Makefiles for Kernel Modules

A kernel module is not an independant executable, but an object file which will be linked into the kernel in runtime. As a result, they should be compiled with the -c flag. Also, all kernel modules have to be compiled with certain symbols defined.  

• __KERNEL__ -- This tells the header files that this code will be run in kernel mode, not as part of a user process.  

• MODULE -- This tells the header files to give the appropriate definitions for a kernel module.  

• LINUX -- Technically speaking, this is not necessary. However, if you ever want to write a serious kernel module which will compile on more than one operating system, you'll be happy you did. This will allow you to do conditional compilation on the parts which are OS dependant.  

There are other symbols which have to be included, or not, depending on the flags the kernel was compiled with. If you're not sure how the kernel was compiled, look it up in /usr/include/linux/config.h      

• __SMP__ -- Symmetrical MultiProcessing. This has to be defined if the kernel was compiled to support symmetrical multiprocessing (even if it's running just on one CPU). If you use Symmetrical MultiProcessing, there are other things you need to do (see chapter 12).  

• CONFIG_MODVERSIONS -- If CONFIG_MODVERSIONS was enabled, you need to have it defined when compiling the kernel module and and to include /usr/include/linux/modversions.h. This can also be done by the code itself.    

ex Makefile   

 
# Makefile for a basic kernel module

CC=gcc
MODCFLAGS := -Wall -DMODULE -D__KERNEL__ -DLINUX

hello.o:	hello.c /usr/include/linux/version.h
		$(CC) $(MODCFLAGS) -c hello.c
		echo insmod hello.o to turn it on
		echo rmmod hello to turn if off
		echo
		echo X and kernel programming do not mix.
		echo Do the insmod and rmmod from outside X.

So, now the only thing left is to su to root (you didn't compile this as root, did you? Living on the edge^1.1...), and then insmod hello and rmmod hello to your heart's content. While you do it, notice your new kernel module in /proc/modules.        

By the way, the reason why the Makefile recommends against doing insmod from X is because when the kernel has a message to print with printk, it sends it to the console. When you don't use X, it just goes to the virtual terminal you're using (the one you chose with Alt-F<n>) and you see it. When you do use X, on the other hand, there are two possibilities. Either you have a console open with xterm -C, in which case the output will be sent there, or you don't, in which case the output will go to virtual terminal 7 -- the one `covered' by X.            

If your kernel becomes unstable you're likelier to get the debug messages without X. Outside of X, printk goes directly from the kernel to the console. In X, on the other hand, printk's go to a user mode process (xterm -C). When that process receives CPU time, it is supposed to send it to the X server process. Then, when the X server receives the CPU, it is supposed to display it -- but an unstable kernel usually means that the system is about to crash or reboot, so you don't want to delay the error messages, which might explain to you what went wrong, for longer than you have to.