EBC Exercise 08a Cross-Compiling

This class is about developing software for embedded Linux. So far we have been doing all of our development on the Beagle. This works well for small (and not so small) programs. However, we are now moving into kernel development and that's best done on a more powerful host computer. In EBC_Exercise_08_Installing_Development_Tools_4.4 you learned how to download and install the cross compilers and the source for kernel and u-boot. Now we'll use those tools.

Cross compiling Hello World
Normally when you compile you compile on the machine that will run the code. You can compile and run on the Bone, but sometimes (like when compiling the kernel) it's better to use a more powerful machine for the compiling. First we'll compile helloWorld.c on the host computer and run it there, then we'll cross compile it on the host to run on the Bone.

If you've set up your git repository you will find it in helloWorld.c when you do a git pull. Compile and run it on your host to be sure it works.

host$ gcc helloWorld.c host$ file a.out a.out: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0x357e34e90f7c32d414368d69cc06d0aed59acf1c, not stripped host$ ./a.out Hello, World! Main is executing at 0x400524 This address (0x7fff8260bdf8) is in our stack frame This address (0x601038) is in our bss section This address (0x601020) is in our data section

Now that you know it's working, let's cross compile it. First figure out what version of the cross compiler was loaded.

host$ cd BeagleBoard/bb-kernel/dl host$ ls gcc-linaro-5.4.1-2017.05-x86_64_arm-linux-gnueabihf gcc-linaro-5.4.1-2017.05-x86_64_arm-linux-gnueabihf.tar.xz gcc-linaro-7.1.1-2017.08-x86_64_arm-linux-gnueabihf gcc-linaro-7.1.1-2017.08-x86_64_arm-linux-gnueabihf.tar.xz

Here we see two versions of the compiler have been loaded. Let's pick the newer one.

Now set the paths to find the cross-compiler. Put the following in a file, call it ~/crossCompileEnv.sh.

export ARCH=arm export CROSS_COMPILE=arm-linux-gnueabihf- export PATH=$PATH:~/BeagleBoard/bb-kernel/dl/gcc-linaro-7.1.1-2017.08-x86_64_arm-linux-gnueabihf/bin Make sure the PATH you use goes to the bin directory where the cross compiler is installed.

Now source the file and compile again. (Note: you only have to source once per terminal session.) host$ source ~/crossCompileEnv.sh host$ ${CROSS_COMPILE}gcc helloWorld.c host$ file a.out a.out: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=b9222cbcee442470c7b89ac294e392a631c82264, not stripped

The file command tells what's in the file. In this case we have an ARM executable. Success! Now copy to your Beagle and run

host$ scp a.out debian@192.168.7.2:. host$ ssh debian@192.168.7.2 ./a.out Hello, World! Main is executing at 0x8374 This address (0xbeb32d4c) is in our stack frame This address (0x10650) is in our bss section This address (0x10648) is in our data section

The scp copies a.out to the beagle and the ssh runs the a.out on the beagle. Notice the addresses are very different from the host version.