Embedded Handbook/General/Creating a cross-compiler

== Creating a cross-compiler == The first thing you should know about building a toolchain is that some versions of toolchain components refuse to work together. Exactly which combinations are problematic is a matter that's constantly in flux as the Portage tree evolves. The only reliable way to determine what works is to run crossdev, adjusting individual component versions as necessary, until crossdev completes the toolchain build successfully. Even then, the cross toolchain may build binaries which break on the target system. Only through trial and error and patience will you arrive at a favorable combination of all factors.

You do not have to worry about the cross-compiler interfering with your native build system. All of the toolchain packages are designed such that they are isolated from each other based on the target. This way you can install cross-compilers for whatever architecture you wish without breaking the rest of your system.

Intro
Generating a cross-compiler by hand is a long and painful process. This is why it has been fully integrated into Gentoo! A frontend called crossdev will run with all of the proper environment variables and install all the right packages to generate arbitrary cross-compilers based on your needs. First you'll need to install :

You'll probably want to install the ~arch keyworded version of crossdev to get all the latest fixes.

We only cover the basic usage of crossdev here, but crossdev can customize the process fairly well for most needs. Run to get some ideas on how to use crossdev. Here are some common usage options:

Installing
The first step is to select the proper tuple for your target. Here we will assume you want to build a cross-compiler for the SH4 (SuperH) processor with glibc running on Linux. We will do this on a PowerPC machine. Generate a SH4 cross-compiler:

Quick test
If everything goes as planned, you should have a shiny new compiler on your machine. Give it a spin!

Use the SH4 cross-compiler:

If the crossdev command failed, you have the log file which you can review to see if the problem is local. If you're unable to fix the issue, you're welcome to file a bug in our Bugzilla. See the Communication section for more information.

Uninstalling
To uninstall a toolchain, simply use the  option. If you modified the sysroot by hand, you'll be prompted to delete things inside of it, so you may want to pipe into the command.

Uninstall the SH4 cross-compiler:

In case you didn't already notice, deleting any and all files in the directory is completely safe.

Options
Obviously crossdev can do a lot more, so to find out more, simply run.

Overview
There are generally two ways to build up your cross-compiler. The "accepted" way, and the cheater's shortcut.

The current "accepted" way is:


 * 1) binutils
 * 2) kernel headers
 * 3) libc headers
 * 4) gcc stage1 (c-only)
 * 5) libc
 * 6) gcc stage2 (c/c++/etc...)

The cheater's shortcut is:


 * 1) binutils
 * 2) kernel headers
 * 3) gcc stage1 (c-only)
 * 4) libc
 * 5) gcc stage2 (c/c++/etc...)

The reason people are keen on the shortcut is that the libc headers step tends to take quite a while, especially on slower machines. It can also be kind of a pain to setup kernel/libc headers without a usuable cross compiler. Note though that if you seek help with cross-compilers, upstream projects will not want to help you if you took the shortcut.

Also note that the shortcut requires the gcc stage1 to be "crippled". Since you're building without headers, you cannot enable the sysroot option nor can you build up proper gcc libs. This is OK if the only thing you use the stage1 is building the C library and a kernel, but beyond that you need a nice sysroot based compiler.

Below I will describe the "accepted" way as the steps are pretty much the same. You just need some extra patches for gcc in order to take the shortcut.

Sysroot
We will be cross-compiling using the sysroot method. But does the sysroot do?

The sysroot tells GCC to consider dir as the root of a tree that contains a (subset of) the root filesystem of the target operating system. Target system headers, libraries and run-time object files will be searched in there.

The top level directory is commonly rooted in

As you can see, it's just like the directory setup in but in. This setup is of course not an accident but designed on purpose so you can easily migrate applications/libraries out of and into  on your target board. If you wanted, you could even be lazy and use the as a quick NFS root!

Binutils
Grab the latest binutils tarball and unpack it.

The  option is to prevent binutils from aborting the compile due to warnings. Great feature for developers, but a pain for users. Configure and build binutils:

The reason we install into the localdir is so we can remove crap that doesn't belong. For example, a normal install will give us which doesn't belong in our host. So clean out stuff first:

And install what's left:

Kernel headers
Grab the latest Linux tarball and unpack it. There are two ways of installing the kernel headers: sanitized and unsanitized. The former is clearly better (but requires a recent version of the Linux kernel), but we'll quickly cover both.

Building and install the unsanitized headers:

Build and install the sanitized headers:

System libc headers
Grab the latest glibc tarball and unpack it. Glibc is picky, so you'll have to compile in a directory separate from the source code. Build and install the glibc headers:

Glibc sucks at life so you have to do a few things by hand:

GCC Stage 1 (C only)
We first have to help gcc find the current libc headers:

Then grab the latest gcc tarball and unpack it:

Same as binutils, gcc leaves some stuff behind we don't want. Clean the gcc stage 1:

System libc
Remove the old glibc build directory and recreate it:

GCC Stage 2 (All frontends)
Build up a full GCC now. Select whichever compiler frontends you like; we'll just do C/C++ for simplicity. Build and install the gcc stage 2:

Core runtime files
There are many random core runtime files that people wonder what they may be for. Let's explain:

The general linking order: