What is-a-computer-process-os

What is a Process
By: Manish Singh
Date – 17th
July, 2015

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A process as seen by the kernel :
process table details
task structure
inode
max no of processes’ management
pid allocation
related system calls :
o fork
o clone
o exec
o wait
File system calls :
how open files are maintained by a process
effect of fork and exec on the files and the task structure with respect
to opened files
open and close calls and their effect in the kernel
discussion on system calls like fcntl, flock, dup
COURSE DETAILS

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Threads :
creation
communication
maintenance by the kernel
Signals and their implementation in kernel :
kill
signal
pause
 alarm
Ipc calls related to :
shared memory
pipes and fifos
semaphores
message queues
Data structures maintained by the kernel :
• system calls responsible for creation and destruction

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Memory management :
swapping and demand paging
page tables
physical and virtual memory
Stress will be given to the effects on the shared memory.
Device drivers :
registration
loading and unloading of modules
mapping to file system functions
interrupt handling and bottom halves
Socket programming

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LINUX …
A Unix clone
Written from scratch by Linus Torvalds
with assistance from a loosely - knit
team of hackers across the Net.
Aims towards POSIX compliance.

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INTRODUCTION TO LINUX
KERNEL
Some of the salient features of the OS:
•Multi user, multi processor, each user can execute
several processes.
•Machine architecture hidden from the user, thus
making it an easy environment for programming.
•Uses a hierarchical file system that allows easy
maintenance and easy implementation
•Uses a consistent format for files, byte streams
etc, making application programs easier to write.
•Supports multiple executable formats - like a.out,
ELF, java)

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SYSTEM OVERVIEW
User
Applications
O/S Services
Linux Kernel
Hardware
User Applications - being used
by a general user.
O/S Services - various services
lik vi, sh, who etc.
Linux Kernel –the set of system
calls and the various services the
system provides to the applications.
Hardware - the underlying
hardware.
Linux kernel forms the heart of the
OS and shall be the area discussed.

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A Process :
•As seen by the user
•As seen by the kernel

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PROCESS
– from user’s view point

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The following system calls are used by the programmer :
fork, clone : to create a new process
exec : to run a different executable on the same process
exit : to end the execution of a process.
wait : to wait for a child process to complete execution.

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The fork system call :
int fork( )
 The fork system call creates a copy of the process that
executes the system call.
 The process executing the call is the parent process.
 The newly created process is the child process.
 The fork system call is called once (in the parent) but it
returns twice (once in the parent and once in the child).
 In the parent it returns the pid of the child.
 In the child it returns with value 0.

Process Description Horizon 12
 In case of failure it returns –1.
 The child is the copy of the parent, i.e., the same program
starts executing as two different processes.
 The address space of the parent is duplicated.
 Parent and the child share the following : text region, opened
files, pipes etc and have a unique copy of the data region and
the stack.
 Except for Process 0 all processes have a single immediate
parent.

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 The files that were open in the parent process before fork are
shared by the child process after fork.
 The child process has a new and unique pid.
 The child process has its own copies of the parent’s file
descriptors.
 The child’s memory pages are generated via copy-on-write
 File locks and pending signals are not inherited.
 The execution in the child continues from the point of fork
and continues till the main function ends.
To obtain details on the complete state of the newly created
child process, run info fork.

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main( ) {
int cpid, fd ; char ch[10] ;
fd = open(“TEST”, O_RDONLY) ; printf(“fd :: %dn”, fd) ;
read(fd, (void *)ch, 5) ; printf(“char read :: %c, %cn”, ch[3], ch[4]) ;
if(cpid = fork()) {
wait4(cpid, NULL, 0, NULL) ; printf(“par :: wait overn”) ;
close(fd) ;}
else {
printf(“i m chldn”) ;
sleep(3) ; printf(“chld exitingn”) ;
close(fd) ; }
}
The following code shows how the child process is sharing the
opened files of the parent process.

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The clone system call :
int clone(int (*fn)(void*arg),void* child_stack, int
flags, void * arg)
 This creates a new process
 The parent and child processes share the same memory pages,
table of file descriptors and the table of the signal handlers.
 The child process starts execution at the function fn ( args )
passed as argument to the system call.
 The child process terminates when this function fn ends and
the integer returned by fn is the exit code for the child
process.
 The child process may also terminate explicitly by calling
exit( ) or after receiving a fatal signal.

