Creating Hello World Kernel Module in Linux

September 26, 2007

2.1. Hello, World: The Simplest Module

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.

Example 2-1. hello-1.c

/* hello-1.c – The simplest kernel module.

*/

#include <linux/module.h> /* Needed by all modules */

#include <linux/kernel.h> /* Needed for KERN_ALERT */

int init_module(void)

{

printk(“<1>Hello world 1.\n”);

// A non 0 return means init_module failed; module can’t be loaded.

return 0;

}

void cleanup_module(void)

{

printk(KERN_ALERT “Goodbye world 1.\n”);

} 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. 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.

2.1.1. Introducing 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.

2.2. Compiling Kernel Modules

Kernel modules need to be compiled with certain gcc options to make them work. In addition, they also need to be

compiled with certain symbols defined. This is because the kernel header files need to behave differently, depending on

whether we’re compiling a kernel module or an executable. You can define symbols using gcc’s -D option, or

with the #define preprocessor command. We’ll cover what you need to do in order to compile kernel modules

in this section.

We use gcc’s -isystem option instead of -I because it tells gcc to surpress some

“unused variable” warnings that -W -Wall causes when you include module.h.

By using -isystem under gcc-3.0, the kernel header files are treated specially, and the warnings are

surpressed. If you instead use -I (or even -isystem under gcc 2.9x), the “unused variable”

warnings will be printed. Just ignore them if they do.

So, let’s look at a simple Makefile for compiling a module named hello-1.c:

Example 2-2. Makefile for a basic kernel module

TARGET := hello-1

WARN := -W -Wall -Wstrict-prototypes -Wmissing-prototypes

INCLUDE := -isystem /lib/modules/`uname -r`/build/include

CFLAGS := -O2 -DMODULE -D__KERNEL__ ${WARN} ${INCLUDE}

CC := gcc-3.0

${TARGET}.o: ${TARGET}.c

.PHONY: clean

clean:

rm -rf {TARGET}.oAs an exercise to the reader, compile hello-1.c and insert it into the kernel with insmod

./hello-1.o (ignore anything you see about tainted kernels; we’ll cover that shortly). Neat, eh? All modules

loaded into the kernel are listed in /proc/modules. Go ahead and cat that file to see that your module

is really a part of the kernel. Congratulations, you are now the author of Linux kernel code! When the novelty wares off,

remove your module from the kernel by using rmmod hello-1. Take a look at

/var/log/messages just to see that it got logged to your system logfile.

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