一、Eclipse Paho介绍

Eclipse Paho 是一个开源项目，由 Eclipse Foundation 主持，提供可靠的开源实现来处理 MQTT（Message Queuing Telemetry Transport）协议以及其他与物联网 (IoT) 相关的协议。MQTT 是一种轻量级的发布/订阅消息传输协议，专为具有低带宽和不可靠网络连接的设备设计。Paho 提供了多种语言的客户端库，使得开发者可以在各种平台上开发基于 MQTT 协议的应用程序。

1.1 主要特点

1. 跨平台支持：Paho 支持多种操作系统和硬件平台，包括Windows、Linux、macOS 以及嵌入式系统。

2. 多语言实现：Paho 客户端库提供了多种编程语言的选择，如 C、C++、Java 和 Python 等。

3. 可靠性：Paho 能够在各种网络条件下可靠地工作，包括高延迟、丢包和间歇性连接等。

4. 安全性：Paho 支持 TLS/SSL 加密通信，以保证数据的安全传输。

5. 灵活性：除了基本的 MQTT 协议实现之外，Paho 还允许扩展和定制以适应特定的需求。

1.2 Eclipse Paho MQTT C客户端库特点

Eclipse Paho MQTT支持多种语言，其中的C客户端库是一个用于实现MQTT协议客户端的开源C语言库。

1. 跨平台支持：该库设计为可移植的，支持多种操作系统和硬件平台，包括Linux、Windows、MacOS以及嵌入式系统。

2. 易于集成：库的设计使得它易于集成到现有的C或C++项目中，为开发者提供了简单而强大的API来构建MQTT客户端。

3. 灵活的连接选项：支持TLS/SSL加密的MQTT连接，提供安全的通信通道。同时，支持QoS（服务质量）级别0（最多一次）、1（至少一次）和2（仅一次）的消息传递，确保消息传递的可靠性。

4. 异步操作：大多数库操作都是异步的，允许应用程序在等待网络响应时继续执行其他任务，提高了应用程序的响应性和效率。

5. 客户端和服务器消息处理：库支持客户端到服务器的消息发布（PUBLISH）以及从服务器到客户端的消息订阅（SUBSCRIBE）和接收（RECEIVE）。

6. 持久会话和遗嘱消息：支持持久会话，即使客户端断开连接也能保持订阅和QoS状态。同时，可以设置遗嘱消息，在客户端异常断开时发送特定消息。

7. 回调函数：通过提供回调函数来处理连接、断开连接、消息接收等事件，使得事件处理逻辑更加灵活。

二、Eclipse Paho源码下载

官网地址：https://projects.eclipse.org/projects/iot.paho/downloads

库的下载地址：https://github.com/eclipse/paho.mqtt.c/releases

在页面上可以看到源码下载和编译好的库文件下载。

提供了Linux下、Windows下编译好的库文件，可以直接使用。 如果你现在嵌入式平台上、其他平台上使用，那需要自己下载源码进行编译，使用。

三、编译Eclipse Paho库文件

3.1 下载编译openSSL

如果要支持ssl加密的支持，那就需要先编译安装 openSSL。

下载地址：https://codeload.github.com/openssl/openssl/tar.gz/refs/tags/OpenSSL_1_1_1g

编译步骤：

1、解压缩

tar zxf openssl-OpenSSL_1_1_1g.tar.gz

2、进入目录，并配置输出目录和编译器

cd openssl-OpenSSL_1_1_1g/
./config no-asm shared no-async --prefix=`pwd`/ssl_result 

如果是交叉编译器，可以指定--cross-compile-prefix=arm-linux-。

3、执行下面命令，删除Makefile文件的 -m64

(如果指定了交叉编译器)

sed -i 's/-m64//' Makefile

执行后，可以避免出现这个编译错误：arm-linux-: error: unrecognized command line option '-m64'

4、编译、安装

make && make install

成功编译后，在openssl-OpenSSL_1_1_1g/目录会生成一个ssl_result目录，可以看到里面生成的库.

编译过程中。

编译完成。

3.2 paho.mqtt.c 编译

当前下载的是 paho.mqtt.c-1.3.13.tar.gz

下载地址：https://github.com/eclipse/paho.mqtt.c/archive/refs/tags/v1.3.13.tar.gz

wget https://github.com/eclipse/paho.mqtt.c/archive/refs/tags/v1.3.13.tar.gz

编译步骤：

1、解压缩，创建要安装目录paho.mqtt.c_result

tar zxf paho.mqtt.c-1.3.13.tar.gz
​
mkdir paho.mqtt.c_result/bin -p
mkdir paho.mqtt.c_result/include -p
mkdir paho.mqtt.c_result/lib -p
mkdir paho.mqtt.c_result/share/man/man1 -p

2、进入目录，交叉编译

cd paho.mqtt.c-1.3.13/
make  CC=gcc CFLAGS:="-I `pwd`/../ssl_result/include" LDFLAGS:="-L `pwd`/../ssl_result/lib"

编译过程中：

编译完成：

如果是交叉编译需要指定交叉编译器完整名字：

cd paho.mqtt.c-1.3.13/
make  CC=arm-linux-gcc CFLAGS:="-I `pwd`/../ssl_result/include" LDFLAGS:="-L `pwd`/../ssl_result/lib"

参数介绍：

CFLAGS:=“-I `pwd`/…/ssl_result/include”：指定前面编译的 openssl 的头文件；
LDFLAGS:=“-L `pwd`/…/ssl_result/lib”：指定前面编译的 openssl 的库文件路径；

3、make install，安装编译结果

make install prefix=`pwd`/../paho.mqtt.c_result 
prefix=`pwd`/…/paho.mqtt.c_result ：指定安装目录路径；

编译完成后，会生成目录。

3.3 paho.mqtt.cpp 编译

当前下载的是paho.mqtt.cpp-1.3.2.tar.gz

下载地址：https://github.com/eclipse/paho.mqtt.cpp/archive/refs/tags/v1.3.2.tar.gz

如果你下载的版本跟我的一样，可以使用下面的脚本进行编译。

编译之前，先创建一个脚本文件，名字为paho.mqtt.cpp_install，将下面代码粘贴进去保存。

#! /bin/sh
RESULT_DIR=$(pwd)/result_dir
RESULT_SSL=${RESULT_DIR}/ssl_result
RESULT_MQTT_C=${RESULT_DIR}/paho.mqtt.c_result
RESULT_MQTT_CPP=${RESULT_DIR}/paho.mqtt.cpp_result
CROSSS_COMPILE_TOOL=arm-linux-   #如果你是需要交叉编译

编译步骤：

1、解压缩

tar zxf paho.mqtt.cpp-1.3.2.tar.gz 

2、进入目录

cd paho.mqtt.cpp-1.3.2/

3、执行 cmake

mkdir build_arm 
cd build_arm

编译不需要 openssl 的库

cmake ..  -DCMAKE_CXX_COMPILER=${CROSSS_COMPILE_TOOL}g++ \
-DCMAKE_INSTALL_PREFIX=${RESULT_MQTT_CPP} \
-DPAHO_MQTT_C_LIBRARIES=${RESULT_MQTT_C}/lib/libpaho-mqtt3a.so \
-DPAHO_MQTT_C_INCLUDE_DIRS=${RESULT_MQTT_C}/include \
-DPAHO_WITH_SSL=OFF \
-DCMAKE_CXX_FLAGS="-std=gnu++11 -mcpu=cortex-a53"

说明：如果你在PC机编译目标是PC机本身使用，-mcpu=cortex-a53 就不用写。 如果交叉编译的目标是嵌入式芯片，就如实写构架。

编译带有 openssl 的库

cmake ..  -DCMAKE_CXX_COMPILER=${CROSSS_COMPILE_TOOL}g++ \
-DCMAKE_INSTALL_PREFIX=${RESULT_MQTT_CPP} \
-DPAHO_MQTT_C_LIBRARIES=${RESULT_MQTT_C}/lib/libpaho-mqtt3a.so \
-DPAHO_MQTT_C_INCLUDE_DIRS=${RESULT_MQTT_C}/include \
-DOPENSSL_SSL_LIBRARY=${RESULT_SSL}/lib/libssl.so  \
-DOPENSSL_INCLUDE_DIR=${RESULT_SSL}/include  \
-DOPENSSL_CRYPTO_LIBRARY=${RESULT_SSL}/lib/libcrypto.so \
-DCMAKE_CXX_FLAGS="-std=gnu++11 -mcpu=cortex-a53"

