User:SwifT/Complete Handbook/Building the system

The build configuration file
As Gentoo is primarily a build the software-distribution it requires a bit more configuration directives than most other distributions. The first and foremost configuration file is (remember, we are now inside the chrooted environment - outside it would be ). Open up the file with nano, an easy to use editor. We add the -w option to turn off word wrapping since this might harm the configuration itself.

Inside this configuration file you can define various configuration directives that affect Portage' behavior or the software building process. We will only discuss a few here - others exist as well, but are not that important at the beginning of the installation procedure.

Each directive is a variable with some specific content. Variables are often used throughout Linux (and this is even more so within Gentoo). To set a certain variable, you define its name followed by an equal sign and then the content of the variable.

The compiler flags
The first directives that we'll discuss are the compiler flags. A compiler is a tool that builds executable code from source code and one of the benefits of using Gentoo is that you can define how the compiler should behave. More precisely, you can tell the compiler to use certain optimizations on your system. Although a compiler can take a lot more options than just optimization options, most Gentoo users only use the optimization options.

The GNU Compiler Collection, the compiler toolchain used by most architectures, supports more than hundred optimization flags. Some of them are interesting, others hardly used. Some of them are harmless, others quite intrusive. We must warn you that using anything beyond the Gentoo-recommended optimization flags is not supported.

CFLAGS and CXXFLAGS
The CFLAGS and CXXFLAGS directives are immediately passed to the C and C++ compilers respectively as command line arguments. They are often used to inform the compiler about the destination architecture and optimization settings.

Quite often, the CXXFLAGS is told to contain the same setting as the CFLAGS variable:

The GNU Compiler Collection has lots of possible directives. The first directive that most users will want to set is the destination CPU type. This is configurable with the -march= setting. For most people, finding out what CPU-TYPE to pick is quite a challenge and indeed, it is not possible to document what you should use. What we can tell you is that it is safe to pick an older type while a more recent type can cause malformed software on your system.

To obtain a list of valid -march= settings, please consult the GCC Manual or the GCC Info pages:

Select GCC Command Options, Submodel Options, and pick your architecture.

A second often used compiler setting is the optimization setting. By adding a -O followed by a number you can ask the compiler to optimize the code in various degrees. Gentoo recommends -O2 (that is "O" for "Optimization", not "0" like in "007"). The highest possible value is -O3 - if you want even stronger optimization settings you'll need to add them to the variable.

If you don't want to optimize the code for speed but for size, use -Os.

The third option we add is -pipe which tells GCC that it can use process pipes instead of temporary files for communication between the various stages of compilation. This considerably improves compiling performance on systems with sufficient memory.

CHOST
The CHOST variable is an identifier for the target host for which the compiler should build software. It is vital that this variable identifies your system, but it is even more vital that you do not edit this variable if you are not performing a bootstrap installation. If you alter this variable, any toolchain rebuild will cause the toolchain to be in an intermediate state, possibly producing malfunctioning libraries.

So your settings would be ...
At the end of this handbook you will find architecture-specific chapters. For each architecture, precise (and valid) examples for various systems are listed so that you can pick one of those if the above explanation isn't sufficient.

USE
The USE variable is probably the most important setting of all inside. With this variable you define what purposes your system serves. Each flag set in the USE variable enables or disables specific support in one or more packages.

The idea is pretty simple: if your system has a DVD reader, you'll probably set dvd as one of the USE flags to add support for DVD readers. Writing DVDs will require the dvdr setting. Similar, if your system will host an IBM DB2 server (or any application you'll use connects to such a server) you'll probably want db2 as a USE flag.

A list of all possible USE flags can be found in. This is a plain text file which is also reproduced online at the Gentoo web site. But starting to dig through this list is cumbersome as many USE flags will have no clear description (which is not because we want to remain vague on the subject, but because the USE flag is either used by different applications for slightly different purposes, or because the topic itself is too technical).

To resolve this issue, Gentoo provides you with a default USE setting. To find out what the default setting is, run emerge --info and filter out the line that starts with USE:

Any setting you place in is added to the USE flag - you don't substitute the default setting but embrace and extend it. Therefore you need to place a hyphen in front of the USE flags you do not want. For instance, the arts USE flag (which enables support for the aRts sound daemon) might be set by default. To deselect it, use -arts. As an example we'll select support for the Enlightened Sound Daemon and disable support for aRts:

There are also USE flags that are specific to a single package. Such USE flags are called local USE flags. Although you can set those USE flags in, it is wiser to enable or disable those USE flags on a per-package basis. We'll refrain from explaining how to do this here - you don't need to set USE flags during the installation, Portage is intelligent enough to handle USE flag changes after an installation.

Before we go on with the next setting there are four remarks we want to make regarding USE flags:


 * 1) Changing the USE flag in this stage of the installation might result in Portage downloading tools you don't want to install at this point yet. A frequently occurring issue is where a USE flag combination triggers the installation of kde-base/kdebase which is a quite huge build. You should consider altering the USE flags at any point later and ask Portage to just rebuild those tools that are affected by the USE flag change.
 * 2) USE flags allow Gentoo to make decisions regarding optional support and features. If any feature or support is not optional but mandatory or inherent to the package, the respective USE flags are ignored. A good example here is the qt USE flag. If a package can (but doesn't have to) support the Qt Graphical Widget library, it uses the qt USE flag to decide whether or not to build in support for Qt. If the package requires Qt to function, it'll install Qt regardless of USE flag.
 * 3) Some packages override the default USE flag settings (not the one you specify in  though) if you install them. For instance, the tetex USE flag is not set by default, but when you install the TeX distribution, Gentoo will automatically enable the tetex USE flag so that other programs can now be built with TeX support if they can handle it.
 * 4) If you want to hard-set a custom USE flag listing regardless of the default USE flags, you can start with deactivating all USE flags and then list those you want to enable. This can be accomplished using USE="-* flag1 flag2 ...".

The Gentoo Portage repository
Any Gentoo package information is stored inside an ebuild. That is a small text file which contains some metadata about the package (like the description, home page, source code location, ...) and instructions for Gentoo on how to successfully build and deploy the package on your system.

The complete set of all supported ebuilds is stored inside the Gentoo Portage Tree, also known as the Gentoo Portage Repository. You will find a snapshot of this repository at where all the ebuilds are categorised by function and name. Gentoo Portage, Gentoo's software management tool, builds decisions such as "What packages should be updated" or "What software is available for installation" based on the content of the repository snapshot on your disk.

It is therefore quite important that you regularly update the snapshot on your disk with the latest Gentoo Portage Repository content released by the Gentoo Developers. By default, Gentoo Portage chooses a random mirror1 but it is more efficient to use a mirror that is either close to you or fastest for your environment. The SYNC variable is where you put the location of the Gentoo Portage Repository where you want to take a snapshot of.

Because you can't know what mirrors are available, Gentoo provides an easy to remember naming scheme for the mirrors. The default setting picks a random mirror. The country-based still picks a random mirror, but only of a pool of mirrors inside that country. The single mirror syntax always picks the mirror you define.

An example setting for could be:

Gentoo source code location
Most Gentoo ebuilds inform Gentoo about the original location of the source code. This could be a mirror set (like with the many sourceforge hosted projects) or a fixed URL. Some ebuilds can't point to this location for whatever possible reason. If this is the case, the source code is pushed on the Gentoo mirrors in a specific location called the directory.

The GENTOO_MIRRORS variable declares what mirrors Gentoo Portage should check to find the required source code. Each mirror declared in this variable should point to the parent location of the mirror (i.e. not the directory but one level higher). A list of possible mirrors can be found online.