<sect1><title>What Is A Kernel Module?</title>

	<para>So, you want to write a kernel module.  You know C, you've written a few normal programs to run as processes, and now
	you want to get to where the real action is, to where a single wild pointer can wipe out your file system and a core dump
	means a reboot.</para>

	<para>What exactly is a kernel module?  Modules are pieces of code that can be loaded and unloaded into the kernel upon
	demand.  They extend the functionality of the kernel without the need to reboot the system.  For example, one type of module
	is the device driver, which allows the kernel to access hardware connected to the system.  Without modules, we would have to
	build monolithic kernels and add new functionality directly into the kernel image.  Besides having larger kernels, this has
	the disadvantage of requiring us to rebuild and reboot the kernel every time we want new functionality.</para>

</sect1>



<sect1><title>How Do Modules Get Into The Kernel?</title>

	<indexterm><primary>/proc/modules</primary></indexterm>
	<indexterm><primary>kmod</primary></indexterm>
	<indexterm><primary>kerneld</primary></indexterm>
	<indexterm><primary><filename>/etc/modules.conf</filename></primary></indexterm>
	<indexterm><primary><filename>/etc/conf.modules</filename></primary></indexterm>

	<para>You can see what modules are already loaded into the kernel by running <command>lsmod</command>, which gets its
	information by reading the file <filename>/proc/modules</filename>.</para>

	<para>How do these modules find their way into the kernel?  When the kernel needs a feature that is not resident in the
	kernel, the kernel module daemon kmod<footnote><para>In earlier versions of linux, this was known as
	kerneld.</para></footnote> execs modprobe to load the module in.  modprobe is passed a string in one of two forms:</para>

	<itemizedlist>
	<listitem><para>A module name like <filename>softdog</filename> or <filename>ppp</filename>.</para></listitem>
	<listitem><para>A more generic identifier like <varname>char-major-10-30</varname>.</para></listitem>
	</itemizedlist>

	<para>If modprobe is handed a generic identifier, it first looks for that string in the file
	<filename>/etc/modules.conf</filename>.  If it finds an alias line like:</para>
	
	<screen>
   alias char-major-10-30 softdog
	</screen>

	<para>it knows that the generic identifier refers to the module <filename>softdog.o</filename>.</para>

	<para>Next, modprobe looks through the file <filename>/lib/modules/version/modules.dep</filename>, to see if other modules
	must be loaded before the requested module may be loaded.  This file is  created by <command>depmod -a</command> and contains
	module dependencies.  For example, <filename>msdos.o</filename> requires the <filename>fat.o</filename> module to be already
	loaded into the kernel.  The requested module has a dependancy on another module if the other module defines symbols
	(variables or functions) that the requested module uses.</para>

	<para>Lastly, modprobe uses insmod to first load any prerequisite modules into the kernel, and then the requested module.
	modprobe directs insmod to <filename role="directory">/lib/modules/version/</filename><footnote><para>If you are modifying the
	kernel, to avoid overwriting your existing modules you may want to use the <varname>EXTRAVERSION</varname> variable in the
	kernel Makefile to create a seperate directory.</para></footnote>, the standard directory for modules.  insmod is intended to
	be fairly dumb about the location of modules, whereas modprobe is aware of the default location of modules.  So for example,
	if you wanted to load the msdos module, you'd have to either run:</para>

	<screen>
    insmod /lib/modules/2.5.1/kernel/fs/fat/fat.o
    insmod /lib/modules/2.5.1/kernel/fs/msdos/msdos.o
	</screen>
	
	<para>or just run "<command>modprobe -a msdos</command>".</para>

	<indexterm><primary>modules.conf</primary><secondary>keep</secondary></indexterm>
	<indexterm><primary>modules.conf</primary><secondary>comment</secondary></indexterm>
	<indexterm><primary>modules.conf</primary><secondary>alias</secondary></indexterm>
	<indexterm><primary>modules.conf</primary><secondary>options</secondary></indexterm>
	<indexterm><primary>modules.conf</primary><secondary>path</secondary></indexterm>


