Module is the piece of binary which can be inserted and removed, to and from Kernel dynamically. I mean into the current running kernel. I may demonstrate simple examples here,

Hello World Module

Module Creation

$ mkdir dummy

$ cd dummy/

$ vim hello.c

/* hello.c */

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

static int hello_init(void)
{
printk(KERN_ALERT “Jack and jill went upto hill\n”);
return 0;
}

static void hello_exit(void)
{
printk(KERN_ALERT “to fetch a pail of water\n”);
}

module_init(hello_init);
module_exit(hello_exit);

Here, hello_init() will get executed when this module is inserted, hello_exit() will get executed when the  module is removed from Kernel.

Creation of Makefile

$ vim Makefile

obj-m += hello.o

KDIR = /usr/src/linux-headers-2.6.39-bpo.2-686-pae

all:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
rm -rf *.o *.ko *.mod.* *.symvers *.order

KDIR may point to path to linux kernel source directory, or only to kernel headers as I mentioned above.

Module Compilation

$ make

make -C /usr/src/linux-headers-2.6.39-bpo.2-686-pae SUBDIRS=/home/elcot/Desktop/Kernel_dd_training/felabs/linux/modules/dummy modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.39-bpo.2-686-pae’
Building modules, stage 2.
MODPOST 1 modules
make[1]: Leaving directory `/usr/src/linux-headers-2.6.39-bpo.2-686-pae’

As a result various files generated as below,

hello.ko

hello.mod.c

hello.mod.o

hello.o

Module.symvers

modules.order

insmod (inserting module inside kernel)

$ sudo insmod hello.ko

dmesg (checking output of module inside kernel)

$ dmesg

<output truncated>

[ 8044.317993] r8169 0000:01:00.0: eth5: link down
[ 9387.926290] r8169 0000:01:00.0: eth5: link up
[13132.548674] r8169 0000:01:00.0: eth5: link down
[14226.600851] r8169 0000:01:00.0: eth5: link up
[15710.119776] hello: module license ‘unspecified’ taints kernel.
[15710.120782] Jack and jill went upto hill

hello_init() function is executed when module is inserted into the Kernel.

rmmod (to remove module from kernel)

$ sudo rmmod hello.ko

again, cheking with dmesg 

$ dmesg

<output truncated> 

[ 8044.317993] r8169 0000:01:00.0: eth5: link down
[ 9387.926290] r8169 0000:01:00.0: eth5: link up
[13132.548674] r8169 0000:01:00.0: eth5: link down
[14226.600851] r8169 0000:01:00.0: eth5: link up
[15710.119776] hello: module license ‘unspecified’ taints kernel.
[15710.120782] Jack and jill went upto hill

[15750.777030] to fetch a pail of water

hello_exit() function is called, when the module is removed..

 Enjoy..!

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