Prerequisites and Directory Setup

This section deals with the packages we need on our system to cross bootstrap our mini distro, as well as the basic directory setup before we get started.

Prerequisites

For compiling the packages you will need:

gcc
g++
make
flex
bison
gperf
makeinfo
ncurses (with headers)
awk
automake
help2man
curl
pkg-config
libtool
openssl (with headers)

In case you wonder: even if you don't build any C++ package, you need the C++ compiler to build GCC. The GCC code base mainly uses C99, but with some additional C++ features. makeinfo is used by the GNU utilities that generate info pages from texinfo. ncurses is mainly needed by the kernel build system for menuconfig. OpenSSL is also requried to compile the kernel later on.

The list should be fairly complete, but I can't guarantee that I didn't miss something. Normally I work on systems with tons of development tools and libraries already installed, so if something is missing, please install it and maybe let me know.

Directory Setup

First of all, you should create an empty directory somewhere where you want to build the cross toolchain and later the entire system.

For convenience, we will store the absolute path to this directory inside a shell variable called BUILDROOT and create a few directories to organize our stuff in:

BUILDROOT=$(pwd)

mkdir -p "build" "src" "download" "toolchain/bin" "sysroot"

I stored the downloaded packages in the download directory and extracted them to a directory called src.

We will later build packages outside the source tree (GCC even requires that nowadays), inside a sub directory of build.

Our final toolchain will end up in a directory called toolchain.

We store the toolchain location inside another shell variable that I called TCDIR and prepend the executable path of our toolchain to the PATH variable:

TCDIR="$BUILDROOT/toolchain"
export PATH="$TCDIR/bin:$PATH"

The sysroot directory will hold the cross compiled binaries for our target system, as well as headers and libraries used for cross compiling stuff. It is basically the / directory of the system we are going to build. For convenience, we will also store its absolute path in a shell variable:

SYSROOT="$BUILDROOT/sysroot"

The Filesystem Hierarchy

You might be familiar with the Linux Filesyste Hiearchy Standard which strives to standardize the root filesytem layout across GNU/Linux distros.

This layout of course goes back to the directory hierarchy on Unix Systems which in turn hasn't been designed in any particular way, but evolved over the course of history.

One issue that we will run into is that there are multiple possible places that libraries and program binaries could be installed to: - /bin - /sbin - /lib - /usr/bin - /usr/sbin - /usr/lib

Yes, I know that there is an additional /usr/local sub-sub-hierarchy, but we'll ignore that once, since nowadays* nobody outside the BSD world actually uses that.

The split between / and /usr has historical reasons. The /usr directory used to be the home directory for the system users (e.g. /usr/ken was Ken Thompsons and /usr/dmr that of Dennis M. Ritchie) and was mounted from a separate disk during boot. At some point space on the primary disk grew tight and programs that weren't essential for system booting were moved from /bin to /usr/bin to free up some space. The home directories were later moved to an additional disk, mounted to /home. So basically this split is a historic artifact.

Anyway, for the system we are building, I will get rid of the pointless /bin and /sbin split, as well as the /usr sub-hiearchy split, but some programs are stubborn and use hard coded paths (remember the last time you used #!/usr/bin/env to make a script "portable"? You just replaced one portabillity problem with another one). So we will set up symlinks in /usr pointing back to /bin and /lib.

Enough for the ranting, lets setup our directory hierarchy:

mkdir -p "$SYSROOT/bin" "$SYSROOT/lib"
mkdir -p "$SYSROOT/usr/share" "$SYSROOT/usr/include"

ln -s "../bin" "$SYSROOT/usr/bin"
ln -s "../lib" "$SYSROOT/usr/lib"