说明：如果你在PC机编译目标是PC机本身使用，-mcpu=cortex-a53 就不用写。 如果交叉编译的目标是嵌入式芯片，就如实写构架。

4、编译

make && make install

5、将下载的源码包 paho.mqtt.cpp-1.3.2.tar.gz 和 上面保存的脚本paho.mqtt.cpp_install 放到同一目录，并且将前面编译好的openssl库、paho.mqtt.c库放在脚本指定的结果目录，当前是放到 result_dir 目录的。

6、执行./paho.mqtt.cpp_install.sh 编译，编译完成后，在result_dir目录下会生成一个名为paho.mqtt.cpp_result的目录。

四、代码案例

4.1 订阅—async_subscribe.cpp

这是使用了 libpaho-mqttpp3.so 进行订阅消息的源码，源码路径在源码的这个路径：paho.mqtt.cpp-1.3.2/src/samples/async_subscribe.cpp，只更改了服务器地址。

完整代码如下：

#include <iostream>
#include <cstdlib>
#include <string>
#include <cstring>
#include <cctype>
#include <thread>
#include <chrono>
#include "mqtt/async_client.h"

const std::string SERVER_ADDRESS("117.78.5.125:1883");
const std::string CLIENT_ID("paho_cpp_async_subcribe");
const std::string TOPIC("hello");

const int	QOS = 1;
const int	N_RETRY_ATTEMPTS = 5;

/

// Callbacks for the success or failures of requested actions.
// This could be used to initiate further action, but here we just log the
// results to the console.

class action_listener : public virtual mqtt::iaction_listener
{
	std::string name_;

	void on_failure(const mqtt::token& tok) override {
		std::cout << name_ << " failure";
		if (tok.get_message_id() != 0)
			std::cout << " for token: [" << tok.get_message_id() << "]" << std::endl;
		std::cout << std::endl;
	}

	void on_success(const mqtt::token& tok) override {
		std::cout << name_ << " success";
		if (tok.get_message_id() != 0)
			std::cout << " for token: [" << tok.get_message_id() << "]" << std::endl;
		auto top = tok.get_topics();
		if (top && !top->empty())
			std::cout << "\ttoken topic: '" << (*top)[0] << "', ..." << std::endl;
		std::cout << std::endl;
	}

public:
	action_listener(const std::string& name) : name_(name) {}
};

/

/**
 * Local callback & listener class for use with the client connection.
 * This is primarily intended to receive messages, but it will also monitor
 * the connection to the broker. If the connection is lost, it will attempt
 * to restore the connection and re-subscribe to the topic.
 */
class callback : public virtual mqtt::callback,
					public virtual mqtt::iaction_listener

{
	// Counter for the number of connection retries
	int nretry_;
	// The MQTT client
	mqtt::async_client& cli_;
	// Options to use if we need to reconnect
	mqtt::connect_options& connOpts_;
	// An action listener to display the result of actions.
	action_listener subListener_;

	// This deomonstrates manually reconnecting to the broker by calling
	// connect() again. This is a possibility for an application that keeps
	// a copy of it's original connect_options, or if the app wants to
	// reconnect with different options.
	// Another way this can be done manually, if using the same options, is
	// to just call the async_client::reconnect() method.
	void reconnect() {
		std::this_thread::sleep_for(std::chrono::milliseconds(2500));
		try {
			cli_.connect(connOpts_, nullptr, *this);
		}
		catch (const mqtt::exception& exc) {
			std::cerr << "Error: " << exc.what() << std::endl;
			exit(1);
		}
	}

	// Re-connection failure
	void on_failure(const mqtt::token& tok) override {
		std::cout << "Connection attempt failed" << std::endl;
		if (++nretry_ > N_RETRY_ATTEMPTS)
			exit(1);
		reconnect();
	}

	// (Re)connection success
	// Either this or connected() can be used for callbacks.
	void on_success(const mqtt::token& tok) override {}

