A Technical Odyssey

You’ve got your kernel module code ready and you want to build it. The following is a simple kernel module makefile which will help you do this:


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  obj-m += hello-1.o

all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules (1)

clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean (2)

1	modules is the target to build the kernel module
2	clean is the target to clean the kernel module

The above makefile will fail if used directly please replace the spaces at lines 4 and 7 with TABS or please download the following from here makefile1.

In this explanation there are two makefiles which I will be referring to. The first is the one shown above and the second is the main kernel makefile which is located in the kernel source directory.

When you compile the kernel the make process assembles a list of kernel drirvers and modules that are to be compiled as loadable kernel modules defined by obj-m.

In our example above we indicate that we want to create a kernel module out of the hello-1.o object file which is eventually built from our hello-1.c source code.

There are two targets for the makefile i.e. all and clean. They invoke the 'modules' and 'clean' targets of the main kernel makefile.

Let’s understand the option switches.

The -C option switches the make command to the kernel source directory. Basically this is the directory of the source code used to build the kernel image with the main kernel makefile. Here $(shell uname -r) is a convenient way of specifying the name of the live kernel being run when the make command is executed.

The M option tells the main kernel makefile to go to the present working directory and read the makefile present there. This is a way of telling the main kernel makefile to read the the makefile shown above. This enables the main kernel makefile to add the hello-1.o object to its list of modules to be built.

Here $(PWD) is a convenient way of specifying the present working directory.

More information is available at:
http://www.tldp.org/LDP/lkmpg/2.6/html/x181.html

Kernel Module Makefiles

by Conrad Gomes on February 3, 2013