====== Linux - Kernel - Kernel Drivers - Hello World ======

A basic Linux Kernel Driver.

----

===== Create a C program =====

<code bash>
vi hello.c
</code>

and populate as

<file c hello.c>
#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);
</file>

Save the file.

----

===== Create a Makefile =====

<code bash>
vi Makefile
</code>

and populate as:

<file bash Makefile>
# To build modules outside of the kernel tree, we run "make"
# in the kernel source tree; the Makefile these then includes this
# Makefile once again.
# This conditional selects whether we are being included from the
# kernel Makefile or not.
ifeq ($(KERNELRELEASE),)

    # Assume the source tree is where the running kernel was built
    # You should set KERNELDIR in the environment if it's elsewhere
    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    # The current directory is passed to sub-makes as argument
    PWD := $(shell pwd)

modules:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

clean:
        rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

.PHONY: modules modules_install clean

else
    # called from kernel build system: just declare what our modules are
    obj-m := hello.o hellop.o
endif
</file>

----

===== Compile the module =====

<code bash>
make
</code>

displays:

<code bash>
make -C /lib/modules/4.15.0-72-generic/build M=/home/peter/prog/kernel/helloworld modules
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-72-generic'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-72-generic'
</code>

----

===== Check the module =====

<code bash>
ls hello,ko
</code>

displays:

<code bash>
hello.ko
</code>

----

===== Load the module =====

<code bash>
sudo insmod hello.ko
</code>

<WRAP info>
**NOTE:**  This will not display anything.
</WRAP>


----

===== Check if the module is loaded =====

<code bash>
lsmod
</code>

returns:

<code bash>
Module                  Size  Used by
hello                  16384  0
....
</code>



----

===== Check message from module =====

As mentioned above when the module was loaded it did not display that "Hello World" message.

This message is actually written to the system log.

<code bash>
tail /var/log/syslog
</code>

returns:

<code bash>
Dec 14 17:05:19 peter-bigmamba kernel: [74429.517770] Hello, world
</code>

----

===== Remove the module =====

<code bash>
sudo rmmod hello.ko
</code>

----