init.h file not found - c

Question

I was trying to do a hello world module from the book Linux Device Drivers.
but I got a fatal error from gcc,
fatal error: linux/init.h: No such file or directory
1 | #include<linux/init.h>
I've installed the linux headers with
sudo apt install linux-headers-$(uname -r)
it says:
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
linux-headers-5.10.0-9-amd64 is already the newest version (5.10.70-1).
The following package was automatically installed and is no longer required:
libc-devtools
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
My code:
#include<linux/init.h>
#include<linux/module.h>
static int hello_init(void)
{
printk(KERN_ALERT,"Hello world\n");
}
static void hello_exit(void)
{
printk(KERN_ALERT,"Goodbye, cruel world\n");
}
module_init(hello_init);
module_exit(hello_exit);
``

Answer 1

Like you, I try understanding linux device drivers, and I tried the first example (hello.c) :
All this happens because we're not in a proper environment, and I think we should use another linux system (a "mainline" one, like advised in the previous chapter if I'm not mistaken, this is important if you want to avoid any problem, or worse making your system unstable/losing important data with the examples shown in the book).
But I finally got the "hello world" working :
go in the examples/misc-modules directory (where "hello.c" resides)
edit Makefile and put all other modules in comment so you should only see "obj-m := hello.o" (put all others in lines below and starting with a "#")
execute command "make" and you should have your "hello.ko"
To see the messages of your module hello.ko, run dmesg -w in another terminal (these are kernel messages, they won't be seen on the standard console !)
rem : "hellop.ko" can also be build but it fails with seq.c (another error, implicit definition of a function, header missing or something like that...)