“Init” is the first program that gets executed when linux kernel boots up. I created a compressed root file system in cpio format that hold only init file inside to it.


To get latest stable kernel click here

For qemu emulator installation click here

Short story

  • compile minimal kernel
  • create init from scratch
  • test using qemu emulator

Big story

Compiling minimal Kernel

$ tar -xvf linux-3.8.5.tar.xz

$ cd linux-3.8.5/

$ make distclean

$ make defconfig

$ make

As a result of above steps you will get kernel image


Creating init from scratch

Create a fresh directory

$ mkdir test

$ cd test/

Create dummy.c file

$ vim dummy.c

FYI, content of dummy.c

#include <stdio.h>
int main(int argc,char *argv[])
char name[100];
printf(“Hello World…!\n”);
puts(“enter your name :”);
printf(“your name is : %s\n”,name);

You can judge it as simple program!

Yes my init will prompt me to enter my name and it will print back to me, nothing more than this.

I am compiling statically linked version of above program, and naming the executable as init

$ gcc -static dummy.c -o init

To create cpio.gz format of executable

$ echo init | cpio -o -H newc | gzip > initramfs.cpio.gz

Copy kernel image, inside test directory

$ cp ../linux-3.8.5/arch/x86/boot/bzImage .

Testing using Qemu emulator

Be aware of kernel image and initramfs.cpio.gz are in same directory, then type the below command

$ qemu-system-i386 -kernel ./bzImage -initrd ./initramfs.cpio.gz

you will get,

qemu-system-i386: pci_add_option_rom: failed to find romfile "efi-e1000.rom"

VNC server running on `::1:5900'

Open a new terminal and type the below command

xvnc4viewer : 5900

Kernel will boot inside qemu emulator, and prompt for name, after insertion of name, it will again print back, as we expected.

Something like,

Hello World…!

enter your name : 

Mohammed Audhil

your name is : Mohammed Audhil

Congrats..! You made kernel to work for you…!



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