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Wiki Cross Compilation

Cross Compilation Anatomy​

Cross-Compilation ecosystem involves the following components:

  • host system
    • cross-Compilation toolchain
      • cross compiler
      • cross linker
      • cross debugger
      • sysroot
        • target system library files
        • target system header files
        • target system other files
  • target system

Cross-Compilation toolchain:

  • GCC
  • Buildroot
  • Yocto Project
  • Crosstool-NG
  • Linaro
  • Clang/LLVM

GCC​

Let's explore what a toolchain is like and what are needed to build something for a aarch64 platform on x86_64 debian-like host.

Obtaining a cross-compilation toolchain for aarch64​

For simplicity and in a super fast way, we will use a prebuilt and ready-on toolchain in x86_64 Ubuntu.

apt install gcc make gcc-aarch64-linux-gnu binutils-aarch64-linux-gnu

Where is cross compiler​

We see cross compiler binary type in host is x86-64,

$ file /usr/bin/aarch64-linux-gnu-gcc-11
/usr/bin/aarch64-linux-gnu-gcc-11: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=b1112487d0dcb759db32e15b8f40f28a05484272, for GNU/Linux 3.2.0, stripped

Where is sysroot​

The sysroot locates in /usr/aarch64-linux-gnu,

$ tree --filelimit=100 /usr/aarch64-linux-gnu
/usr/aarch64-linux-gnu
├── bin
│   ├── ar -> ../../bin/aarch64-linux-gnu-ar
│   ├── as -> ../../bin/aarch64-linux-gnu-as
│   ├── ld -> ../../bin/aarch64-linux-gnu-ld
│   ├── ld.bfd -> ../../bin/aarch64-linux-gnu-ld.bfd
│   ├── ld.gold -> ../../bin/aarch64-linux-gnu-ld.gold
│   ├── nm -> ../../bin/aarch64-linux-gnu-nm
│   ├── objcopy -> ../../bin/aarch64-linux-gnu-objcopy
│   ├── objdump -> ../../bin/aarch64-linux-gnu-objdump
│   ├── ranlib -> ../../bin/aarch64-linux-gnu-ranlib
│   ├── readelf -> ../../bin/aarch64-linux-gnu-readelf
│   └── strip -> ../../bin/aarch64-linux-gnu-strip
├── include  [139 entries exceeds filelimit, not opening dir]
└── lib
    ├── Mcrt1.o
    ├── Scrt1.o
    ├── crt1.o
    ├── crti.o
    ├── crtn.o
    ├── gcrt1.o
    ├── grcrt1.o
    ├── ld-linux-aarch64.so.1

As you see, the binutils-aarch64-linux-gnu will install binutils tools in /usr/aarch64-linux-gnu/bin,

These binutils are also x86_64 binaries,

 file $(readlink -f /usr/aarch64-linux-gnu/bin/ar)
/usr/bin/aarch64-linux-gnu-ar: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=4f75b6dc6fe5ae92c78a51e6479ca2c65bbf5335, for GNU/Linux 3.2.0, stripped

While target libraries are aarch64 type,

file /usr/aarch64-linux-gnu/lib/crt1.o
/usr/aarch64-linux-gnu/lib/crt1.o: ELF 64-bit LSB relocatable, ARM aarch64, version 1 (SYSV), for GNU/Linux 3.7.0, not stripped

Compile hello.c​

$ aarch64-linux-gnu-gcc-11 hello.c -o a.out
$ file a.out
a.out: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha1]=367c436db0697f16039d9249e4a4e809ef9e68b3, for GNU/Linux 3.7.0, not stripped

Clang/LLVM​

Cross compilation with Clang and LLVM tools

Cross compiling made easy, using Clang and LLVM · mcilloni's blog

apt install lld clang llvm
$ wget https://releases.linaro.org/components/toolchain/binaries/7.5-2019.12/aarch64-linux-gnu/sysroot-glibc-linaro-2.25-2019.12-aarch64-linux-gnu.tar.xz

$ tar -xvf sysroot-glibc-linaro-2.25-2019.12-aarch64-linux-gnu.tar.xz

$ mv sysroot-glibc-linaro-2.25-2019.12-aarch64-linux-gnu aarch64-linux-gnu
$ ll aarch64-linux-gnu
total 20K
drwxr-xr-x 2 11827 9000 4.0K Dec  4  2019 etc
drwxr-xr-x 3 11827 9000 4.0K Dec  4  2019 lib
drwxr-xr-x 2 11827 9000 4.0K Dec  4  2019 sbin
drwxr-xr-x 8 11827 9000 4.0K Dec  4  2019 usr
drwxr-xr-x 3 11827 9000 4.0K Dec  4  2019 var
$ cat > hello.c << EOL
#include <stdio.h>
int main(int argc, char *argv[])
{
  printf("Hello cross-compilation world!\n");
  return 0;
}
EOL
sysroot=~/Documents/sysroot/aarch64-linux-gnu/usr
clang --target=aarch64-linux-gnu hello.c -o hello_aarch64 -v
clang --target=aarch64-linux-gnu hello.c -o hello_aarch64 --sysroot=$sysroot -v
clang --target=aarch64-linux-gnu  -fsanitize=undefined \
    -fuse-ld=lld \
    --rtlib=compiler-rt -stdlib=libc++ \
    -nostdinc++ -nostdlib \
    -I${sysroot}/usr/include/ \
    -Wl,-L${sysroot}/usr/lib \
    --sysroot=$sysroot \
    --verbose \
    hello.c -o hello

Resources​

https://wiki.osdev.org/GCC_Cross-Compiler

https://github.com/generia/buildroot-osx

https://crosstool-ng.github.io/docs/

https://github.com/messense/homebrew-macos-cross-toolchains/blob/main/.github/workflows/aarch64.yml