Introduction to Cross Compilation, Part 1

This post is the first in a series on cross compilation. In this series I'll introduce the concept of cross compilation, and how to used it. Although there are many different uses for cross compilation, I'll focus in this series in its use for embedded Linux systems development.

What is Cross Compilation?

When you develop a desktop or server application, almost always the development platform (the machine that runs your compiler) and the target platform (the machine that runs your application) are the same. By "platform" I mean the combination of CPU architecture, and Operating System. The process of building executable binaries on one machine, and run them on another machine when the CPU architecture or the Operating System are different is called "cross compilation". A special compiler is needed for doing cross compilation that is called "cross compiler", and sometimes just "toolchain".

For example, desktop PC application developers for Windows or Linux can build and run their binaries on the very same machine. Even developers of server applications generally have the same basic architecture and Operating System on both their development machine and server machine. The compiler used in these cases is called "native compiler".

On the other hand, developers of an embedded Linux application that runs on a non PC architecture (like ARM, PowerPC, MIPS, etc.) tend to use a cross compiler to generate executable binaries from source code. The cross compiler must be specifically tailored for doing cross compilation from the development machine's architecture (sometimes called "host"), to the embedded machine's architecture (called "target").

Note: cross compilation is only needed when generating binary executables from source code written in a compiled language, like C or C++. Programs written in interpreted language, like Perl, Python, PHP, or JavaScript, do not need a cross compiler. In most cases interpreted programs should be able run unchanged on any target. You do need, however, a suitable interpreter running on the target machine.

What is Cross Compilation Good for?

I have covered above one reason for doing cross compilation, that is, the target machine has a different CPU architecture that the development host. In this case cross compilation is necessary because the binaries that the native compiler generates won't run on the target embedded machine.

Sometimes cross compilation is not strictly necessary, but native compilation in not practical, or inconvenient. Consider, for example, a slow ARM9 based target machine running Linux. Having the compiler run on this target will make the build process painfully slow. In many cases target machine is just under-powered, in terms of storage and RAM, for the task of running a modern compiler.

Practically speaking, almost all embedded Linux development is being done with cross compilers. Strong PC workstation machines are used as development hosts to run the development environment (text editor, IDE), and the cross compiler.

In the next post in this series I'll show how get a cross compiler for embedded Linux development.