Linux Tips: Modules, Who Are They, And How Do They Work?

Reading time: 2 – 3 minutes

[ Via Migraine Heartache ] A very nice feature of the Linux kernel is the modules. Modules are pieces of software that can be loaded and unloaded from the kernel at runtime. That way you can save kernel memory (and that’s important for small machines, since kernel memory is never swapped out on disk), and keep your kernel clean when you’re not working with some of your hardware.

Especially sound and network cards are really good to compile as modules, since sometimes they are a bit hard to configure, and when you have a module, you don’t have to recompile and reboot your kernel to configure some irq or io setting.

The Tools Used

The tools you use to handle modules are all located in /sbin. They are insmod, modprobe, lsmod, depmod and rmmod.. Notice the names, and compare them to some often used unix commands such as ls and rm and you’ll immediatelly understand what each command will do for you. As always I recommend reading the manual pages. Anyway, here’s what they do:

  • lsmod – List the modules currently loaded. A completely harmless command, even a normal user can run it.
  • rmmod – Removes a loaded module from memory. Checks dependencies, ie if a module needs another module, the other module may not be unloaded before the first one.
  • insmod – Inserts a specific module in memory. You may give a path, or just a filename with or without extension. In the latter case, insmod will search some standard module paths to find the module.
  • modprobe – Inserts a module in memory, but checks dependencies. If you modprobe module A, and module A needs module B to run, modprobe will insert module B and then module A.
  • depmod – Calculates dependencies between modules needed by the other module commands. This is most probably done by your init scripts at system startup, but some time you may need to execute a depmod -a.
  • modinfo – ens dona informació sobre les funcions del mòdul, els paràemtres que suporta, etc.(se l’havien deixat)

How do I make modules?

When you compile your kernel, you choose to make things as modules, instead of into the kernel itself.

The modules will be installed when you do a make modules ; make modules_install after your kernel is compiled. They will reside in a directory called /lib/modules/xx.yy.zz where xx.yy.zz is your kernel version.