Cross compile for Raspberry Pi Zero from Ubuntu?

Question

I've got a CMake project I'd like to cross-compile for Raspberry Pi zero.

I'm able to compile my code using the gcc-linaro-7.4.1-2019.02-x86_64_arm-linux-gnueabihf toolchain and run it over Raspberry Pi 3B+, but on the zero I'm getting Illegal instruction.

I tried to verify that the binary is legit:

pi@raspberrypi:~/mylib $ file mylib.so
mylib.so: ELF 32-bit LSB pie executable, ARM, EABI5 version 1 (SYSV), dynamically linked, BuildID[sha1]=478ce6fb9de7f4dfbc87199c3f7618a17afacf9e, with
debug_info, not stripped

What am I doing wrong? I'm invoking CMake with:

CMAKE_DEFINES="$CMAKE_DEFINES -DCMAKE_SYSTEM_NAME=Linux"                                                             
CMAKE_DEFINES="$CMAKE_DEFINES -DCMAKE_C_COMPILER=$TOOLCHAIN_PATH/bin/arm-linux-gnueabihf-gcc"
CMAKE_DEFINES="$CMAKE_DEFINES -DCMAKE_CXX_COMPILER=$TOOLCHAIN_PATH/bin/arm-linux-gnueabihf-g++"

cmake ../src -G"Unix Makefiles" $CMAKE_DEFINES

Answer 1

I think the issue is down to the differences in the processors. The Rasberry Pi 3B+ has a ARM Cortex-A53 and the Pi Zero has a ARM1176JZF-S processor. Both are ARM CPUs, but the 3B+'s CPU is a much newer ARMv8 architecture compared to the Zeros' ARMv6 architecture. They are similar, but not exactly the same. Think Intel Core 2 Duo vs Core i7. The i7 can run applications specifically compiled for the Core 2 Duo, but not necessarily vice-versa. gcc-linaro-7.4.1-2019.02-x86_64_arm-linux-gnueabihf targets ARMv7 (the Raspberry Pi 2 and 3's ARM Cortex-A7 CPU architecture), which is compatible with the Pi 3+ and 4, but not Zero.

To cross-compile for ARMv6 it looks like there are a couple of options. The first is to use the older official cross-compiler available from https://github.com/raspberrypi/tools. This is gcc 4.9.3 which isn't recent, but should still be fine for a lot of code. A more recent option appears to be available on SourceForge at https://sourceforge.net/projects/raspberry-pi-cross-compilers/files/Raspberry%20Pi%20GCC%20Cross-Compiler%20Toolchains/GCC%209.1.0/Raspberry%20Pi%201%2C%20Zero/ (via https://github.com/abhiTronix/raspberry-pi-cross-compilers).

The only other option that I can find for now would be to be compile your own cross-compiler. Unless you're fairly confident with Linux I wouldn't go this route, but broadly speaking download and extract the binutils, glibc, and gcc source archives from https://ftpmirror.gnu.org/binutils/, etc., the compile them in that order. Specify --target=arm-linux-gnueabihf --with-arch=armv6 --with-fpu=vfp --with-float=hard --disable-multilib as configure options. More detailed directions are available with a bit of searching, e.g. https://gist.github.com/sol-prog/94b2ba84559975c76111afe6a0499814.