Compiling Kernel Modules

Kernel modules need to be compiled a bit differently from regular userspace apps. Former kernel versions required us to care much about these settings, which are usually stored in Makefiles. Although hierarchically organized, many redundant settings accumulated in sublevel Makefiles and made them large and rather difficult to maintain. Fortunately, there is a new way of doing these things, called kbuild, and the build process for external loadable modules is now fully integrated into the standard kernel build mechanism. To learn more on how to compile modules which are not part of the official kernel (such as all the examples you'll find in this guide), see file linux/Documentation/kbuild/modules.txt

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

obj-m += hello-1.o  all:  make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules  clean:  make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

From a technical point of view just the first line is really necessary, the "all" and "clean" targets were added for pure convenience.

Now you can compile the module by issuing the command make . You should obtain an output which resembles the following:

$ makemake -C /lib/modules/2.6.11/build M=/root/lkmpg-examples/02-HelloWorld modules

make[1]: Entering directory `/usr/src/linux-2.6.11'   CC [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.o  Building modules, stage 2.   MODPOST   CC      /root/lkmpg-examples/02-HelloWorld/hello-1.mod.o   LD [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.ko make[1]: Leaving directory `/usr/src/linux-2.6.11'

$

Note that kernel 2.6 introduces a new file naming convention: kernel modules now have a .ko extension (in place of the old .o extension) which easily distinguishes them from conventional object files. The reason for this is that they contain an additional .modinfo section that where additional information about the module is kept. We'll soon see what this information is good for.

Use modinfo hello-*.ko to see what kind of information it is.

$ modinfo hello-1.ko filename:       hello-1.ko vermagic:       2.6.11 preempt PENTIUMII 4KSTACKS gcc-3.3 depends:

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