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 The main use of clone is to implement threads – multiple
threads of control in a program that run concurrently in a
shared memory space.
 The clone() system call leaves all memory management up
to the programmer. The first thing to be done is allocating
space for the stack of the new child thread with malloc().
 NOTE : clone should not be used if you are writing
portable code. it is a linux only system call. if you want
portable threads, use a POSIX threads implementation
such as LinuxThreads.]

int create_thread(int stack_size, int (*start_routine)(void*), void* arg) {
void* child_stack; int pid;
if(!stack_size) stack_size = 16384;
/* allocate some memory for a new stack */
if(!(child_stack = malloc(stack_size))) return 0;
/* stacks grow downwards on 99% of linux implementations so point to end of it */
child_stack = (void*)(stack_size + (char*)child_stack);
pid = clone(start_routine, child_stack,
CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND , arg);
if(pid < 0) { /* failed so free the new stack */
child_stack = (void*)((char*)child_stack - stack_size);
free(child_stack); } return pid; }
int thread(void* arg) { while(1) {
printf("threadn"); sleep(200); } }
int main(void) {
create_thread(0, thread, NULL);
while(1) { printf("mainn"); sleep(200); } }
Example : the following shows that the linux system call 'clone' can be
used to create a thread.

The exec system call :
int execve ( char *filename, char * argv[ ],
char * envp[ ])
 This executes a new program.
 It replaces the current program of the executing process with
the new program.
 It does not change the pid or the parent’s pid of the executing
process.
 Any signals that were set to terminate the original program
will terminate the new program also and the signals that were
ignored in the original program will be ignored by the new
program also.

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 Any signals that were set to be caught in the original
program are handled as per the default action in the
program.
This is because the new program does not contain the
signal handler function as was defined in the old program.

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Example :
main( )
{
char *args[] = {“cp”, “new.c”, “dd.c”, NULL} ;
execvp(“cp”, args) ;
printf(“exec failn”) ;
}

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main( ) {
char *args[3] ;
int i ;
args[0] = “./run” ;
args[1] = “hello” ;
args[2] = NULL ;
printf(“before execn”) ;
execvp(“./run”, args) ;
printf(“exec failedn”) ;
}
Example :
compile to a.out
main( int argc, char **argv) {
printf(“string sent is %sn”, argv[1]) ;
}
compile to run

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The wait system call :
pid_t wait4 ( pid_t pid, int * status, int
options, struct rusage *rusage
 This suspends execution of the current process until :
a child as specified by the pid argument has exited
OR
an unignored signal is delivered.
 If no child as specified by pid is existing at the time the call is
made, then the function returns immediately.
 It frees up the system resources that were used by the child.

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Example : The following code shows use of fork and wait :
main( )
{
int i, cpid ;
if(cpid = fork()) {
wait4(cpid, NULL, 0, NULL) ;
printf(“parent finished waitingn”) ;
}
else {
for(i = 0 ;i < 10; i++)
printf(“i am childn”) ; }
}

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The following have been taken from man wait(2) :
A child that terminates, but has not been waited for,
becomes a "zombie".
The kernel maintains a minimal set of information about
the zombie process (PID, termination status, resource
usage information) in order to allow the parent to later
perform a wait to obtain information about the child.
As long as a zombie is not removed from the system via a
wait, it will consume a slot in the kernel process table, and
if this table fills, it will not be possible to create further
processes.
If a parent process terminates, then its "zombie" children
are adopted by init process, which automatically performs a
wait to remove the zombies.

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The wait set of system calls are used to wait for state
changes in a child of the calling process, and obtain
information about the child whose state has changed.
Child's state change means :
– child terminated
– child stopped by a signal
– child resumed by a signal.
If child is terminated, when parent wait( )s, the system
releases the resources associated with the child.
Till the time a wait( ) is performed by the parent, the
terminated child remains in a "zombie" state.
If a child has already changed state, these calls return
immediately. Else they block the caller until either a child
changes state or a signal handler interrupts the call.

