Writing a built in object for Linux Kernel?

Question

Everywhere I search for Linux Kernel Development, I get answers for creating Linux Kernel modules. Example

 /*
* hello−1.c − The simplest kernel module.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
int init_module(void)
{
printk(KERN_INFO "Hello world 1.\n");
/*
* A non 0 return means init_module failed; module can't be loaded.
*/
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "Goodbye world 1.\n");
}

Here, there is init_module and cleanup_module functions which i understand contains things to be executed when the kernel is initialized and cleaned up. There are made by adding obj-m += hello-1.c to the makefile.

But I dont want this. I want to add a built in program, not a driver, basically a service to facilitate cloud uploading of some data from the kernel level. I dont event want the module option for the program when compiling the kernel.

I understand for just programs I should use obj-y not obj-m. But there is no manual to write these kind of programs. Why? Am I missing something? Does these programs also have the init_module and cleanup_module functions even though they are not modules?

These are not programs. Just object files for the kernel. And you should modify some existing kernel files to call them! — Basile Starynkevitch
Just because your driver or whatever is a module, does not mean it has to be a dynamically loadable module. We often compile modules statically, as part of the kernel. Just consider your thing a device driver (for starters), and go read LDD3. Then start poring over the kernel sources, and see how the non-driver parts, like kernel tasks (see LANG=C LC_ALL=C ps axfu | sed -ne '1p; / \\_ \[/p' for the ones running on your own machine) differ. However, I do suspect you eventually find out your starting premise is wrong: all this is better done in userspace. — Nominal Animal

Santosh A Santosh A · Accepted Answer · 2015-01-13T07:14:10

For example consider that your source is under driver/new in the linux kernel source tree. You need to modify Makefile's under drivers and new to build your module statically into linux kernel.

Under drivers/Makefile add the below line at the end.

obj-y   += new/

Under drivers/new/Makefile add the below line at the end.

obj-y   += hello.o

After build the linux kernel. And load to see that your module has printed the printk messages using dmesg command.

Note: When building module statically into linux, change

int init_module(void)

to

int __init init_module(void)

and change

void cleanup_module(void)

to

void __exit cleanup_module(void)

Writing a built in object for Linux Kernel?

2 Answers