How to Cross-Compile Software for ARM Architecture on FreeBSD Operating System

Introduction

Cross-compilation is an essential technique when working with embedded systems and different CPU architectures. It allows developers to build software on one platform (host) for execution on another (target). This is especially useful for ARM-based devices, which often have limited resources that make native compilation impractical.

FreeBSD provides robust support for cross-compiling, making it an excellent choice for building ARM-targeted applications. In this guide, we will cover the process of setting up a cross-compilation environment on FreeBSD and compiling software for an ARM-based target system.

Prerequisites

Before starting, ensure you have the following:

• A FreeBSD system (version 12 or later is recommended)
• Root or sudo access
• Basic familiarity with shell commands
• An ARM-based target device (such as a Raspberry Pi or an embedded board)
• The FreeBSD Ports Collection (optional but useful for dependencies)

Step 1: Installing Cross-Compilation Toolchain

FreeBSD provides a native toolchain that supports cross-compilation. To install the required toolchain, follow these steps:

1.1 Install the LLVM or GCC Compiler

The base system includes Clang, but you may need to install GCC if your target software requires it. You can install it using:

pkg install gcc

Alternatively, to use LLVM:

pkg install llvm

1.2 Install binutils for Cross-Compilation

GNU binutils provides essential tools for cross-compilation. Install it with:

pkg install binutils

1.3 Set Up the Cross-Compilation Environment

Define the cross-compilation toolchain variables:

export TARGET=arm
export TARGET_ARCH=armv7
export CROSS_TOOLCHAIN=llvm
export CC=clang
export CXX=clang++
export AS=llvm-as
export LD=ld.lld

If you are using GCC instead:

export CC=arm-none-eabi-gcc
export CXX=arm-none-eabi-g++
export AS=arm-none-eabi-as
export LD=arm-none-eabi-ld

Step 2: Obtaining Source Code

Download the source code of the software you wish to compile. You can use git or fetch the tarball manually:

git clone https://github.com/example/project.git
cd project

Ensure the source supports cross-compilation. If it uses CMake, configure, or Makefiles, check for options to specify a cross-compiler.

Step 3: Configuring the Build System

Most projects use either configure scripts, CMake, or Makefiles.

3.1 Configuring with Autotools

If the project uses configure, run:

./configure --host=arm-none-eabi --build=x86_64-unknown-freebsd12.1 --prefix=/usr/local/arm

This tells the script that we are building for an ARM target while running on FreeBSD.

3.2 Configuring with CMake

For CMake-based projects:

cmake -DCMAKE_SYSTEM_NAME=Generic \
      -DCMAKE_SYSTEM_PROCESSOR=armv7 \
      -DCMAKE_C_COMPILER=arm-none-eabi-gcc \
      -DCMAKE_CXX_COMPILER=arm-none-eabi-g++ .

3.3 Configuring with Makefiles

For a simple Makefile-based project, specify the compiler explicitly:

make CC=arm-none-eabi-gcc CXX=arm-none-eabi-g++

Step 4: Building the Software

Compile the software using the make command:

make -j$(nproc)

If using CMake:

cmake --build .

This process should generate binaries compatible with the ARM architecture.

Step 5: Testing the Build

To ensure the binary is for ARM, use:

file myprogram

It should return output indicating that the binary is for the ARM architecture.

You can also use QEMU to test the execution:

qemu-arm-static ./myprogram

Step 6: Deploying to Target Device

Use scp or rsync to transfer the compiled binaries to the target ARM system:

scp myprogram user@target-ip:/home/user/

Once transferred, log in to the device via SSH and execute:

chmod +x myprogram
./myprogram

Step 7: Debugging Issues

7.1 Common Errors

• “Command not found” for cross-compilers: Ensure that the toolchain is correctly installed and the PATH variable includes it.
• “Exec format error” on execution: This means the binary is incompatible with the target. Verify the target architecture.
• Undefined references: Ensure the correct libraries are available or cross-compiled.

7.2 Using GDB for Debugging

You can use gdb-multiarch on FreeBSD to debug ARM binaries remotely:

pkg install gdb

Run the debug server on the target device:

gdbserver :1234 ./myprogram

Then, connect from FreeBSD:

gdb-multiarch -ex "target remote target-ip:1234" myprogram

Conclusion

Cross-compiling for ARM on FreeBSD is straightforward with the right toolchain and configuration. By setting up the appropriate compiler, configuring the build system, and debugging potential issues, you can efficiently develop software for ARM-based devices. This method is particularly useful for embedded development, IoT projects, and ARM-based servers.

By following this guide, you should be well-equipped to cross-compile software for ARM architecture using FreeBSD. Happy coding!