2.1. Terminology

Loadable kernel modules are often called just kernel modules or just modules, but those are rather misleading terms because there are lots of kinds of modules in the world and various pieces built into the base kernel can easily be called modules. We use the term loadable kernel module or LKM for the particular kinds of modules this HOWTO is about.

Some people think of LKMs as outside of the kernel.

They speak of LKMs communicating with the kernel. This is a mistake; LKMs (when loaded) are very much part of the kernel. The correct term for the part of the kernel that is bound into the image that you boot, i.e. all of the kernel except the LKMs, is "base kernel." LKMs communicate with the base kernel.

In some other operating systems, the equivalent of a Linux LKM is called a "kernel extension."

Now what is "Linux"? Well, first of all, the name is used for two entirely different things, and only one of them is really relevant here:

• The kernel and related items distributed as a package by Linus Torvalds.
  
• A class of operating systems that traditionally are based on the Linux kernel.
  

Only the first of these is really useful in discussing LKMs. But even choosing this definition, people are often confused when it comes to LKMs. Is an LKM part of Linux or not? Though an LKM is always part of the kernel, it is part of Linux if it is distributed in the Linux kernel package, and not otherwise. Thus, if you have loaded into your kernel a device driver LKM that came with your device, you can't, strictly speaking, say that your kernel is Linux.

Rather, it's a slight extension of Linux. As you might expect, it is commonplace to use the name "Linux" approximately -- Lots of variations on Linux are in use and are widely distributed, and referred to as "Linux." In this document, though, we will stick to the strictest definition in the interest of clarity.

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