Hello World C program on the EV3

I have been programming the EV3 using LeJOS (basically Java) for a few weeks now, but the desire to use a truly low-level language such as C to control the robot has not gone away. I took a significant first step in that direction by acquiring a cross-compiler and using it to successfully write and compile a Hello World program that basically prints the infamous string on stdout. Since my computer runs Linux on the x86 architecture while the EV3 runs on an ARM 9 architecture, a cross-compiler is needed on my x86 to take source code (in C) and compile it for the ARM 9 architecture. After compilation the executable can be placed in the EV3 and executed at will. Having a cross-compiler allows me to develop on my PC (a much faster approach) and simply push binaries (executables) to the EV3 for execution. This site/blog explains how to acquire the cross-compiler while this associated section explains how to write and compile the Hello World program. The blog has more complex examples to do with controlling the sensors and motors but that will have to wait for another day.

Using make to build and deploy LeJOS programs on the EV3

As discussed earlier my aim is to automate the repetitive nature of the edit-build-deploy-run cycle for programming the EV3 using LeJOS. To that end I set up build.gradle to build the project from the command-line as explained here. Furthermore I set up wifi on the EV3 (using a usb dongle). The next step was to edit ~/.ssh/config and add an entry for the EV3:

  Host ev3

      User root
      HostName 192.168.1.16
      IdentityFile ~/.ssh/abid_ev3

Note: Since the wifi setup on the EV3 is DHCP (not static) the IP address is liable to change, in which event one has to modify ~/.ssh/config accordingly.

The next step is to create a Makefile in your project root. Here is a link to the one I'm using for my "Rover" project. Note the extensive use of PHONY and EMPTY targets to ensure that build and scp tasks are only performed when needed. make once correctly set up takes care of the dependencies. I only need to issue make run from the terminal and the source is compiled and the jar file copied to the EV3 only if needed. Sweet and simple, just the way I like it.

You can take a look at the complete Rover source code on github. There you will also find a number of other repos for my various LeJOS EV3 projects.

Building LeJOS programs for EV3 using gradle

With the wifi connection (including ssh) to the EV3 running and IntelliJ set up to write programs, development for the EV3 was coming along nicely. There was still an issue, however, with the time it took to make a change in the code, compile it, copy it to the EV3 (using scp), run it on the EV3 and then repeat. Since repetition is the bane of all programmers I started looking in to ways to automate these tasks. Of course the ubiquitous make tool came instantly to mind. I would just write a make script (Makefile) to automate these tasks. But how to automate the build task that creates the jar file from the source code, and in such a way that make could do it.

The solution is gradle. Gradle is a project automation tool with excellent support for Java (Google now recommends using gradle and Android Studio - a port of IntelliJ - for Android development). Though gradle has somewhat of a learning curve, it is well worth it, since only after using gradle have I finally begun to understand how a java project comes together. Furthermore with gradle one can build projects from the command line by simply issuing the command gradle build.

Gradle installation is straightforward and is explained here. Once installed the next step is to create a build.gradle file in the root folder of your LeJOS project (your source code). The build.gradle for one of my earlier projects looks like this:

apply plugin: 'java'

def main_class = "Rover"          // We define the main class of the project

// We set the Version of the Java SDK to the one on the system: 1.6
sourceCompatibility = 1.6

sourceSets {

    main {

        // We specify the source directories for the project. The first one is created by us while the latter two are part of the LeJOS source and provide
        // essential classes for controlling the EV3.
        java {
            srcDirs = ['src', '/home/abid/applications/lejos/ev3/DBusJava/src', '/home/abid/applications/lejos/ev3/ev3classes/src']
        }

        // We specify the jar library required to compile the project. This provides the com.sun.jna package
        dependencies {
            compile files('/home/abid/applications/lejos/ev3/ev3classes/lib/jna-3.2.7.jar')
        }

        // We specify manifest attributes (key-value pairs that must appear in the META-INF/MANIFEST>MF file inside the output Jar for the EV3 to run it correctly.
        // Additionally we supply the name of the Main-Class which is the starting point of the program execution.
        jar {
            manifest {
                attributes("Main-Class": main_class, "Class-Path": "/home/root/lejos/lib/ev3classes.jar /home/root/lejos/libjna/usr/share/java/jna.jar")
            }
        }
    }
}

This is pretty much a template build.gradle file for any LeJOS EV3 project. The one-time alterations needed are to change the latter two srcDirs to point to the locations in the local LeJOS git repo as well as the jna jar dependency in the same.

The only thing one needs to alter (from project to project) is the name of the main class defined in the variable main_class near the beginning. And you are good to go. Execute gradle build and the project will compile and place the results in the build/ sub-folder.

Next time I will show you how to incorporate all of this in to one epic Makefile complete with dependencies.

Using LeJOS to write write programs for the EV3

Over the years I have thought many times of getting in to Android development; buying books, reading up online tutorials and such. But every time I would be driven away by the sheer ugliness, unusability and slowness of Eclipse. It wasn't until I discovered IntelliJ that I could start making sense of Android and now that is what I do for a living.

What this means is that when I bought the Lego Mindstorms EV3 and installed LeJOS on it the next step was figuring out how to develop programs for it on IntelliJ (and not Eclipse as suggested by the guys at LeJOS, but what do they know). This post explains how to do it on Windows and since IntelliJ is multi-platform the instructions can be ported trivially to Linux.

