When the first caveman programmer chiseled the first program on the walls of the first cave computer, it was a program to paint the string `Hello, world' in Antelope pictures. Roman programming textbooks began with the `Salut, Mundi' program. I don't know what happens to people who break with this tradition, but I think it's safer not to find out. We'll start with a series of hello world programs that demonstrate the different aspects of the basics of writing a kernel module.
Here's the simplest module possible. Don't compile it yet; we'll cover module compilation in the next section.
Kernel modules must have at least two functions: a "start" (initialization) function called
init_module() which is called when the module is insmoded into the kernel, and an "end" (cleanup)
function called cleanup_module() which is called just before it is rmmoded. Actually, things have
changed starting with kernel 2.3.13. You can now use whatever name you like for the start and end functions of a module, and
you'll learn how to do this in Section 2.3. In fact, the new method is the preferred method. However, many
people still use init_module() and cleanup_module() for their start and end
functions.
Typically, init_module() either registers a handler for something with the kernel, or it replaces
one of the kernel functions with its own code (usually code to do something and then call the original function). The
cleanup_module() function is supposed to undo whatever init_module() did, so the
module can be unloaded safely.
Lastly, every kernel module needs to include linux/module.h. We needed to
include linux/kernel.h only for the macro expansion for the
printk() log level, KERN_ALERT, which you'll learn about in Section 2.1.1.
printk()Despite what you might think, printk() was not meant to communicate information to the user,
even though we used it for exactly this purpose in hello-1! It happens to be a logging
mechanism for the kernel, and is used to log information or give warnings. Therefore, each printk()
statement comes with a priority, which is the <1> and KERN_ALERT you see.
There are 8 priorities and the kernel has macros for them, so you don't have to use cryptic numbers, and you can view them
(and their meanings) in linux/kernel.h. If you don't specify a priority level, the
default priority, DEFAULT_MESSAGE_LOGLEVEL, will be used.
Take time to read through the priority macros. The header file also describes what each priority means. In practise, don't use number, like <4>. Always use the macro, like KERN_WARNING.
If the priority is less than int console_loglevel, the message is printed on your current
terminal. If both syslogd and klogd are running, then the message will also
get appended to /var/log/messages, whether it got printed to the console or not. We use a high
priority, like KERN_ALERT, to make sure the printk() messages get printed to your
console rather than just logged to your logfile. When you write real modules, you'll want to use priorities that are
meaningful for the situation at hand.