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Eclipse Paho MQTT C++ Client Library

This repository contains the source code for the Eclipse Paho MQTT C++ client library for memory-managed operating systems such as Linux, MacOS, and Windows.

This code builds a library which enables Modern C++ applications (C++11 and beyond) to connect to an MQTT broker, publish messages, subscribe to topics, and receive messages from the broker.

The library has the following features:

• Support for MQTT v3.1, v3.1.1, and v5.
• Network Transports:
  • Standard TCP
  • Secure sockets with SSL/TLS
  • WebSockets
    • Secure and insecure
    • Proxy support
• Message persistence
  • User configurable
  • Built-in File persistence
  • User-defined key/value persistence easy to implement
• Automatic Reconnect
• Offline Buffering
• High Availability
• Blocking and non-blocking APIs
• Modern C++ interface (C++11 and beyond)

This code requires the Paho C library by Ian Craggs, et al., specifically version 1.3.13 or possibly later.

Latest News

To keep up with the latest announcements for this project, or to ask questions:

Twitter: @eclipsepaho and @fmpagliughi

Email: Eclipse Paho Mailing List

What's New in v1.4.0

The v1.4.0 release is primarily concerned with reorganizing the sources and fixing a number of CMake build issues, particularly to get the Paho C submodule build working with the existing C library, fix transient dependencies, and get the Windows DLL (maybe, finally) working properly.

• Ability to build the Paho C library automatically (now working)
  • Reworked the CMake build so that 'PAHO_WITH_MQTT_C' option properly compiles the existing Paho C v1.3.13
  • Moved 'src/externals/' to top-level
• Reorganized the source tree:
  • Moved header files to top-level 'include/' directory.
  • Moved 'src/sampless/' to top-level and renamed 'examples/'
  • Removed the ob
• Fixed and optimized 'topic_matcher' trie collection
• Added some missing Eclipse/Paho legal documents to the repo.

For a full list of updates see the CHANGELOG

Coming Next

The next release, v1.5, will upgrade the library to C++17 and start adding features the the newer C++ version supports, like an API that uses std::variant<>, std::option<>, constexpr, and so on.

Contributing

Contributions to this project are gladly welcomed and appreciated Before submitting a Pull Request, please keep three things in mind:

• This is an official Eclipse project, so it is required that all contributors sign an Eclipse Contributor Agreement (ECA)
• Please submit all Pull Requests against the develop branch (not master).
• Please sign all commits.

For full details, see CONTRIBUTING.md.

Building from source

CMake is a cross-platform build system suitable for Unix and non-Unix platforms such as Microsoft Windows. It is now the only supported build system.

The Paho C++ library requires the Paho C library, v1.3.13 or greater, to be built and installed. That can be done before building this library, or it can be done here using the CMake PAHO_WITH_MQTT_C build option.

CMake allows for options to direct the build. The following are specific to Paho C++:

In addition, the C++ build might commonly use CMAKE_PREFIX_PATH to help the build system find the location of the Paho C library.

Build the Paho C++ and Paho C libraries together

The quickest and easiest way to build Paho C++ is to buid it together with Paho C in a single step using the included Git submodule. This requires the CMake option PAHO_WITH_MQTT_C set.

$ git clone https://github.com/eclipse/paho.mqtt.cpp
$ cd paho.mqtt.cpp
$ git co v1.4.0

$ git submodule init
$ git submodule update

$ cmake -Bbuild -H. -DPAHO_WITH_MQTT_C=ON -DPAHO_BUILD_EXAMPLES=ON
$ sudo cmake --build build/ --target install

This assumes the build tools and dependencies, such as OpenSSL, have already been installed. For more details and platform-specific requirements, see below.

Unix and Linux

On *nix systems CMake creates Makefiles.

The build process currently supports a number of Unix and Linux flavors. The build process requires the following tools:

• CMake v3.5 or newer
• A fully-compatible C++11 compiler (GCC, Clang, etc)

On Debian based systems this would mean that the following packages have to be installed:

$ sudo apt-get install build-essential gcc make cmake

If you will be using secure sockets (and you probably should if you're sending messages across a public netwok):

$ sudo apt-get install libssl-dev

Building the documentation requires doxygen and optionally graphviz to be installed:

$ sudo apt-get install doxygen graphviz

Unit tests are built using Catch2.

Catch2 can be found here: Catch2. You must download and install Catch2 to build and run the unit tests locally.

Building the Paho C library

The Paho C library can be built automatically when building this library by enabling the CMake build option, PAHO_WITH_MQTT_C. That will build and install the Paho C library from a Git submodule, using a known-good version, and the proper build configuration for the C++ library. But iIf you want to manually specify the build configuration of the Paho C library or use a different version, then it must be built and installed before building the C++ library. Note, this version of the C++ library requires Paho C v1.3.13 or greater.

