Cross-Compiling for Raspberry Pi can become pretty challenging as soon as third-party libraries get involved. Using the sysroot concept together with rsync, it’s easy to keep those libraries synchronized between your host and target system.

Note: This is a follow-up to How to Cross-Compile for Raspberry Pi on Ubuntu Linux in 5 Steps. Some parts are taken from the Qt Wiki.

In this post, we will attempt to cross-compile a self-contained example (for Raspbian Stretch) that relies on some third-party library. You will see how to get development packages ready on your Pi and how to synchronize them to your host computer.

1 Install development libraries on the Pi

Before starting cross-compilation, we want to make sure we’ve got all the required libraries at hand.

To achieve that, we will first install them on the target machine (the Pi). Usually, there’s a *-dev package for every library, containing headers and object files.

Let’s use ncurses, a library for creating text-based user interfaces, as an example. SSH into your Pi and install the corresponding package:

sudo apt-get install libncurses5-dev

At this point, compiling against libncurses would probably already work locally on the Pi, assuming a working toolchain. Our cross-compiler, however, isn’t yet aware of that change.

2 Synchronize the Pi’s directories to the host machine

For making cross-compilation work, we may copy those development libraries over to the host system so we can compile & link against them.

We can let rsync do the job. That way, it’s easy to keep everything in sync in the future as well.

Assuming you already got a working directory on your host under /opt/pi, as introduced in How to Cross-Compile for Raspberry Pi, create a sysroot directory and fetch all relevant content from the Pi:

cd /opt/pi
mkdir sysroot
cd sysroot

rsync -avz pi@pi.local:/lib .
rsync -avz pi@pi.local:/usr/include usr
rsync -avz pi@pi.local:/usr/lib usr

Note: Replace pi.local by your Pi’s host name or IP address.

You should now have a copy of the relevant headers and libraries:

$ find -name libncurses.so.*
./lib/arm-linux-gnueabihf/libncurses.so.5.9
./lib/arm-linux-gnueabihf/libncurses.so.5

$ find -name ncurses.h
./usr/include/ncurses.h

The output of the find command might vary depending on your Raspbian version. Just make sure you got the header file and the shared object (note that one of the *.so files is just a symlink).

3 Attempt to Cross-Compile without setting sysroot

You’re now ready for cross-compiling.

Here’s an example from the NCURSES Programming Howto. Save the following snippet as demo.c.

#include <ncurses.h>

int main()
{	
  initscr();
  printw("Hello World !!!");
  refresh();
  getch();
  endwin();

  return 0;
}

Let’s determine which cross-toolchain to use by setting the PATH environment variable accordingly:

export PATH="$PATH:/opt/pi/tools/arm-bcm2708/\
gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin"

Now you should able to launch the cross-compiler. Make sure you can start it like this:

$ arm-linux-gnueabihf-gcc --version
arm-linux-gnueabihf-gcc (crosstool-NG linaro-1.13.1+bzr2650 - Linaro GCC 2014.03) 4.8.3 20140303 (prerelease)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Let’s just try to build without further considerations:

$ arm-linux-gnueabihf-gcc demo.c -o demo -lncurses
demo.c:1:21: fatal error: ncurses.h: No such file or directory
 #include <ncurses.h>
                     ^
compilation terminated.

This is failing as expected: the compiler is missing a certain header file.

4 Cross-Compile with setting sysroot

We can fix the build by adding the --sysroot option when calling gcc. This tells the compiler to use the specified location as the logical root directory for headers and libraries, as stated in the GCC docs.

Call the compiler like this:

arm-linux-gnueabihf-gcc --sysroot ./sysroot demo.c -o demo -lncurses

And verify the executable has been created successfully.

$ file demo
demo: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=0f88dffd576bab754342516b3566868fd64f07a1, not stripped

The demo program is now ready to be executed on your Pi:

scp demo pi@pi.local:~/
ssh pi@pi.local
pi@pi $ ./demo

References