Building linux kernel module - c

Question

Im a windows driver programmer who is a complete newbie in linux kernel development.I have installed linux kernel headers. I am trying my helloworld module in linux kernel.
#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);
following is the code for my module. makefile for my build is
obj-m +=tryout.o
KDIR =/usr/src/linux-headers-4.13.0-37-generic
all:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
clean:
rm -rf *.o *.ko *.mod.* *.symvers *.order
but im getting
'fatal error: linux/init.h: No such file or directory while making this module'. What could be the possible reason? and how can i resolve it?

Answer 1

Your Makefile is mis-configured. In particular you used SUBDIRS whereas you're supposed to use M and your $(PWD) is meaningless, you should use pwd to be simple (or $$PWD); Here's how you should set it up:
ifneq ($(KERNELRELEASE),)
# kbuild part of makefile
obj-m := tryout.o
# any other c files that you would like to include go into
# yourmodule-y := <here> e.g.:
# tryout-y := tryout-1.o tryout-2.o
else
# normal makefile
KDIR ?= /usr/src/linux-headers-4.13.0-37-generic
# you really should set KDIR up as:
# KDIR := /lib/modules/`uname -r`/build
all::
$(MAKE) -C $(KDIR) M=`pwd` $#
# Any module specific targets go under here
#
endif
Configuring your makefile like this will allow you to simply type make in your module directory and it will invoke the Kernel's kbuild subsystem which will in-turn use the kbuild part of your Makefile.
Read up on https://www.kernel.org/doc/Documentation/kbuild/modules.txt for all the different permutations on how to do this. It comes with examples.