	// (Re)connection success
	void connected(const std::string& cause) override {
		std::cout << "\nConnection success" << std::endl;
		std::cout << "\nSubscribing to topic '" << TOPIC << "'\n"
			<< "\tfor client " << CLIENT_ID
			<< " using QoS" << QOS << "\n"
			<< "\nPress Q<Enter> to quit\n" << std::endl;

		cli_.subscribe(TOPIC, QOS, nullptr, subListener_);
	}

	// Callback for when the connection is lost.
	// This will initiate the attempt to manually reconnect.
	void connection_lost(const std::string& cause) override {
		std::cout << "\nConnection lost" << std::endl;
		if (!cause.empty())
			std::cout << "\tcause: " << cause << std::endl;

		std::cout << "Reconnecting..." << std::endl;
		nretry_ = 0;
		reconnect();
	}

	// Callback for when a message arrives.
	void message_arrived(mqtt::const_message_ptr msg) override {
		std::cout << "Message arrived" << std::endl;
		std::cout << "\ttopic: '" << msg->get_topic() << "'" << std::endl;
		std::cout << "\tpayload: '" << msg->to_string() << "'\n" << std::endl;
	}

	void delivery_complete(mqtt::delivery_token_ptr token) override {}

public:
	callback(mqtt::async_client& cli, mqtt::connect_options& connOpts)
				: nretry_(0), cli_(cli), connOpts_(connOpts), subListener_("Subscription") {}
};

/

int main(int argc, char* argv[])
{
	// A subscriber often wants the server to remember its messages when its
	// disconnected. In that case, it needs a unique ClientID and a
	// non-clean session.

	mqtt::async_client cli(SERVER_ADDRESS, CLIENT_ID);

	mqtt::connect_options connOpts;
	connOpts.set_clean_session(false);

	// Install the callback(s) before connecting.
	callback cb(cli, connOpts);
	cli.set_callback(cb);

	// Start the connection.
	// When completed, the callback will subscribe to topic.

	try {
		std::cout << "Connecting to the MQTT server..." << std::flush;
		cli.connect(connOpts, nullptr, cb);
	}
	catch (const mqtt::exception& exc) {
		std::cerr << "\nERROR: Unable to connect to MQTT server: '"
			<< SERVER_ADDRESS << "'" << exc << std::endl;
		return 1;
	}

	// Just block till user tells us to quit.

	while (std::tolower(std::cin.get()) != 'q')
		;

	// Disconnect

	try {
		std::cout << "\nDisconnecting from the MQTT server..." << std::flush;
		cli.disconnect()->wait();
		std::cout << "OK" << std::endl;
	}
	catch (const mqtt::exception& exc) {
		std::cerr << exc << std::endl;
		return 1;
	}

 	return 0;
}

编译指令：

g++ async_subscribe.cpp -I result_dir/paho.mqtt.cpp_result/include/ -I result_dir/paho.mqtt.c_result/include/ -L result_dir/paho.mqtt.cpp_result/lib/ -L result_dir/paho.mqtt.c_result/lib/ -l paho-mqttpp3 -l paho-mqtt3a 

4.2 发布——async_publish.cpp

这是使用了 libpaho-mqttpp3.so 进行发布消息的源码，源码路径在源码的这个路径：paho.mqtt.cpp-1.3.2/src/samples/async_publish.cpp，只更改了服务器地址。

完整代码如下：

#include <iostream>
#include <cstdlib>
#include <string>
#include <thread>
#include <atomic>
#include <chrono>
#include <cstring>
#include "mqtt/async_client.h"

using namespace std;

const string DFLT_SERVER_ADDRESS	{ "117.78.5.125:1883" };
const string CLIENT_ID				{ "paho_cpp_async_publish" };
const string PERSIST_DIR			{ "./persist" };

const string TOPIC { "hello" };

const char* PAYLOAD1 = "Hello World!";
const char* PAYLOAD2 = "Hi there!";
const char* PAYLOAD3 = "Is anyone listening?";
const char* PAYLOAD4 = "Someone is always listening.";

const char* LWT_PAYLOAD = "Last will and testament.";

const int  QOS = 1;

const auto TIMEOUT = std::chrono::seconds(10);

/

/**
 * A callback class for use with the main MQTT client.
 */
class callback : public virtual mqtt::callback
{
public:
	void connection_lost(const string& cause) override {
		cout << "\nConnection lost" << endl;
		if (!cause.empty())
			cout << "\tcause: " << cause << endl;
	}

	void delivery_complete(mqtt::delivery_token_ptr tok) override {
		cout << "\tDelivery complete for token: "
			<< (tok ? tok->get_message_id() : -1) << endl;
	}
};