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Process priority :
 The nicer a process, the less CPU it will try to take from other
processes.
Thus higher the nice value, the lower the priority of the process.
 nice() function is used to modify the nice value of the calling
process.
 Only the superuser may specify a negative increment, or
priority increase.
Example :
#include <stdio.h>
#include <sys/resource.h>
main() { printf("%dn", getpriority(PRIO_PROCESS, 0)) ;
nice(-2) ; printf("%dn", getpriority(PRIO_PROCESS, 0)) ;
}
Execute the binary as a superuser. Eg, on ubuntu → sudo ./a.out

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 nice() becomes useful when :
– several processes are demanding more resources than the
CPU can provide.
– a higher priority process will get a larger chunk of the
CPU time than a lower priority process.
 If the CPU can deliver more resources than the processes are
requesting, then even the lowest priority process can get up to
99% of the CPU.
Nice value and static priority :
Conventional process's static priority = (120 + Nice value)
So user can use nice/renice command to change nice value in order
to change conventional process's priority.
By default, conventional process starts with nice value of 0 which
equals static priority 120.

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Scheduling priority depends on scheduling class.
Scheduling classes :
- SCHED_FIFO: A First-In, First-Out real-time process
- SCHED_RR: A Round Robin real-time process
- SCHED_NORMAL: A conventional, time-shared process
Most processes are SCHED_NORMAL
Using ps command, scheduling class can be obtained as follows :
ps -o class,cmd
Interpretation :
TS SCHED_OTHER (SCHED_NORMAL)
FF SCHED_FIFO
RR SCHED_RR

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Scheduling priorities :
For real-time process (SCHED_FIFO/SCHED_RR) :
– it is the real-time priority
– ranging from 1 (lowest priority) to 99 (higest priority).
For conventional process (SCHED_NORMAL ) :
– It is dynamic priority which depends on →
• It is static priority
ranging from 100 (highest priority) to 139 (lowest priority)
that is, (120 + (-20) ) to ( 120 + (19) )
AND
• Bonus
ranging from 0 to 10
is set by scheduler
depends on the past history of the process – it is related to the
average sleep time of the process.

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The ps command can be used to know the real-time priority and the
dynamic priority of the processes as follows :
ps -o class,rtprio,pri,nice,cmd
The sched_setscheduler( ) function can be used to set the
scheduling policy and the associated parameters for the process.
For conventional processes, the policy could be :
SCHED_OTHER the standard round-robin time-sharing policy
SCHED_BATCH for "batch" style execution of processes
SCHED_IDLE for running very low priority background jobs
For real-time processes, policy could be :
SCHED_FIFO a first-in, first-out policy
SCHED_RR a round-robin policy
In this case the sched_priority can be set to indicate real-time
priority.

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PROCESS
– the kernel side picture

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A process as an entity has the following features :
 It is the execution of a program.
 It consists of a pattern of bytes (interpreted by the CPU as
machine instructions ) – the text, data and stack.
 It executes by following a strict sequence of instructions that is
self contained.
 It can read and write its own data and stack but cannot access
the data and stack of other processes.
…contd.
General Features :

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 It has a life – spent partly in user mode and partly in system
mode.
 It is uniquely known by its process id ( pid). The pid
remains the same throughout the life of the process.
 It is a dynamic entity constantly changing as the machine
code instructions are executed by the CPU.
 It can communicate with the other processes running on the
system
…contd.

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It is allocated a special structure by the kernel (This is the
structure task_struct).
It holds an entry in the task table
On Redhat 6.0 the limit on the total number of processes is 512,
and the limit on the number of tasks per user is half of that.
Since Redhat 6.1, using the standard redhat supplied kernel, the
total number of process is 2560, and the max per user is 2048.
(If you need to increase the limits, you will need to modify the
/usr/src/linux/include/tasks.h file. The parameters to change are
NR_TASKS and MAX_TASKS_PER_USER.) In all cases, the
maximum value for these parameters is 4092.

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The program executed by a process during its life can be
changed.
Every process has a parent process.
The parent of a process may change during the life of a process.
There exist some special processes in the system which are
known as daemon processes.

