Question

I am following the following tutorial, trying to learn how to develop device drivers, and in Chapter 2, the focus is to develop a working module and insert it into the kernel. I used the following code (hello.c):

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("Dual BSD/GPL");

static int hello_init(void)
{
    printk(KERN_ALERT "Hello World!\n");
    return 0;
}

static void hello_exit(void)
{
    printk(KERN_ALERT "Goodbye, cruel world!\n");
}

module_init(hello_init);
module_exit(hello_exit);

And my this is the Makefile:

obj-m += hello.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

I then run the following in LXTerminal:

brian@brian-desktop:~/driver_stuff/hello$ su
root@brian-desktop:/home/brian/driver_stuff/hello# make
make -C /lib/modules/2.6.32-21-generic/build M=/home/brian/driver_stuff/hello modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.32-21-generic'
Building modules, stage 2.
MODPOST 1 modules
make[1]: Leaving directory `/usr/src/linux-headers-2.6.32-21-generic'
root@brian-desktop:/home/brian/driver_stuff/hello# insmod ./hello.ko
root@brian-desktop:/home/brian/driver_stuff/hello# rmmod hello
root@brian-desktop:/home/brian/driver_stuff/hello# exit

However, after the insmod ./hello.ko command, one should expect that the terminal would print “Hello world!” and then “Goodbye, cruel world!” after the rmmod hello command. The book mentioned that this happens when you run the commands in the console, but not in an emulated terminal, could this be the problem?

I also checked under /var/log/messages and /var/log/messages.1 which had no record of either “Hello World!” nor “Good bye, cruel world!”. Is it possible that these messages are in a different file, or is the issue that the messages aren’t being pushed to the kernel in the first place?

If you need an info about the kernel I am running (Lubuntu 10.04, inside a VM):

brian@brian-desktop:~/driver_stuff/hello$ uname -r
2.6.32-21-generic

Thank you.

Answer 1

The output from kprintf is always to the kernel log. This may happen to also go to the system console (and, therefore, to the terminal you’re using if you’re on the console), but there’s nothing to link it back to the specific terminal that you loaded the module in.

To read the kernel log, run dmesg.