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Chapter 5.  Configuring and Building

Now that you have downloaded the source for your selected kernel version and installed it into a local directory, it is time to build the code. The first step is to configure the kernel with the appropriate options; the kernel can then be compiled. Both tasks are done through the standard make utility.

Creating a configuration

The kernel configuration is kept in a file called .config in the top directory of the kernel source tree. If you have just expanded the kernel source code, there will be no .config file, so it needs to be created. It can be created from scratch, created by basing it on the "default configuration," taken from a running kernel version, or taken from a distribution kernel release. We will cover the first two methods here, and the last two methods in Chapter 8, Customizing a Kernel .

Configuring from scratch

The most basic method of configuring a kernel is to use the make config method:

$ 
cd linux-2.6.17.10

$ 
make config

 make config
scripts/kconfig/conf arch/i386/Kconfig
*
* Linux Kernel Configuration
*
*
* Code maturity level options
*
Prompt for development and/or incomplete code/drivers (EXPERIMENTAL) [Y/n/?] 
Y

*
* General setup
*
Local version - append to kernel release (LOCALVERSION) [] 
 

Automatically append version information to the version string (LOCALVERSION_AUTO) [Y/n/?] 
Y

...

The kernel configuration program will step through every configuration option and ask you if you wish to enable this option or not. Typically, your choices for each option are shown in the format [Y/m/n/?] The capitalized letter is the default, and can be selected by just pressing the Enter key. The four choices are:

y

Build directly into the kernel.

n

Leave entirely out of the kernel.

m

Build as a module, to be loaded if needed.

?

Print a brief descriptive message and repeat the prompt.

The kernel contains almost two thousand different configuration options, so being asked for every individual one will take a very long time. Luckily, there is an easier way to configure a kernel: base the configuration on a pre-built configuration.

Default configuration options

Every kernel version comes with a "default" kernel configuration. This configuration is loosely based on the defaults that the kernel maintainer of that architecture feels are the best options to be used. In some cases, it is merely the configuration that is used by the kernel maintainer himself for his personal machines. This is true for the i386 architecture, where the default kernel configuration matches closely what Linus Torvalds uses for his main development machine.

To create this default configuration, do the following:

$ 
cd linux-2.6.17.10

$ 
make defconfig

A huge number of configuration options will scroll quickly by the screen, and a .config file will be written out and placed in the kernel directory. The kernel is now successfully configured, but it should be customised to your machine in order to make sure it will operate correctly.


 
 
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