Yesterday I blogged about building Emacs on the Beaglebone. While this is great Sunday night fun, it’s not the most efficient way to compile applications for this device. The original Beaglebone has a 720 MHz processor and 256MB of RAM. My desktop has a 2.8GHz processer and 12GB of RAM. The turtle does not beat the hare in this race. It would be nice if I can compile an application on my desktop and send the compiled binary to the Beaglebone to run. Well, welcome to cross-compiling!

Now, there is a lot of information out there on building your own custom kernel, getting Ubuntu on the Beaglebone, etc… But that’s not what this post is about. This post attempts to answer the question, “I have an application in C/C++, how do I compile this for the Beaglebone.”

Sometimes, the simplest questions are the hardest to answer. Fortunately, I found this well-written summary. Basically, you can follow those instructions verbatim, but I’m going to outline the steps here in the case that site goes down and simultaneously, Google cache clears 🙂  Of course, after I found those instructions, I read the README on the setup-scripts distro itself, which has slightly different steps.  I have not yet gone back and tried those instructions…

I’ve got a Mac and while I love homebrew, I found it easier to work in Linux. So:

1. I picked up an Ubuntu ISO.
2. Loaded it up in Virtual Box.
3. Updated packages, installed Emacs, etc…
4. Download the Angstrom distro from github:

   git clone git://github.com/Angstrom-distribution/setup-scripts.git

5. cd setup-scripts
6. Followed the advice in the article and went to setup-scripts/conf/local.conf and commented out the line INHERIT += rm_work.
7. Ran the following commands (the last one takes a while):

   MACHINE=beaglebone ./oebb.sh config beaglebone 
   MACHINE=beaglebone ./oebb.sh update 
   MACHINE=beaglebone ./oebb.sh bitbake virtual/kernel

8. You may have to apt-get install packages if any of the above scripts grumble.

Now you’ve got the compiled kernel and target machine compilers. Let’s write a hello world program in C and send it to the board.

1. Add the target compiler bin directory to your path (assuming your cloned from your home directory).  You’ll probably also want to put this in your appropriate bash config file as this only sets the PATH for your current session.

   PATH=$PATH:~/setup-scripts/build/tmp-angstrom_v2012_12-eglibc/sysroots/x86_64-linux/usr/bin/armv7a-vfp-neon-angstrom-linux-gnueabi

2. Write a simple hello world program in C:

   #include <stdio.h>
   int main(void) 
   { 
     printf("Hello Beaglebone world!\n");
     return 0; 
   }

3. To compile: arm-angstrom-linux-gnueabi-gcc -o hello hello.c
4. Run: ./hello. Cannot execute binary file?!?! It worked! See, most likely your host computer is of an x86 architecture, which has different machine code instructions than an ARM (Beaglebone).  In this case, not working is a good thing.
5. Fine, let’s see it really work. Put the binary on the board somehow (I’m using ssh / sftp).
6. Run on the board!

That’s great if you have a small application.  But normally, a C program will have a makefile.  In this case type:

make ARCH=arm CROSS_COMPILE=arm-angstrom-Linux-gnueabi-

instead of the vanilla make and it will build for your Beaglebone. Getting the make install part might be a bit more tricky though since you still have to get the binaries on the board. Emacs built in about 7 minutes, running on VM on my mac compared to the 1 hour plus building directly on the Beaglebone. So, it pays to know how to cross-compile 🙂