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

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?

When I `cat dmesg` I see lots of stuff, but nothing related to the module. I assume it should be in the end of file but not only its not in the end, its not in the middle or beginning either. — Varaquilex, Dec 09 '13 at 16:52
However, `cat kern.log` did the trick: ` Dec 9 18:42:46 Varaquilex kernel: [ 2314.937815] <1>Hello, world! Dec 9 18:42:49 Varaquilex kernel: [ 2317.873400] Goodbye, world.. ` — Varaquilex, Dec 09 '13 at 16:54

Varaquilex · Accepted Answer · 2013-12-09T21:03:07.280

5

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..

edited Dec 09 '13 at 21:03

answered Dec 09 '13 at 16:56

Varaquilex

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Note they aren't *necessarily* there; this is determined in `/etc/syslog.conf` (but look for `/etc/rsyslog.conf` first, that will take precedence if it exists). You'll find some reference to `kern.log` in that file. You'll also notice the priorities (1 = alert, etc) are used there. Strange this didn't end up in `dmesg` tho. – CodeClown42 Dec 09 '13 at 17:00
Hmmm -- notice `<1>Hello, world!`. Methinks the API has changed a bit since your reference was written, and that's perhaps not being processed as a priority, so the default is used. Something to investigate, anyway. – CodeClown42 Dec 09 '13 at 17:07
I looked up `rsyslog.conf`. Nothing related to `kern.log` there. When I search for `kern` there is only 1 line that has a match: `$ModLoad imklog # provides kernel logging support`. Perhaps this is the reference to `kern.log` file? – Varaquilex Dec 09 '13 at 17:08
Maybe, although the directives should still be directives; it could be that `imklog` includes its own defaults (although the same module is used here, but I have no `/var/log/kern.log`...). There's probably also an `/etc/rsyslog.d/` directory with stuff included by `rsyslog.conf`; those files may or may not contain much. – CodeClown42 Dec 09 '13 at 17:12
Also possible: your system isn't actually using `rsyslogd` -- try `ps -A | grep syslog`, you'll get either `syslogd` or `rsyslogd`. They use different conf files (hence `/etc/syslog.conf`). – CodeClown42 Dec 09 '13 at 17:17

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

1 Answers1