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