To download and build the Paho C library:

$ git clone https://github.com/eclipse/paho.mqtt.c.git
$ cd paho.mqtt.c
$ git checkout v1.3.13

$ cmake -Bbuild -H. -DPAHO_ENABLE_TESTING=OFF -DPAHO_WITH_SSL=ON -DPAHO_HIGH_PERFORMANCE=ON
$ sudo cmake --build build/ --target install

This builds the C library with SSL/TLS enabled. If that is not desired, omit the -DPAHO_WITH_SSL=ON.

It also uses the "high performance" option of the C library to disable more extensive internal memory checks. Remove the PAHO_HIGH_PERFORMANCE option (i.e. turn it off) to debug memory issues, but for most production systems, leave it on for better performance.

The above will install the library to the default location on the host, which for Linux is normally /usr/local. To install the library to a non-standard location, use the CMAKE_INSTALL_PREFIX to specify a location. For example, to install into a directory under the user's home directory, perhaps for local testing, do this:

$ cmake -Bbuild -H. -DPAHO_ENABLE_TESTING=OFF \
    -DPAHO_WITH_SSL=ON -DPAHO_HIGH_PERFORMANCE=ON \
    -DCMAKE_INSTALL_PREFIX=$HOME/install

Building the Paho C++ library

If the Paho C library is not already installed, the recommended version can be built along with the C++ library in a single step using the CMake option PAHO_WITH_MQTT_C set on.

$ git clone https://github.com/eclipse/paho.mqtt.cpp
$ cd paho.mqtt.cpp
$ git co v1.4.0
$ git submodule init
$ git submodule update

$ cmake -Bbuild -H. -DPAHO_WITH_MQTT_C=ON -DPAHO_BUILD_EXAMPLES=ON
$ sudo cmake --build build/ --target install

If a recent version of the Paho C library is available on the build host, and it's installed to a default location, it does not need to be built again. Omit the PAHO_WITH_MQTT_C option:

$ cmake -Bbuild -H. -DPAHO_BUILD_SAMPLES=ON

If the Paho C library is installed to a non-default location, or you want to build against a different version, use the CMAKE_PREFIX_PATH to specify its install location. Perhaps something like this:

$ cmake -Bbuild -H. -DPAHO_BUILD_SAMPLES=ON \
    -DCMAKE_PREFIX_PATH=$HOME/install

Building a Debian/Ubuntu package

A Debian/Ubuntu install .deb file can be created as follows:

$ cmake -Bbuild -H. -DPAHO_WITH_SSL=ON -DPAHO_ENABLE_TESTING=OFF -DPAHO_BUILD_DEB_PACKAGE=ON
$ cmake --build build
$ (cd build && cpack)

Windows

On Windows, CMake creates Visual Studio project files for use with MSVC. Currently, other compilers like clang or MinGW are not directly supported.

Using Paho C++ as a Windows DLL

The project can be built as a static library or shared DLL on Windows. If using it as a DLL in your application, you should define the macro PAHO_MQTTPP_IMPORTS before including any Paho C++ include files. Preferably, make it a global definition in the application's build file, like in CMake:

target_compile_definitions(myapp PUBLIC PAHO_MQTTPP_IMPORTS)

It's better not to mix DLLs and static libraries, but if you do link the Paho C++ DLL against the Paho C static library, you may need to manually resolve some system dependencies, like adding the WinSock library as a dependency to your application:

target_link_libraries(myapp ws2_32)

Building the Library on Windows

The build process currently supports a number Windows versions. The build process requires the following tools:

• CMake GUI v3.5 or newer
• Visual Studio 2017 or newer

The libraries can be completely built at an MSBuild Command Prompt. Download the Paho C and C++ library sources, then open a command window and first compile the Paho C library:

> cd paho.mqtt.c
> cmake -Bbuild -H. -DCMAKE_INSTALL_PREFIX=C:\mqtt\paho-c
> cmake --build build/ --target install

Then build the C++ library:

> cd ..\paho.mqtt.cpp
> cmake -Bbuild -H. -DCMAKE_INSTALL_PREFIX=C:\mqtt\paho-cpp -DPAHO_BUILD_SAMPLES=ON -DPAHO_WITH_SSL=OFF -DCMAKE_PREFIX_PATH=C:\mqtt\paho-c
> cmake --build build/ --target install

This builds and installs both libraries to a non-standard location under C:\mqtt. Modify this location as desired or use the default location, but either way, the C++ library will most likely need to be told where the C library was built using CMAKE_PREFIX_PATH.

