A basic Linux Kernel Driver.

vi hello.c

and populate as

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);

Save the file.

vi Makefile

and populate as:

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