h
• MQTTProperties_getNumericValue() : MQTTProperties.h
• MQTTProperties_getNumericValueAt() : MQTTProperties.h
• MQTTProperties_getProperty() : MQTTProperties.h
• MQTTProperties_getPropertyAt() : MQTTProperties.h
• MQTTProperties_hasProperty() : MQTTProperties.h
• MQTTProperties_len() : MQTTProperties.h
• MQTTProperties_propertyCount() : MQTTProperties.h
• MQTTProperties_read() : MQTTProperties.h
• MQTTProperties_write() : MQTTProperties.h
• MQTTProperty_getType() : MQTTProperties.h
• MQTTPropertyName() : MQTTProperties.h
• MQTTReasonCode_toString() : MQTTReasonCodes.h

###代码使用案例

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

#if !defined(_WIN32)
#include <unistd.h>
#else
#include <windows.h>
#endif

#if defined(_WRS_KERNEL)
#include <OsWrapper.h>
#endif

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

int finished = 0;

void connlost(void *context, char *cause)
{
	MQTTAsync client = (MQTTAsync)context;
	MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
	int rc;

	printf("\nConnection lost\n");
	printf("     cause: %s\n", cause);

	printf("Reconnecting\n");
	conn_opts.keepAliveInterval = 20;
	conn_opts.cleansession = 1;
	if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to start connect, return code %d\n", rc);
 		finished = 1;
	}
}

void onDisconnectFailure(void* context, MQTTAsync_failureData* response)
{
	printf("Disconnect failed\n");
	finished = 1;
}

void onDisconnect(void* context, MQTTAsync_successData* response)
{
	printf("Successful disconnection\n");
	finished = 1;
}

void onSendFailure(void* context, MQTTAsync_failureData* response)
{
	MQTTAsync client = (MQTTAsync)context;
	MQTTAsync_disconnectOptions opts = MQTTAsync_disconnectOptions_initializer;
	int rc;

	printf("Message send failed token %d error code %d\n", response->token, response->code);
	opts.onSuccess = onDisconnect;
	opts.onFailure = onDisconnectFailure;
	opts.context = client;
	if ((rc = MQTTAsync_disconnect(client, &opts)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to start disconnect, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}
}

void onSend(void* context, MQTTAsync_successData* response)
{
	MQTTAsync client = (MQTTAsync)context;
	MQTTAsync_disconnectOptions opts = MQTTAsync_disconnectOptions_initializer;
	int rc;

	printf("Message with token value %d delivery confirmed\n", response->token);
	opts.onSuccess = onDisconnect;
	opts.onFailure = onDisconnectFailure;
	opts.context = client;
	if ((rc = MQTTAsync_disconnect(client, &opts)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to start disconnect, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}
}


void onConnectFailure(void* context, MQTTAsync_failureData* response)
{
	printf("Connect failed, rc %d\n", response ? response->code : 0);
	finished = 1;
}


void onConnect(void* context, MQTTAsync_successData* response)
{
	MQTTAsync client = (MQTTAsync)context;
	MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
	MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
	int rc;

	printf("Successful connection\n");
	opts.onSuccess = onSend;
	opts.onFailure = onSendFailure;
	opts.context = client;
	pubmsg.payload = PAYLOAD;
	pubmsg.payloadlen = (int)strlen(PAYLOAD);
	pubmsg.qos = QOS;
	pubmsg.retained = 0;
	if ((rc = MQTTAsync_sendMessage(client, TOPIC, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to start sendMessage, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}
}

int messageArrived(void* context, char* topicName, int topicLen, MQTTAsync_message* m)
{
	/* not expecting any messages */
	return 1;
}

int main(int argc, char* argv[])
{
	MQTTAsync client;
	MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
	int rc;

	if ((rc = MQTTAsync_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to create client object, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}

	if ((rc = MQTTAsync_setCallbacks(client, NULL, connlost, messageArrived, NULL)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to set callback, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}

	conn_opts.keepAliveInterval = 20;
	conn_opts.cleansession = 1;
	conn_opts.onSuccess = onConnect;
	conn_opts.onFailure = onConnectFailure;
	conn_opts.context = client;
	if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
	{
		printf("Failed to start connect, return code %d\n", rc);
		exit(EXIT_FAILURE);
	}

	printf("Waiting for publication of %s\n"
         "on topic %s for client with ClientID: %s\n",
         PAYLOAD, TOPIC, CLIENTID);
	while (!finished)
		#if defined(_WIN32)
			Sleep(100);
		#else
			usleep(10000L);
		#endif

	MQTTAsync_destroy(&client);
 	return rc;
}