It seems quite odd, but even on a 64-bit system using a 64-bit compiler, MSVC seems to default to a 32-bit build target.

The 64-bit target can be selected using the CMake generator switch, -G, at configuration time. The full version must be provided.

For MSVS 2019 and beyond:

> cmake -G "Visual Studio 16 2019" -Ax64 -Bbuild -H. -DCMAKE_INSTALL_PREFIX=C:\mqtt\paho-c
> ...

Note that it is very important that you use the same generator (target) to build BOTH libraries, otherwise you will get lots of linker errors when you try to build the C++ library.

Supported Network Protocols

The library supports connecting to an MQTT server/broker using TCP, SSL/TLS, and websockets (secure and insecure). This is chosen by the URI supplied to the connect() call. It can be specified as:

"mqtt://<host>:<port>"   - TCP, unsecure
 "tcp://<host>:<port>"    (same)

"mqtts://<host>:<port>"  - SSL/TLS
 "ssl://<host>:<port>"     (same)

"ws://<host>:<port>"    - Unsecure websockets
"wss://<host>:<port>"   - Secure websockets

The "mqtt://" and "tcp://" schemas are identical. They indicate an insecure connection over TCP. The "mqtt://" variation is new for the library, but becoming more common across different MQTT libraries.

Similarly, the "mqtts://" and "ssl://" schemas are identical. They specify a secure connection over SSL/TLS sockets.

Note that to use any of the secure connect options, "mqtts://, "ssl://", or "wss://" you must compile the library with the PAHO_WITH_SSL=ON CMake option to include OpenSSL. In addition, you must specify ssl_options when you connect to the broker - i.e. you must add an instance of ssl_options to the connect_options when calling connect().

Catch2 Unit Tests

Unit tests use Catch2 for the test framework. Versions 2.x and 3.x are supported.

Catch2 can be found here: Catch2

Basics of Thread Safety

Some things to keep in mind when using the library in a multi-threaded application:

• The clients are thread-safe. You can publish/subscribe/etc from multiple threads simultaneously. There are internal mutexes to protect multi-threaded access.
• You should not make a blocking call from within a callback from the library, i.e. anything registered with set_callback(), set_message_callback(), etc. Callbacks are invoked from the one internal thread that is processing incoming packets from the network. If you make a blocking call that expects an ACK, you will deadlock.
• You can only register one on_message() callback per client to receive incoming messages for all of your registered subscriptions. That callback runs in the context of the library thread. If you want to process incoming messages from a different (or multiple) threads:
  • Use a consumer queue, or create one or more instances of a thread-safe queue to move the messages around.
  • The thread_queue class in the library is a thread-safe queue that you can use for this.
  • To route incoming messages by topic:
    • Use an instance of the (topic_matcher)[https://github.com/eclipse/paho.mqtt.cpp/blob/master/include/mqtt/topic_matcher.h] collection to create a collection of queues or callback functions to receive messages that match a set of topic filters.
    • For MQTT v5 consider using Subscription Identifiers to map incoming messages to callbacks or queues.
• The various data and options structs (like connect_options) are simple data structs. They are not thread protected.

Example

Sample applications can be found in the source repository at src/samples: https://github.com/eclipse/paho.mqtt.cpp/tree/master/src/samples

This is a partial example of what a typical example might look like:

int main(int argc, char* argv[])
{
    sample_mem_persistence persist;
    mqtt::client cli(ADDRESS, CLIENT_ID, &persist);

    callback cb;
    cli.set_callback(cb);

    auto connOpts = mqtt::connect_options_builder()
        .keep_alive_interval(20);
        .clean_session()
        .finalize();

    try {
        cli.connect(connOpts);

        // First use a message pointer.

        mqtt::message_ptr pubmsg = mqtt::make_message(PAYLOAD1);
        pubmsg->set_qos(QOS);
        cli.publish(TOPIC, pubmsg);

        // Now try with itemized publish.

        cli.publish(TOPIC, PAYLOAD2, strlen(PAYLOAD2)+1, 0, false);

        // Disconnect

        cli.disconnect();
    }
    catch (const mqtt::persistence_exception& exc) {
        cerr << "Persistence Error: " << exc.what() << " ["
            << exc.get_reason_code() << "]" << endl;
        return 1;
    }
    catch (const mqtt::exception& exc) {
        cerr << "Error: " << exc.what() << " ["
            << exc.get_reason_code() << "]" << endl;
        return 1;
    }

    return 0;
}

This code requires:

The Paho C library by Ian Craggs, et al. https://github.com/eclipse/paho.mqtt.c