The Linux kernel will automatically call "init." init will in turn launch any other processes. You can add a process in by creating a script for it in the appropriate run level.
Kernel mode is considered a 'privileged' mode, meaning that code executing in that mode can have access to any part of the system, memory, devices, etc. There are no limitations on what it can do....
include<stdio.h>
include<conio.h>
void main()
{
int age;
clrscr();
printf("Enter the Age:");
scanf("%d",&age);
printf("Your age is %d",age);
getch();
}