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Daemon Processes :
 They execute in the background without an associated
terminal or login shell.
 They are started once when the system is initialized.
 Their lifetime is the entire time that the system is operating,
usually they do not die or get restarted later.
 They spend most of their time waiting for some event to
occur at which time they perform their service.
 They frequently spawn other processes to handle service
requests.
 There exist some special conditions that must be taken care
of while writing a daemon.

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Rules while writing Daemons :
 All unnecessary file descriptors must be closed.
 The current working directory must be changed to “/”.
A daemon would not require for example stdin,
stdout, stderr.
These file descriptors are inherited by the
daemon from its parent by default.
The current working directory is also inherited
by a process from its parent.
If the pwd of the daemon is a mounted
filesystem then it can not be unmounted as long
as the daemon is running.
 The daemon must do a fork and have the parent exit allowing
the actual daemon to run in the child process.
 The daemon must be dissociated from its process group.
This is required so that the daemon does not
receive the signals sent to the entire process
group.
This is necessary so as to disassociate the
daemon with a terminal. Thus it need not be
started as a background process.An exception to this rule are the daemons
which execute a chdir to the directory
where they do all the work.
 The daemon must execute umask(0) to reset the file access
creation mask.
A process inherits this mask from parent process.
Why the need ?
If the daemon created a file with mode 0660, so that only the user and
the group could read and write the file, but the umask value was 060,
the group read write permissions wuld be turned off.

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Process States :
STATES IN WHICH A PROCESS COULD BE :
1. The process is executing in USER MODE.
1. The process is executing in SYSTEM (or KERNEL) MODE.
1. The process is READY TO RUN waiting for the CPU.
1. The process is SLEEPING and is residing in the main
memory.
1. The process is ready to run but the swapper must swap the
process into main memory before it could be scheduled.
…contd.

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1. The process is sleeping and the swapper has swapped it to
secondary storage to make space for other processes.
1. The process is returning from the user to kernel mode when
the kernel preempts it and does a context switch to schedule
another process.
1. The process is newly CREATED. It is in a transition state –
neither it is ready to run nor is it sleeping.
1. The process has exited and is in a ZOMBIE STATE.

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8
2
9 7
34
56
1
fork
wakeup
Sleep, Swapped Ready to Run, Swapped
Created
Not enough memory
(swapping)
Preempted
Swap
out
Asleep
in
memory
wakeup
Swap
out
Swap
in
Enough memory
Sleep
Zombie
User Running
Ready to Run in Memory
exit
preempt
reschedule process
return to user
Kernel
Running
interrupt,
interrupt return
return
sys call,
interrupt`

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 A process is CREATED when a fork ( ) is done.
 A process switches from the USER MODE to KERNEL
MODE when a system call is executed.
 A process executing in KERNEL MODE can be preempted
only when it is about to return to the USER MODE
 During the execution of a system call if a process has to wait
for some system resource, e.g, for a disk I/O, the process goes
in the SLEEP state.
 This waiting / sleeping always occurs when the process is in
the kernel mode.
Process State Transition Details :

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Apart from this self giving up of the CPU by a process (while
waiting for a system resource), Linux also uses pre-emptive
scheduling.
Pre – emptive scheduling :
a process executes for a small amount of time
after this time another process is picked up to run
the original process waits till the CPU again selects it.
A runnable process is one waiting only for CPU to run.
The SCHEDULER selects the most deserving process to run
out of all the runnable processes in the system.

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Some Important Constants
HZ : no. of clock ticks received by the system in 1 second = 100
TASK_SIZE : user process space size = 0xC0000000
NR_TASKS : maximum number of processes in the system =
512 (in version 7) / 4098 (in version 9)
PID_MAX : maximum pid allocated to a process = 0x8000

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TASK_RUNNING :
 process is runnable
 process may or may not be running
 many processes could be in TASK_RUNNING
 only one process is running at any given time (for single
CPU) and it is marked by the global variable current
TASK_INTERRUPTIBLE :
 process is sleeping
 process can be interrupted
Process State Related Constants :

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TASK_UNINTERRUPTIBLE :
 process is sleeping
 process can not be interrupted
TASK_ZOMBIE :
 process has finished execution
 the parent of the process has not yet executed wait

