As a dramatic closing exercise to the Enchanted Objects parade, I chose to try and build a C MQTT client for the SAMA5D4.

It turned out to be not dramatic at all. It was rather easy. Here's my log of the activities.

My ToolSet

I'm cross-building the library and executable on a Windows machine.

I used Eclipse for DS-5 - Community Edition as my IDE, and used the built-in ARM GCC 4.x toolchain.

My telnet client is PuTTY, and WinSCP does the file transfer.

I used the GIT command line to retrieve the Paho client code.

The Paho MQTT client utility was my testbed.

I've made a how-to on using these tools as part of my blog series.

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1958 Turntable from the Black Forest - Grande Finale: Paho MQTT Client on the SAMA5D4

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Building the MQTT Shared Library

The source code for the eclipse Paho libraries is available from github.

You can find the instructions on eclipse's MQTT C Client for Posix and Windows.

Because I'm using DS-5 for the build (and also because the makefile didn't work on my Windows PC), I've only followed the steps to download the code.

C:\Users\Jan\Documents\elektronica\atmel\mqtt_lib\paho_from_source>git clone http://git.eclipse.org/gitroot/paho/org.eclipse.paho.mq
tt.c.git
Cloning into 'org.eclipse.paho.mqtt.c'...
remote: Counting objects: 1719, done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 1719 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (1719/1719), 787.24 KiB | 31.00 KiB/s, done.
Resolving deltas: 100% (1177/1177), done.
Checking connectivity... done.

Then I created a new Shared Library project in DS-5. I've used the makefile source as guideline to check what files to compile and what command line options to pass to the toolchain.

I carefully studied the makefile to understand what files to include/exclude from build, and what settings to port from make to DS-5.

I'm building the async lib version without SSL, so I took over the settings from that lib's make commands.

I excluded the files listed in this variable from compilation, just like in the make script.

SOURCE_FILES_A = $(filter-out $(srcdir)/MQTTClient.c $(srcdir)/MQTTVersion.c $(srcdir)/SSLSocket.c, $(SOURCE_FILES))

I also checked where this particular makefile variable was used:

MQTTASYNC_INIT = MQTTAsync_init

and incorporated it in the linker flags:

Then I built the project. That gave me a compiled .so library.

Build the MQTT Subscriber App

I used the  async subscription example that's available in the same git extract. MQTTAsync_subscribe.c

I created a new project in DS-5, and added the source  to that project.

I took care that the async shared library was available to the linker:

I modified the source to connect to eclipse's mqtt sandbox, and to listen to my turntable's topic:

#define ADDRESS     "tcp://iot.eclipse.org"
#define CLIENTID    "1cfee8dd0afc_SAMA5D4"
#define TOPIC       "PerpetuumEbnerSpectrum"

Build, and done!

Load and Run

I deliberately didn't do all the steps to properly register the shared library.

I handled the part of setting the lib search path on the command line when invoking my executable.

There's a nice article on how to do a full-fledged install of a .so here: Shared Libraries.

I moved both the .so and the executable to root's home on the SAMA5D4 with WinSCP,

made both the program objects executable:

$ chmod +x Enchanted_Player_MQTT_Client

note to self: does an .so really need the exex flag set?

answer to self: no; explanation updated

and invoked the whole chebang using:

$ LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./Enchanted_Player_MQTT_Client

Nothing more to say. Success. It just worked. My audio spectrum, sampled by the Yún inside my turntable, arrives nicely on the SAMA5D4.