/

/**
 * A base action listener.
 */
class action_listener : public virtual mqtt::iaction_listener
{
protected:
	void on_failure(const mqtt::token& tok) override {
		cout << "\tListener failure for token: "
			<< tok.get_message_id() << endl;
	}

	void on_success(const mqtt::token& tok) override {
		cout << "\tListener success for token: "
			<< tok.get_message_id() << endl;
	}
};

/

/**
 * A derived action listener for publish events.
 */
class delivery_action_listener : public action_listener
{
	atomic<bool> done_;

	void on_failure(const mqtt::token& tok) override {
		action_listener::on_failure(tok);
		done_ = true;
	}

	void on_success(const mqtt::token& tok) override {
		action_listener::on_success(tok);
		done_ = true;
	}

public:
	delivery_action_listener() : done_(false) {}
	bool is_done() const { return done_; }
};

/

int main(int argc, char* argv[])
{
	// A client that just publishes normally doesn't need a persistent
	// session or Client ID unless it's using persistence, then the local
	// library requires an ID to identify the persistence files.

	string	address  = (argc > 1) ? string(argv[1]) : DFLT_SERVER_ADDRESS,
			clientID = (argc > 2) ? string(argv[2]) : CLIENT_ID;

	cout << "Initializing for server '" << address << "'..." << endl;
	mqtt::async_client client(address, clientID, PERSIST_DIR);

	callback cb;
	client.set_callback(cb);

	auto connOpts = mqtt::connect_options_builder()
		.clean_session()
		.will(mqtt::message(TOPIC, LWT_PAYLOAD, strlen(LWT_PAYLOAD), QOS, false))
		.finalize();

	cout << "  ...OK" << endl;

	try {
		cout << "\nConnecting..." << endl;
		mqtt::token_ptr conntok = client.connect(connOpts);
		cout << "Waiting for the connection..." << endl;
		conntok->wait();
		cout << "  ...OK" << endl;

		// First use a message pointer.

		cout << "\nSending message..." << endl;
		mqtt::message_ptr pubmsg = mqtt::make_message(TOPIC, PAYLOAD1);
		pubmsg->set_qos(QOS);
		client.publish(pubmsg)->wait_for(TIMEOUT);
		cout << "  ...OK" << endl;

		// Now try with itemized publish.

		cout << "\nSending next message..." << endl;
		mqtt::delivery_token_ptr pubtok;
		pubtok = client.publish(TOPIC, PAYLOAD2, strlen(PAYLOAD2), QOS, false);
		cout << "  ...with token: " << pubtok->get_message_id() << endl;
		cout << "  ...for message with " << pubtok->get_message()->get_payload().size()
			<< " bytes" << endl;
		pubtok->wait_for(TIMEOUT);
		cout << "  ...OK" << endl;

		// Now try with a listener

		cout << "\nSending next message..." << endl;
		action_listener listener;
		pubmsg = mqtt::make_message(TOPIC, PAYLOAD3);
		pubtok = client.publish(pubmsg, nullptr, listener);
		pubtok->wait();
		cout << "  ...OK" << endl;

		// Finally try with a listener, but no token

		cout << "\nSending final message..." << endl;
		delivery_action_listener deliveryListener;
		pubmsg = mqtt::make_message(TOPIC, PAYLOAD4);
		client.publish(pubmsg, nullptr, deliveryListener);

		while (!deliveryListener.is_done()) {
			this_thread::sleep_for(std::chrono::milliseconds(100));
		}
		cout << "OK" << endl;

		// Double check that there are no pending tokens

		auto toks = client.get_pending_delivery_tokens();
		if (!toks.empty())
			cout << "Error: There are pending delivery tokens!" << endl;

		// Disconnect
		cout << "\nDisconnecting..." << endl;
		client.disconnect()->wait();
		cout << "  ...OK" << endl;
	}
	catch (const mqtt::exception& exc) {
		cerr << exc.what() << endl;
		return 1;
	}

 	return 0;
}

编译指令：

g++ async_publish.cpp -I result_dir/paho.mqtt.cpp_result/include/ -I result_dir/paho.mqtt.c_result/include/ -L result_dir/paho.mqtt.cpp_result/lib/ -L result_dir/paho.mqtt.c_result/lib/ -l paho-mqttpp3 -l paho-mqtt3a -o async_publish