The Hello World Module

The following code is a complete "hello world" module:

#include <linux/init.h>
#include <linux/module.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);

• This module defines two functions,
  • one to be invoked when the module is loaded into the kernel ( $hello\_init$)
  • and one for when the module is removed ( $hello\_exit$).
• The $module\_init$ and $module\_exit$ lines use special kernel macros to indicate the role of these two functions. Another special macro
  ( $MODULE\_LICENSE$) is used to tell the kernel that this module bears a free license; without such a declaration, the kernel complains when the module is loaded.
• The $printk$ function is defined in the Linux kernel and made available to modules; it behaves similarly to the standard C library function $printf$. The kernel needs its own printing function because it runs by itself, without the help of the C library.
• The module can call $printk$ because, after $insmod$ has loaded it, the module is linked to the kernel and can access the kernel's public symbols (functions and variables).
• The string $KERN\_ALERT$ is the priority of the message. We've specified a high priority in this module, because a message with the default priority might not show up anywhere useful, depending on the kernel version you are running, the version of the $klogd$ daemon, and your configuration.
• You can test the module with the $insmod$ and $rmmod$ utilities, as shown below. Note that only the superuser can load and unload a module.

  [root@ozdogan week7]# make
  make -C /lib/modules/2.6.19.2/build M=/home/ozdogan/week7 modules
  make[1]: Entering directory `/usr/src/kernels/linux-2.6.19.2'
    CC [M]  /home/ozdogan/week7/hello.o
    Building modules, stage 2.
    MODPOST 1 modules
    CC      /home/ozdogan/week7/hello.mod.o
    LD [M]  /home/ozdogan/week7/hello.ko
  make[1]: Leaving directory `/usr/src/kernels/linux-2.6.19.2'
  % su
  root# insmod ./hello.ko
  Hello, world
  root# rmmod hello
  Goodbye cruel world
  root#
  

• For the above sequence of commands to work, you must have a properly configured and built kernel tree in a place where the makefile is able to find it (/usr/src/linux-2.6.10 in the example shown). According to the mechanism your system uses to deliver the message lines, your output may be different. In particular, the previous screen dump was taken from a text console; if you are running $insmod$ and $rmmod$ from a terminal emulator running under the window system, you won't see anything on your screen. The message goes to one of the system log files, such as /var/log/messages (the name of the actual file varies between Linux distributions).