Eclipse Paho C Client Library for the MQTT Protocol

This repository contains the source code for the Eclipse Paho MQTT C client library.

This code builds libraries which enable applications to connect to an MQTT broker to publish messages, and to subscribe to topics and receive published messages.

Synchronous and various asynchronous programming models are supported.

Information About MQTT

• MQTT website
• The MQTT 3.1.1 standard
• The MQTT 5.0 standard
• HiveMQ introduction to MQTT
• OASIS Introduction to MQTT presentation

Libraries

The Paho C client comprises four variant libraries, shared or static:

• paho-mqtt3a - asynchronous (MQTTAsync)
• paho-mqtt3as - asynchronous with SSL/TLS (MQTTAsync)
• paho-mqtt3c - “classic” / synchronous (MQTTClient)
• paho-mqtt3cs - “classic” / synchronous with SSL/TLS (MQTTClient)

Which Paho C API to use, with some history, for context

Usage and API

Detailed API documentation is available online. It is also available by building the Doxygen docs in the doc directory.

Samples are available in the Doxygen docs and also in src/samples for reference. These are:

• paho_c_pub.c and paho_c_sub.c: command line utilities to publish and subscribe, -h will give help
• paho_cs_pub.c and paho_cs_sub.c: command line utilities using MQTTClient to publish and subscribe
• MQTTClient_publish.c, MQTTClient_subscribe.c and MQTTClient_publish_async.c: MQTTClient simple code examples
• MQTTAsync_publish.c and MQTTAsync_subscribe.c: MQTTAsync simple code examples

Some potentially useful blog posts:

• Paho client MQTT 5.0 support and command line utilities
• MQTT, QoS and persistence
• A story of MQTT 5.0

Various MQTT and MQTT-SN talks I’ve given.

Supported Network Protocols

The library supports connecting to an MQTT server using TCP, SSL/TLS, Unix-domain sockets, and websockets (secure and insecure). This is chosen by the client using the URI supplied in the connect options. It can be specified as:

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

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

"unix:///path/to/socket        - UNIX-domain socket (*nix systems only)

"ws://<host>:<port>[/path]"    - Websockets, unsecure
"wss://<host>:<port>[/path]"   - Websockets, secure

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. The use any of the secure connect options requires that you compile the library with the PAHO_WITH_SSL=TRUE 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().

The use of Unix-domain sockets requires the build option of PAHO_WITH_UNIX_SOCKETS=TRUE is required. This is only available on *nix-style systems like Linux and macOS. It is not vailable on Windows.

Runtime tracing

A number of environment variables control runtime tracing of the C library.

Tracing is switched on using MQTT_C_CLIENT_TRACE (a value of ON traces to stdout, any other value should specify a file to trace to).

The verbosity of the output is controlled using the MQTT_C_CLIENT_TRACE_LEVEL environment variable - valid values are ERROR, PROTOCOL, MINIMUM, MEDIUM and MAXIMUM (from least to most verbose).

The variable MQTT_C_CLIENT_TRACE_MAX_LINES limits the number of lines of trace that are output.

export MQTT_C_CLIENT_TRACE=ON
export MQTT_C_CLIENT_TRACE_LEVEL=PROTOCOL

Reporting bugs

Please open issues in the Github project: https://github.com/eclipse-paho/paho.mqtt.c/issues.

More information

Discussion of the Paho clients takes place on the Eclipse paho-dev mailing list.

Follow Eclipse Paho on Twitter: @eclipsepaho

General questions about the MQTT protocol are discussed in the MQTT Google Group.

There is more information available via the MQTT community site.

Building with CMake

The build process currently supports a number of Linux “flavors” including ARM and s390, OS X, AIX and Solaris as well as the Windows operating system. The build process requires the following tools:

• CMake
• GNU Make or Ninja
• A conforming C compiler, such as gcc, Clang, etc

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

$ apt-get install build-essential gcc make cmake cmake-gui cmake-curses-gui

Also, in order to build a debian package from the source code, the following packages have to be installed

$ apt-get install fakeroot devscripts dh-make lsb-release

Ninja can be downloaded from its github project page in the “releases” section. Optionally it is possible to build binaries with SSL/TLS support. This requires the OpenSSL libraries and includes to be available. E. g. on Debian:

$ apt-get install libssl-dev

The documentation requires doxygen and optionally graphviz:

$ apt-get install doxygen graphviz

Building your application with CMake

If the Paho C library was built with CMake and is already installed on the system, it is relatively easy to set up a CMake build for your application. (If it’s not already built and installed read the next section).

The library can be built with several options which create variations of the library for asynchronous or synchronous use; encryption (SSL/TLS) support or not; and whether the library is shared or static. CMake exports all of the libraries that were built as targets, and the user can chose which is best suited for an application.

