Vital Unix IPC Interview Preparation Guide

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A system that has been taken over using Remote Control
Software. Zombies are often used to send spam or to attack
remote servers with an overwhelming amount of traffic (a
Distributed Denial of Service Attack).

The processes like vhand, bdflush, sched are housed in
kernel file or /unix system which are known as daemons.
These files run in the background without users
request.These are created when the system boots up and
remains active till it shut down or hang. These are not
linked to any user or any terminal .We can't kill a daemon.

When command is given then unix os will fork the shell i.e
will creat a new process and will execute the command using
exec command...something like suppose you gave command "ls"
in the shell then...
fork();
exec(ls);
It will give you the result and after this the child process
will die.

There r two main advantage :
1.you can use console to execute your command.
2.More importantly if one is connected through remote
console and as there is no controlling terminal attached to
background process even if your terminal get disconnected
the process continues (with help of nohup in shell script
or by using setsid in C)

by calling system call fork() to create child process which
handles the other.

Inter Process Communication. IPC is used to pass information
between two or more processes.

Schemes are pipes, shared memory & semaphore.

Below are the different IPC methods

1. Semaphores
2. FIFO's (Also called Named Pipes)
3. Message Queues
4. Shared Memory

–Hardware Boot
–Read boot block (block 0) and load to memory
–Transfer control to kernel
–Kernel initialization
–Mount the root file system on / and create
environment for process 0
–Process 0 invokes process 1(init process)
–init process invokes process getty
–gettyinvokes login

The system calls for process management are
Fork() > create child process
Vfork() > create child process (copy-on-write only)
exec() > exec to do a different task
wait() > wait for a process to complete execution
kill() > to send a signal to a process
signal() > to handle a signal
sigaction() > handle signal
exit() > exit from a process execution
_exit() > same as exit() but with out clean up

and many more ................

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/ipc.h>
#include<sys/types.h>
int main()
{
int p1[2],p2[2],i;
char buf[5];
pipe(p1);
pipe(p2);
if(fork()==0)
{
printf("\n this is child(the input text is text)\n");
close(p1[0]);
close(p2[1]);
write(p1[1],"text",5);
read(p2[0],buf,5);
write(1,buf,5);
}
else
{
printf("\n this is parent(the output text is text)\n");
close(p2[0]);
close(p1[1]);
read(p1[0],buf,5)
for(i=0;i<4;i++)
buf[i]=to upper(buf[i]);
write(p2[1],buf,5)
}
}

a semaphor is nothing but a term used in unix for a
variable which acts as a counter.for instance there may be
times when two processes try to access the same file
simultaneously.in this event we must control the access of
the when the other process is accessing
this is done by assigning value to a semaphore.
the value of the semaphore is initialized by the 1st
process when the file is in access by it.when the 2nd
process try to access the file it checks the value of the
semaphore and if it finds the value as initialized it does
not access the file.

The Process id are sequentially numbered and the first process id created is ' PID 0', which loads the data structures and resources of Kernel which also forks the next process "INIT" which has the process id '1'.

zero(0)

The first process in Unix is INIT.

It's possible through both symbolic and hard links. However,
hard links are only possible if both source and destination
belong to the same volume.

Linking is also two types.

1>Hard link.
2>soft link.

For hard :-

ln <source> <destination>

for softlink :-

ln -s <source> <destnation>

Fork system call is used to create a new process. Fork is
called onces and returns twice.It will return 0 to the newly
created process (child process) and process id of child to
the calling process(parent process).

The child process gets copy of parents data, stack and heap
segement.The code segement is common for both.Both the
processes will resume execution from the command next to
fork.Scheduler decide which process to run first.

chmod 755 <filename>

File is identified by the I-node number in linux.Kernel uses
inode number to access the file. It is unique for the
perticular filesystem. Inode contains all the information
about the file like , file size, access permissions, time
stamp , uid , gid , pointers to data blocks..etc.

deadlock occurs because of
1.no preembtion
2.circular waiting i.e.. when A process is waiting for the
resources which are held by the procees B,which is waiting
for the resources engaged by process A.

setuid short for set user ID upon execution is Unix access
rights flag that allow users to run an executable with the
permissions of the executable's owner. This is often used to
allow users on a computer system to run programs with
temporarily elevated privileges in order to perform a
specific task. While the assumed user id privileges provided
are not always elevated, at a minimum that is specific.
setuid and setgid are needed for tasks that require higher
privileges than those which a common user has, such as
changing his or her login password. Some of the tasks that
require elevated privileges may not immediately be obvious,
though — such as the ping command, which must send and
listen for control packets on a network interface.

While booting, special process called the 'swapper'
or 'scheduler' is created by the Process-ID 0. The swapper
manages memory allocation for processes and influences CPU
allocation. The swapper in turn creates 3 children: the
process dispatcher, vhand, etc...

Real User id, Effective User Id, saved user id.
Real Group id, Effective Group Id, saved Group id.

fork is a system call used for creating child processes of a
parent process.it returns the process id of the created
child process.after that pid(process id ) is checked if it
is negative,it means no child process is created,pid==0
implies the id of the newly created process and pid>0 is the
id of child process given to the parent process.the
statements following fork system call are executed by both
the parent and child process.and one more thing,the parent
and child process have the exact copy of address space but
it exist separately for the two processes.

prints Hello world Hello world
All the statements after the call to fork() will be
executed twice

This should not allow to increase process priority.