A few clarifications to the instructions follow:

The LeJOS source code (the classes required for development - think of it as the SDK) can be acquired using:

    git clone git://git.code.sf.net/p/lejos/ev3

Once the repo has been cloned switch to the latest release (0.5.0-alpha):

    git checkout 0.5.0-alpha

This will place you in a HEAD-less state but that is alright.

Your main class doesn't need to be labelled "Main". It just needs to implement the function public static void main(String[] args). This class serves as the entry point in to your application (execution starts in the main function/method. Just remember to use the actual name of the class in the MANIFEST.MD file when specifying the Main-Class value.

Setting up Wifi on an EV3 running LeJOS

What is the point of having an awesome robotics kit running Linux if you cannot access the shell? That is a question I found myself facing once I had installed LeJOS on the EV3. Although I could place programs (Java .jar files) on the sd card directly) and execute them on the EV3 this became quite tedious quite fast. Fortunately, of the few Wifi dongles supported by LeJOS is the Edimax EW-7811Un which sells for $10 at Amazon. This amazingly small wifi dongle is an excellent add-on for the EV3 and works on the Raspberry Pi as well.

Once again the internet is full off conflicting advice on how to get the EV3 running LeJOS to automatically detect the Wifi dongle (Note: The stock Lego EV3 firmware never did detect it). The advice that worked for me came from this source.

The common pitfalls to avoid are: do NOT edit /etc/network/interfaces (no matter how many tutorials tell you to), you only need to modify /etc/wpa_supplicant.conf. And you can generate the psk in hex form by running the wpa_passphrase command as follows:

    wpa_passphrase <SSID>

and simply enter the passphrase when prompted to get the psk in hex format.

In summary load the microSD card (with the LeJOS partitions for EV3) in to your computer, mount the non-boot partition and edit /etc/wpa_supplicant.conf to enter your wireless network credentials. Once done, unmount the microSD card and load it in to the EV3. Start it up (it should boot in to LeJOS) and the EV3 should connect automatically to your specified wireless network.

Note: Once wifi is set up you can ssh in to the EV3 using the IP Address displayed by LeJOS on the EV3's screen. The username is root and the password is blank/empty. To allow scripts to ssh in to the EV3 I placed my public key in the /home/root/.ssh/authorized_keys file (which I had to create) and then simply added an entry for EV3 in my computers ~/.ssh/config file. Now I can access my EV3 over wifi effortlessly.

Programming Lego Mindstorms EV3 using LeJOS on Ubuntu Linux

I have been coveting a Lego Mindstorms Robotic kit ever since I learned there was one to be had and learning that the latest version, the EV3 runs Linux was the last straw. I was still hesitant because though the EV3 ran Linux there was no support for programming it in Linux, that is, from a PC running Linux. Out of the box the EV3 can only be programmed using a Windows or Mac machine, and even then only in a GUI suite ("visual programming") geared towards children.

What I needed was the power of a fully-formed language where I could program the EV3 using my Linux desktop (it is what I call my 7 year old laptop that I run only as a box). Lego had chosen to place Linux on the EV3 to make it more hackable and their foresight paid dividends when the LeJOS project was able to fork the open-source Lego kernel allowing them to place a JVM (Java Virtual Machine) and give access to the complete functionality of the EV3 using Java classes. Since I am an Android Developer by day, this was perfect.

The really great thing about the EV3 is that it comes with a micro-SD slot on which you can place your own bootable partition which allows you to change the firmware of the device without actually having to mod it. Basically, write a suitable image to a micro-SD card and the EV3 will boot from it. Remove the sd card and the EV3 will return to booting from the Lego firmware.

The LeJOS project provides a complete tutorial on how to create a micro-sd card that will run LeJOS on the EV3:

Stay tuned for more information on how to use a (supported) Wifi dongle (USB), program the EV3 using IntelliJ and automate the deployment of custom programs using ssh, make and gradle.

Happy hacking on the EV3.

How To quote source code in Blogger posts

Update (06/27/17): This post is now out of date since the underlying js library is no longer hosted. The same can be achieved, more simply in fact, using the highlight.js library (https://highlightjs.org/).

The tutorial for the updated technique can be found at: http://abidmujtaba.blogspot.com/2017/06/highlighted-syntax-code-snippets-in.html

The original post (now defunct) is provided for the sake of posterity:


Since I write this watching "Community" it feels appropriate that my first Blogger post be a meta How-To explaining how to elegantly quote source code in Blogger posts. This feature is a necessity for someone like me starting a blog dedicated to technical topics.

There is a lot of discussion about how to achieve this online. The simplest solution I found and the one I actually applied in this blog itself comes from StackOverflow of course: http://stackoverflow.com/a/10848816/2926226

I often wonder how they managed to program before StackOverflow came along. Verily we stand on the shoulders of giants.

Don't forget to (HTML) escape your code if it contains offending characters like <, >, & .etc. using http://accessify.com/tools-and-wizards/developer-tools/quick-escape/default.php

And now a sample Java Hello World function to show you how it looks:

public String greeting()
{
    return "Hello, World";
}

To create the code block above I entered HTML mode (in Blogger) and entered the following HTML code:

<pre class="java" name="code">
public String greeting()
{
    return "Hello, World";
}
</pre>

Delightfully simple and therefore conducive to efficient technical blogging.