The package is named: eclipse-paho-mqtt-c

The namespace for all the targets is also: eclipse-paho-mqtt-c

The target names are the same as the library names. The static libraries append -static to the target name even for platforms that use the same base name for shared and static libraries. So:

Remember, though, that not all of these targets may be available. It depends on how the library was built.

A sample CMakeLists.txt for an application that uses the asynchronous library with encryption support (paho-mqtt3as) might look like this:

cmake_minimum_required(VERSION 3.5)
project(MyMQTTApp VERSION 1.0.0 LANGUAGES C)

find_package(eclipse-paho-mqtt-c REQUIRED)

add_executable(MyMQTTApp MyMQTTApp.c)
target_link_libraries(MQTTVersion eclipse-paho-mqtt-c::paho-mqtt3as) 

If the library was installed to a non-traditional location, you may need to tell CMake where to find it using CMAKE_PREFIX_PATH. For example, if you installed it in /opt/mqtt/paho.mqtt.c

$ cmake -DCMAKE_PREFIX_PATH=/opt/mqtt/paho.mqtt.c ..

Building the Paho C library with CMake

Before compiling, determine the value of some variables in order to configure features, library locations, and other options:

Using these variables CMake can be used to generate your Ninja or Make files. Using CMake, building out-of-source is the default. Therefore it is recommended to invoke all build commands inside your chosen build directory but outside of the source tree.

An example build session targeting the build platform could look like this:

$ mkdir /tmp/build.paho ; cd /tmp/build.paho
$ cmake -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=TRUE \
    -DPAHO_BUILD_SAMPLES=TRUE ~/paho.mqtt.c

Invoking cmake and specifying build options can also be performed using cmake-gui or ccmake (see https://cmake.org/runningcmake/). For example:

$ ccmake ~/paho.mqtt.c

To compile/link the binaries, to install, or to generate packages, use these commands:

$ cmake --build .

$ cmake --build . --target install

$ cmake --build . --target package

To build, install, or generate packages, you can also use the generated builder like ninja or make directly after invoking the initial CMake configuration step, such as ninja package or make -j <number-of-jpbs> package.

Debug builds

Debug builds can be performed by defining the value of the CMAKE_BUILD_TYPE option to Debug. For example:

$ cmake -DCMAKE_BUILD_TYPE=Debug ~/paho.mqtt.c

Running the tests

Test code is available in the test directory. The tests can be built and executed with the CMake build system. The test execution requires a MQTT broker running. By default, the build system uses localhost, however it is possible to configure the build to use an external broker. These parameters are documented in the Build Requirements section above.

After ensuring a MQTT broker is available, it is possible to execute the tests by starting the proxy and running ctest as described below:

$ python ../test/mqttsas.py &
$ ctest -VV

Cross compilation

Cross compilation using CMake is performed by using so called “toolchain files” (see: https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html).

The path to the toolchain file can be specified by using CMake’s -DCMAKE_TOOLCHAIN_FILE option. In case no toolchain file is specified, the build is performed for the native build platform.

For your convenience toolchain files for the following platforms can be found in the cmake directory of Eclipse Paho:

• Linux x86
• Linux ARM11 (a.k.a. the Raspberry Pi)
• Windows x86_64
• Windows x86

The provided toolchain files assume that required compilers/linkers are to be found in the environment, i. e. the PATH-Variable of your user or system. If you prefer, you can also specify the absolute location of your compilers in the toolchain files.

Example invocation for the Raspberry Pi:

$ cmake -GNinja -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_SAMPLES=TRUE \
    -DPAHO_BUILD_DOCUMENTATION=TRUE \
    -DOPENSSL_LIB_SEARCH_PATH=/tmp/libssl-dev/usr/lib/arm-linux-gnueabihf \
    -DOPENSSL_INC_SEARCH_PATH="/tmp/libssl-dev/usr/include/openssl;/tmp/libssl-dev/usr/include/arm-linux-gnueabihf" \
    -DCMAKE_TOOLCHAIN_FILE=~/paho.mqtt.c/cmake/toolchain.linux-arm11.cmake \
    ~/paho.mqtt.c

Compilers for the Raspberry Pi and other ARM targets can be obtained from ARM (https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/downloads)

This example assumes that OpenSSL-libraries and includes have been installed in the /tmp/libssl-dev directory.

