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
CC      := gcc-3.0
${TARGET}.o: ${TARGET}.c

.PHONY: clean

    rm -rf ${TARGET}.o

As 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.

Here's another exercise to the reader. See that comment above the return statement in init_module()? Change the return value to something non-zero, recompile and load the module again. What happens?