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

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

Synchronous and various asynchronous programming models are supported.

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

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

Detailed API documentationis available online. It is also available by building the Doxygen docs in thedoc directory.

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

• paho_c_pub.c andpaho_c_sub.c: command line utilities to publish and subscribe, -h will give help
• paho_cs_pub.c andpaho_cs_sub.c: command line utilities using MQTTClient to publish and subscribe
• MQTTClient_publish.c, MQTTClient_subscribe.c andMQTTClient_publish_async.c: MQTTClient simple code examples
• MQTTAsync_publish.c andMQTTAsync_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.

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 thePAHO_WITH_SSL=TRUE CMake option to include OpenSSL. In addition, youmust specifyssl_options when you connect to the broker - i.e. you must add an instance ofssl_options to theconnect_options when callingconnect().

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

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

Tracing is switched on usingMQTT_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 theMQTT_C_CLIENT_TRACE_LEVEL environment variable - valid values are ERROR, PROTOCOL, MINIMUM, MEDIUM and MAXIMUM (from least to most verbose).

The variableMQTT_C_CLIENT_TRACE_MAX_LINES limits the number of lines of trace that are output.

export MQTT_C_CLIENT_TRACE=ONexport MQTT_C_CLIENT_TRACE_LEVEL=PROTOCOL

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

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

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

There is more information available via theMQTT community site.

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 orNinja
• A conforming C compiler, such asgcc,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

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 sampleCMakeLists.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 usingCMAKE_PREFIX_PATH. For example, if you installed it in/opt/mqtt/paho.mqtt.c

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

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 (seehttps://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 likeninja ormake directly after invoking the initial CMake configuration step, such asninja package ormake -j <number-of-jpbs> package.

Debug builds can be performed by defining the value of theCMAKE_BUILD_TYPE option toDebug. For example:

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

Test code is available in thetest 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 useslocalhost, 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 runningctest as described below:

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

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 thecmake 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

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 thebuild directory within the source-tree of Eclipse Paho. Generated binares, libraries, and the documentation can be found in thebuild/output directory after completion.

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

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

git clone https://github.com/Microsoft/vcpkg.gitcd 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, pleasecreate an issue or pull request on the vcpkg repository.

(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_publishPublishing a message on the 'test' topic

$ ldd async_publishnot 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.

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.