Example invocation for Windows 64 bit:

$ cmake -DPAHO_BUILD_SAMPLES=TRUE \
    -DCMAKE_TOOLCHAIN_FILE=~/paho.mqtt.c/cmake/toolchain.win64.cmake \
    ~/paho.mqtt.c

In this case the libraries and executable are not linked against OpenSSL Libraries. Cross compilers for the Windows platform can be installed on Debian like systems like this:

$ apt-get install gcc-mingw-w64-x86-64 gcc-mingw-w64-i686

Build instructions for GNU Make

Ensure the OpenSSL development package is installed. Then from the client library base directory run:

$ make
$ sudo make install

This will build and install the libraries. To uninstall:

$ sudo make uninstall

To build the documentation requires doxygen and optionally graphviz.

$ make html

The provided GNU Makefile is intended to perform all build steps in the build directory within the source-tree of Eclipse Paho. Generated binares, libraries, and the documentation can be found in the build/output directory after completion.

Options that are passed to the compiler/linker can be specified by typical Unix build variables:

Building paho-mqtt - Using vcpkg

You can download and install paho-mqtt using the vcpkg dependency manager:

git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
./vcpkg install paho-mqtt

The paho-mqtt port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please create an issue or pull request on the vcpkg repository.

Fully static builds with musl libc

(By Frank Pagliughi)

musl libc is is an implementation of the C standard library built on top of the Linux system call API, including interfaces defined in the base language standard, POSIX, and widely agreed-upon extensions.

Users of the Rust library, which wraps this one, had been complaining that they could not compile using the musl build tools. Musl is a small std C lib that can be statically linked. With the latest Paho C library (and a very minor tweak to the build), we’re now able to build Rust apps using musl and Paho C that are fully static; no runtime dependencies on the platform; not even on the standard C lib.

$ ./async_publish Publishing a message on the ‘test’ topic

$ ldd async_publish not a dynamic executable

So, for example, if maintaining a suite of apps for some newer and older embedded Linux boards, the same executables could be deployed without worry about the C ABI on the particular boards.

Certainly C apps using the Paho library could do this also.

Microsoft Windows

Calling convention

As is normal for C programs on Windows, the calling convention is __cdecl. See the Microsoft documentation here:

https://docs.microsoft.com/en-us/cpp/cpp/cdecl?view=vs-2019

If you call this library from another language, you may need to take this into account.

Contributing to Paho

Thanks for your interest in this project!

You can contribute bugfixes and new features by sending pull requests through GitHub.

Legal

In order for your contribution to be accepted, it must comply with the Eclipse Foundation IP policy.

Please read the Eclipse Foundation policy on accepting contributions via Git.

1. Sign the Eclipse ECA
2. Register for an Eclipse Foundation User ID. You can register here.
3. Log into the Eclipse projects forge, and click on ‘Eclipse Contributor Agreement’.
4. Go to your account settings and add your GitHub username to your account.
5. Make sure that you sign-off your Git commits in the following format:

Signed-off-by: Alex Smith <alexsmith@nowhere.com> This is usually at the bottom of the commit message. You can automate this by adding the ‘-s’ flag when you make the commits. e.g.
```git commit -s -m “Adding a cool feature”

4. Ensure that the email address that you make your commits with is the same one you used to sign up to the Eclipse Foundation website with.

## Contributing a change

1. [Fork the repository on GitHub](https://github.com/eclipse/paho.mqtt.c/fork)
2. Clone the forked repository onto your computer: 
``` git clone https://github.com/<your username>/paho.mqtt.c.git 

1. Create a new branch from the latest

   ``` branch with 
   ```git checkout -b YOUR_BRANCH_NAME origin/develop
   

2. Make your changes
3. If developing a new feature, make sure to include JUnit tests.
4. Ensure that all new and existing tests pass.
5. Commit the changes into the branch: git commit -s Make sure that your commit message is meaningful and describes your changes correctly.
6. If you have a lot of commits for the change, squash them into a single / few commits.
7. Push the changes in your branch to your forked repository.
8. Finally, go to https://github.com/eclipse/paho.mqtt.c and create a pull request from your “YOUR_BRANCH_NAME” branch to the develop one to request review and merge of the commits in your pushed branch.

What happens next depends on the content of the patch. If it is 100% authored by the contributor with less than 1000 lines of new product code that meets the needs of the project (refactored, test code and sample code is excluded from the count), then it can be pulled into the main repository. When there are more than 1000 lines of new product code, more steps are required. More details are provided in the handbook.

Developer resources:

Information regarding source code management, builds, coding standards, and more.

• https://projects.eclipse.org/projects/iot.paho/developer

Contact:

Contact the project developers via the project’s development mailing list.

Search for bugs:

This project uses GitHub Issues here: github.com/eclipse/paho.mqtt.c/issues to track ongoing development and issues.

Create a new bug:

Be sure to search for existing bugs before you create another one. Remember that contributions are always welcome!

• Create new Paho bug