	<para>Linux distros provide modprobe, insmod and depmod as a package called modutils or mod-utils.</para>

	<para>Before finishing this chapter, let's take a quick look at a piece of <filename>/etc/modules.conf</filename>:</para>

	<screen>
    #This file is automatically generated by update-modules
    path[misc]=/lib/modules/2.4.?/local
    keep
    path[net]=~p/mymodules
    options mydriver irq=10
    alias eth0 eepro
	</screen>

	<para>Lines beginning with a '#' are comments.  Blank lines are ignored.</para>
	
	<para>The <literal>path[misc]</literal> line tells modprobe to replace the search path for misc modules with the directory
	<filename role="directory">/lib/modules/2.4.?/local</filename>.  As you can see, shell meta characters are honored.</para>

	<para>The <literal>path[net]</literal> line tells modprobe to look for net modules in the directory <filename
	role="directory">~p/mymodules</filename>, however, the "keep" directive preceding the <literal>path[net]</literal> directive
	tells modprobe to add this directory to the standard search path of net modules as opposed to replacing the standard search
	path, as we did for the misc modules.</para>

	<para>The alias line says to load in <filename>eepro.o</filename> whenever kmod requests that the generic identifier `eth0' be
	loaded.</para>

	<para>You won't see lines like "alias block-major-2 floppy" in <filename>/etc/modules.conf</filename> because modprobe already
	knows about the standard drivers which will be used on most systems.</para>

	<para>Now you know how modules get into the kernel.  There's a bit more to the story if you want to write your own modules
	which depend on other modules (we calling this `stacking modules').  But this will have to wait for a future chapter.  We have
	a lot to cover before addressing this relatively high-level issue.</para>



		<sect2><title>Before We Begin</title>

			<para>Before we delve into code, there are a few issues we need to cover.  Everyone's system is different and everyone has
			their own groove.  Getting your first "hello world" program to compile and load correctly can sometimes be a trick.  Rest
			assured, after you get over the initial hurdle of doing it for the first time, it will be smooth sailing
			thereafter.</para>



				<sect3><title>Modversioning</title>

					<para>A module compiled for one kernel won't load if you boot a different kernel unless you enable
					<literal>CONFIG_MODVERSIONS</literal> in the kernel.  We won't go into module versioning until later in this guide.
					Until we cover modversions, the examples in the guide may not work if you're running a kernel with modversioning
					turned on.  However, most stock Linux distro kernels come with it turned on.  If you're having trouble loading the
					modules because of versioning errors, compile a kernel with modversioning turned off.</para>

				</sect3>



				<sect3 id="usingx"><title>Using X</title>

					<para>It is highly recommended that you type in, compile and load all the examples this guide discusses.  It's also
					highly recommended you do this from a console.  You should not be working on this stuff in X.</para>

					<para>Modules can't print to the screen like <function>printf()</function> can, but they can log information and
					warnings, which ends up being printed on your screen, but only on a console.  If you insmod a module from an xterm,
					the information and warnings will be logged, but only to your log files.  You won't see it unless you look through
					your log files.  To have immediate access to this information, do all your work from console.</para>

				</sect3>



				<sect3><title>Compiling Issues and Kernel Version</title>

					<para>Very often, Linux distros will distribute kernel source that has been patched in various non-standard ways,
					which may cause trouble.</para>

					<para>A more common problem is that some Linux distros distribute incomplete kernel headers.  You'll need to compile
					your code using various header files from the Linux kernel.  Murphy's Law states that the headers that are missing are
					exactly the ones that you'll need for your module work.</para>

					<para>To avoid these two problems, I highly recommend that you download, compile and boot into a fresh, stock Linux
					kernel which can be downloaded from any of the Linux kernel mirror sites.  See the Linux Kernel HOWTO for more
					details.</para>

					<para>Ironically, this can also cause a problem.  By default, gcc on your system may look for the kernel headers in
					their default location rather than where you installed the new copy of the kernel (usually in <filename
					role="directory">/usr/src/</filename>.  This can be fixed by using gcc's <literal>-I</literal> switch.</para>
					
				</sect3>	

		</sect2>

</sect1>



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