Why doesn't my hello world driver module print anything? - c

Question

I'm very new to kernel module programming and right now I'm trying to run the most basic hello world module program, however I could not get any output.
I have written the hello world program introduced in Linux Device Drivers 3rd ed and got some help from this website and this one.
hello.c
#include <linux/module.h>
#include <linux/kernel.h>
MODULE_LICENSE("GPL");
static int hello_init(void){
printk("<1>Hello, world!\n");
return 0;
}
static void hello_exit(void){
printk(KERN_ALERT "Goodbye, world..\n");
}
module_init(hello_init);
module_exit(hello_exit);
The file is in /home/volkan/drive directory. Along with the c file, I have my Makefile
Makefile
obj-m += hello.o
From the terminal, I execute this command for compiling the module:
sudo make -C /lib/modules/3.8.0-19-generic/build M=/home/volkan/drive/ modules
Resulting in:
make: Entering directory `/usr/src/linux-headers-3.8.0-19-generic'
CC [M] /home/volkan/drive/hello.o
Building modules, stage 2.
MODPOST 1 modules
CC /home/volkan/drive/hello.mod.o
LD [M] /home/volkan/drive/hello.ko
make: Leaving directory `/usr/src/linux-headers-3.8.0-19-generic'
I assume up to this point, nothing went wrong. Now, I insert my module and then remove:
volkan#Varaquilex ~/drive $ sudo insmod ./hello.ko
volkan#Varaquilex ~/drive $ sudo rmmod hello
volkan#Varaquilex ~/drive $
There is no output. I also have little experience in linux, so explanatory answers are more than welcome. Am I doing something wrong? Why cannot I see any output?

Answer 1

The kernel messages are logged in the kern.log file located in /var/log. Depending on your system, it may also be in dmesg. So you have to cat accordingly.
Use the command cat /var/log/kern.log
Dec 9 18:51:10 Varaquilex kernel: [ 2818.079572] <1>Hello, world!
Dec 9 18:55:02 Varaquilex kernel: [ 3050.256134] Goodbye, world..