Understanding Cross-Compilation
“Building for One Machine, Compiling on Another — The Power of Cross-Compilation Explained”
I recently came across LFS, which stands for Linux From Scratch. LFS is a book and set of instructions on how to build your own Linux system (basic and minimal) entirely from source. This idea really excited me, and I started following the steps using a mirror of the build environment.
Building my own Linux system felt empowering. And, as with any technical journey, I ran into concepts I had heard of before but never fully understood. One such concept was cross-compilation.
At the time, I didn’t quite grasp what cross-compiling meant. But now that I do, here’s how I would explain it—starting from some basic ideas.
What is a Compiler?
Before diving into cross-compilation, let’s refresh the concept of a compiler.
A compiler is a program that takes your code (written in a high-level language like C or C++) and converts it into assembly-level code, or machine code, that your CPU can understand. This translation step is crucial—it’s what makes your code executable by the hardware.
Think of it this way:
[source code]
int main() {
printf("Hello, world!");
}
↓ (compiler)
[machine code]
mov r1, r2
mov r3, ...What Does “CPU Understand” Even Mean?
Good question.
Every CPU has its own instruction set architecture (ISA). This is the language that the CPU understands—its own set of commands or instructions. So when your compiler converts your code, it must translate it into a form that matches the CPU's instruction set.
That means the final output of a compiler is tailored for a specific type of CPU.
Wait, But… The Compiler is a Program Too!
This leads to an interesting puzzle: if the compiler is just a program, how is it compiled to run on your CPU in the first place?
More importantly, how can you compile a program for a different CPU?
For instance, suppose I want to compile a program to run on an iPod (if you remember those!), or on your phone—but I’m working on my laptop, which has a completely different CPU architecture. Most of our phones run on ARM processors, while laptops generally run on x86 or x86_64 processors.
If the program is compiled on your laptop, how does it end up running on your phone?
Enter Cross-Compilers
This is where cross-compilation comes in.
A cross-compiler is a compiler that runs on one system (host) but generates code for another system (target). So, you might run a cross-compiler on your x86 laptop, and it will generate binaries for an ARM-based phone or embedded device.
This is essential in scenarios like:
Embedded systems development (e.g., for Raspberry Pi, routers, microcontrollers)
Building an OS from scratch (like in LFS)
Compiling for iOS or Android on a development machine
Developing for game consoles or legacy systems
Final Thoughts
Cross-compilation might sound complicated at first, but at its heart, it's about telling your compiler: "I want to build software here that runs there." Understanding this unlocks a lot of possibilities in systems programming, embedded development, and OS building.
If you're exploring LFS or working with embedded Linux, you'll eventually run into this concept. Hopefully, now it won’t be a mystery.
Reference
For those who are interested in building your Own Linux From Scratch: https://www.linuxfromscratch.org/lfs/