Eclipse Paho MQTT C client

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.

Both synchronous and asynchronous modes of operation are supported.

Build requirements / compilation using CMake

There 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 (http://cmake.org)
Ninja (https://martine.github.io/ninja/; preferred) or GNU Make (https://www.gnu.org/software/make/), and
gcc (https://gcc.gnu.org/).

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

Ninja can be downloaded from its github project page in the "releases" section. Optionally it is possible to build binaries with SSL 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

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

Variable	Default Value	Description
PAHO_WITH_SSL	FALSE	Flag that defines whether to build ssl-enabled binaries too.
OPENSSL_INC_SEARCH_PATH	"" (system default)	Directory containing OpenSSL includes
OPENSSL_LIB_SEARCH_PATH	"" (system default)	Directory containing OpenSSL libraries
PAHO_BUILD_DOCUMENTATION	FALSE	Create and install the HTML based API documentation (requires Doxygen)
PAHO_BUILD_SAMPLES	FALSE	Build sample programs

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 -GNinja -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=TRUE -DPAHO_BUILD_SAMPLES=TRUE ~/git/org.eclipse.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 -GNinja ~/git/org.eclipse.paho.mqtt.c

To compile/link the binaries and to generate packages, simply invoke ninja package or make -j <number-of-cores-to-use> package after CMake. To simply compile/link invoke ninja or make -j <number-of-cores-to-use>.

Debug builds

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

cmake -GNinja -DCMAKE_BUILD_TYPE=Debug git/org.eclipse.paho.mqtt.c

Cross compilation

Cross compilation using CMake is performed by using so called "toolchain files" (see: http://www.vtk.org/Wiki/CMake_Cross_Compiling).

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=~/git/org.eclipse.paho.mqtt.c/cmake/toolchain.linux-arm11.cmake ~/git/org.eclipse.paho.mqtt.c

Compilers for the Raspberry Pi can be obtained from e. g. Linaro (see: http://releases.linaro.org/15.06/components/toolchain/binaries/4.8/arm-linux-gnueabihf/). This example assumes that OpenSSL-libraries and includes have been installed in the /tmp/libssl-dev directory.

Example invocation for Windows 64 bit:

cmake -GNinja -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_TOOLCHAIN_FILE=~/git/org.eclipse.paho.mqtt.c/cmake/toolchain.win64.cmake ~/git/org.eclipse.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 (deprecated)

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:

Variable	Description
CC	Path to the C compiler
CFLAGS	Flags passed to compiler calls
LDFLAGS	Flags passed to linker calls

Libraries

The Paho C client comprises four shared libraries:

libmqttv3a.so - asynchronous
libmqttv3as.so - asynchronous with SSL
libmqttv3c.so - "classic" / synchronous
libmqttv3cs.so - "classic" / synchronous with SSL

Usage and API

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

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

Note that using the C headers from a C++ program requires the following declaration as part of the C++ code:

    extern "C" {
    #include "MQTTClient.h"
    #include "MQTTClientPersistence.h"
    }

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 report bugs under the "MQTT-C" Component in Eclipse Bugzilla for the Paho Technology project.

More information

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

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

There is much more information available via the MQTT community site.

multisensor/paho.mqtt.c

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