Cross-GCC

Purpose

Guide agents through setting up and using cross-compilation GCC toolchains: triplets, sysroots, pkg-config, QEMU-based testing, and common failure modes.

Triggers

• "How do I compile for ARM on my x86 machine?"
• "I'm getting 'wrong ELF class' or 'cannot execute binary file'"
• "How do I set up a sysroot for cross-compilation?"
• "pkg-config returns host libraries in my cross build"
• "How do I debug a cross-compiled binary with QEMU + GDB?"

Workflow

1. Understand the triplet

A GNU triplet has the form <arch>-<vendor>-<os>-<abi> (often 3 or 4 parts):

Triplet	Target
aarch64-linux-gnu	64-bit ARM Linux (glibc)
arm-linux-gnueabihf	32-bit ARM Linux hard-float
arm-none-eabi	Bare-metal ARM (no OS)
riscv64-linux-gnu	64-bit RISC-V Linux
x86_64-w64-mingw32	Windows (MinGW) from Linux
mipsel-linux-gnu	Little-endian MIPS Linux

2. Install the toolchain

# Debian/Ubuntu
sudo apt install gcc-aarch64-linux-gnu g++-aarch64-linux-gnu binutils-aarch64-linux-gnu

# For bare-metal ARM (Cortex-M)
sudo apt install gcc-arm-none-eabi binutils-arm-none-eabi

# Verify
aarch64-linux-gnu-gcc --version

3. Basic cross-compilation

# C
aarch64-linux-gnu-gcc -O2 -o hello hello.c

# C++
aarch64-linux-gnu-g++ -O2 -std=c++17 -o hello hello.cpp

# Bare-metal (no stdlib, no OS)
arm-none-eabi-gcc -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16 \
    -ffreestanding -nostdlib -T linker.ld -o firmware.elf startup.s main.c

4. Sysroot

A sysroot is a directory containing the target's headers and libraries. Required when your code links against target-specific libraries.

# Use a sysroot
aarch64-linux-gnu-gcc --sysroot=/path/to/aarch64-sysroot -O2 -o prog main.c

# Common sysroot sources:
# - Raspberry Pi: download from raspbian/raspios
# - Debian multiarch: debootstrap --arch arm64 bullseye /tmp/sysroot
# - Yocto/Buildroot: generated automatically in build output

Verify the sysroot is correct:

aarch64-linux-gnu-gcc --sysroot=/path/to/sysroot -v -E - < /dev/null 2>&1 | grep sysroot

5. pkg-config for cross builds

pkg-config will return host library paths by default. Override:

export PKG_CONFIG_SYSROOT_DIR=/path/to/sysroot
export PKG_CONFIG_LIBDIR=${PKG_CONFIG_SYSROOT_DIR}/usr/lib/aarch64-linux-gnu/pkgconfig:${PKG_CONFIG_SYSROOT_DIR}/usr/share/pkgconfig
export PKG_CONFIG_PATH=   # clear host path

pkg-config --libs libssl  # now returns target paths

6. CMake cross-compilation

Create a toolchain file aarch64.cmake:

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)

set(CMAKE_C_COMPILER   aarch64-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER aarch64-linux-gnu-g++)

set(CMAKE_SYSROOT /path/to/aarch64-sysroot)
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=aarch64.cmake
cmake --build build

7. Test with QEMU

# User-mode emulation (Linux binaries, no full OS)
sudo apt install qemu-user-static

qemu-aarch64-static ./hello
# Or set binfmt_misc for transparent execution:
# Then just: ./hello

# GDB remote debug via QEMU
qemu-aarch64-static -g 1234 ./hello &
aarch64-linux-gnu-gdb -ex "target remote :1234" ./hello

8. Common errors

Error	Cause	Fix
cannot execute binary file: Exec format error	Running target binary on host without QEMU	Use qemu-<arch>-static
wrong ELF class: ELFCLASS64 (or 32)	Wrong-architecture object linked	Check triplet; ensure all objects use same toolchain
/usr/bin/ld: cannot find -lfoo	Host library path used for cross-link	Set --sysroot; fix PKG_CONFIG_LIBDIR
undefined reference to '__aeabi_*'	Missing ARM ABI runtime	Link with -lgcc or -lclang_rt.builtins
relocation R_AARCH64_ADR_PREL_PG_HI21 out of range	Distance too large	Use -mcmodel=large or restructure
unrecognized opcode	Wrong -mcpu or -march	Set correct CPU flags for target

9. Environment variables

# Tell build systems to use cross-compiler
export CC=aarch64-linux-gnu-gcc
export CXX=aarch64-linux-gnu-g++
export AR=aarch64-linux-gnu-ar
export STRIP=aarch64-linux-gnu-strip
export OBJDUMP=aarch64-linux-gnu-objdump

# For autoconf projects
./configure --host=aarch64-linux-gnu --prefix=/usr

For a reference on ARM-specific GCC flags, see references/arm-flags.md.

Related skills

• Use skills/compilers/gcc for GCC flag details
• Use skills/debuggers/gdb for remote debugging with gdbserver
• Use skills/low-level-programming/assembly-arm for AArch64 assembly specifics
• Use skills/build-systems/cmake for toolchain file setup