akoskovacs/kmod_example

Kernel module example, using sysfs

Sysfs kernel module example project

This kernel module will create two files, under /sys/kernel/hello:

• hello, which contains the string 'Hello, world!'
• hello_num, which reads 500 by default

Both are writable by root. hello will dump it's contents to the kernel log, while hello_num will save the number given by the users to a variable.

How to build

 git clone https://github.com/akoskovacs/kmod_example.git
 cd kmod_example
 ./install-deps.sh
 make insert

Currently the install-deps.sh script can install all the neccessary dependencies only for the Raspbian Linux distribution.

The insert command will build the module using the current kernel's headers, then it will replace the any other module named hello (if any) with the newly built one.

Usage

Reading hello sysfs attribute

cat /sys/kernel/hello/hello

Will give the string Hello, world!, proving that the module does work.

Reading hello_num

cat /sys/kernel/hello/hello_num

Reads 500 initially when the module's internal variable is going live, but it can be modified later to any integer number.

Writing to hello

sudo sh -c "echo -n 'this is a test' > /sys/kernel/hello/hello" The sh -c is needed for sudo to work properly. If you are root, the echo -n 'this is a test' > /sys/kernel/hello/hello should also work.

The result could be seen in the kernel logs, by issuing: dmesg | tail, you would get:

[12449.884238] hello: Hello, I am a cool small module!
[12712.550500] hello: I got 'this is a test' :D

If the -n is not used the end quote mark will start in the next line due to the end line mark also written to the file.

Writing to hello_num

The line: sudo sh -c "echo -n '900' > /sys/kernel/hello/hello_num", will change the internal integer variable to 900 instead of the initial 500. This can be tested, by reading the sysfs attribute again: cat /sys/kernel/hello/hello_num