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Modules provide a cleaner interface for writing drivers. To some extent, they require the code to be somewhat modular and to follow a certain coding convention. (Note that this doesn't actually prevent a programmer from abusing the convention and writing nonmodular code. Once the module has been loaded, it is just as free to wreak havoc as if it were compiled directly into the kernel.) Using modules makes drivers easier to debug; you can simply unload a module, recompile it, and reload it without having to reboot the system or rebuild the kernel as a whole. Modules can be used for other parts of the kernel, such as filesystem types, in addition to device drivers.

Most device drivers, and a lot of other kernel functionality under Linux, are implemented as modules. One of them is the parallel port driver for PCs (or parport_pc driver), for devices that connect to the parallel port (there are also additional drivers for special devices such as printers that report back status to the computer). If you plan to use this driver on your system, it is good to know how to build, load, and unload modules. Although nothing is stopping you from compiling this module statically into your kernel, a parallel port driver is something that you need only rarely (whenever you print to a directly connected printer, maybe a couple of times a day), and its driver shouldn't occupy valuable RAM during the times it is not needed. See the Linux Printing HOWTO for more about parallel port printing.

Installing the Kernel

Now we'll talk about how to load and unload modules from the kernel. The first thing you'll need is the module-init-tools package, which contains the commands used to load and unload modules from the kernel. On the FTP archive sites, this is usually found as module-init-tools-versionnumber.tar.bz2 in the directory where the kernel sources are kept. This package contains the sources to the commands insmod, modprobe, rmmod, and lsmod. All reasonably recent Linux distributions include these commands (found in sbin) already, so if you already have these commands installed, you probably don't need to get the modules package. However, it can't hurt to get the package and rebuild these commands to be sure that you have the most up-to-date version.

To rebuild these commands, unpack module-init-tools-versionnumber.tar.bz2 (say, in a subdirectory of /usr/src). Follow the installation instructions contained there; usually all you have to do is execute make followed by make install (as root). The three commands will now be installed in /sbin and will be ready to use.

Compiling Modules

A module is simply a single object file containing all the code for the driver. For example, the parport_pc module might be called parport_pc.ko. On most systems, the modules themselves are stored in a directory tree below /lib/modules /kernelversion, where you can find different directories for the various types of modules. For example, the modules compiled for the 2.6.8 kernel would be below /lib/modules/2.6.8. You might already have a number of modules on your system; check the appropriate directory. Notice that kernel modules, unlike other compiled object files, have the filename extension .ko to show their status as kernel modules. If you are running an older version of Linux, your modules might still have the extension .o.

Modules can be either in the kernel sources or external to it. The former is the case for those device drivers, filesystems, and other functionality that are used most often and are maintained as part of the official kernel sources. Using these modules is very easy: during the make config, make menuconfig, or make xconfig step, select to build a certain feature as a module. Repeat this for everything you want to compile as a module. Then, after the make bzImage step, execute the following commands:

# make modules

#make modules_install

This will compile the modules and install them in /lib/modules/kernelversion.

New modules that are not yet integrated