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SCHED_FIFO : first in first out scheduling policy
SCHED_RR : round robin scheduling policy
SCHED_OTH : any other scheduling policy
Scheduling Policy Related
Constant

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 NSIG : total number of signals = 32 (in version 7)
64 (in version 9)
Signal Processing Related
Constants :

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.
.
.
.
.
.
task table
task_struct of idle task pid hash table

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The task_struct structure
(contents)
 state : TASK_RUNNING, TASK_INTERRUPTIBLE etc.
 priority :
priority given by scheduler to the process.
it is the amount of time for which the process will run for
when allowed.
effected by nice system call (a large nice value means a
low priority, priority can be incremented only by the
superuser, others can use it to decrease the priority of
processes
 time_slice :
it is the amount of time that the process is allowed to run
for
it is set to priority when the process is first run and is
decremented on each clock tick.

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unsigned long rt_priority : relative priority of a real time
process
tarray_ptr :
 pointer to task[ ] array linkage
 used to release the task slot when the process dies
policy : scheduling policy (SCHED_FIFO etc)
tty :
 terminal to which the task is associated.
 if 0, impies no terminal e.g., for a daemon process
p_opptr : original parent
p_pptr : parent
p_cptr : youngest child
p_ysptr : younger sibling
p_osptr : older sibling

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 fs : filesystem information
 files : information of all the open files of the process.
 mm : memory management information
 pid
 pgrp
 sig : holds action to be taken on the various trapped signals.
 signal : holds the signal number information of a received
signal
 pdeath_signal :
•the signal send to the task when its original parent dies.
•it is set by using the prctl system call, the value is cleared
upon a fork

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Related Global Data
 task [ NR_TASKS ] :
 this task vector is an array of pointers to every
task_struct structure in the system
 task [ 0 ] is the idle task which gets called when no other
task can run. It can not be killed and never sleeps. Its
state field is never used.
 init_task is the run queue which contains pointers to only
those tasks which are in the TASK_RUNNING state.
 pidhash [ PIDHASH_SZ ] :
 maintains a hash table of pids of the tasks

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What Happens During fork/clone
A new task_struct data structure is created from the system’s
physical memory.
The new task_struct is entered in the task vector.
The contents of the old process’s task_struct are copied in the
cloned task_struct.
The pid for the new task is obtained.
Pids keep increasing till the maximum limit is reached after which
again the kernel starts allocating (released) pids from the
beginning. For user processes the pids start from value 300
onwards. (All the lower pids are reserved for the daemon
processes)

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 The new process shares the resources of the parent process :
 process’s files and file system information
 signal handlers
 virtual memory
 All pending unhandled signals (inherited from the parent) are
deleted for the child.
 The start_time for the new process is set to jiffies (number of
clock ticks since the system started)
 The dynamic priority of the parent task ( held in counter ) is
shared between parent and child tasks so that the total amount
of dynamic prioirties in the system doesn’t change.

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 The following relationship related processing takes place :
 The parent pointers in the new task are set to current task.
 The child pointer of the new task is set to NULL.
 The new process has no younger sibling.
 the parent’s current youngest child becomes the older
sibling of the new process.
 The new process becomes parent’s youngest child.
 The new process is added to the run queue and its state is
marked as TASK_RUNNABLE.
 At this point the parent completes its part of fork( )
 The child task is later scheduled by the normal scheduling
algorithm. It then returns from the fork ( ) function.

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1st
fork
2nd
fork
3rd
fork
4th
fork
p_cptr
p_osptrp_osptr
p_ysptr
Relationship between the parent task and the
various child tasks
p_osptrp_osptr
NULL
p_ysptr p_ysptr
NULL
p_ysptr

57
kernel stack
kernel stack
open files
current directory
and root
open files
current directory
and root
files_struct
fs_struct
task_struct for parent
task_struct for child
mm_struct
mm_struct
shared text
user stack
user stack
child
data
parent
data

58
Difference Between fork and clone
Both the system calls basically have the same implementation
within the kernel.
The following differences are there between the two system
calls :
 clone allows the child process to share parts of its
execution context with its parent eg, memory space, file
descriptor table, signal handler table and the pid.
 The clone system call is thus providing support for
creating threads managed by the kernel while fork creates
a new task.

59
mm
files
virtual memory areas
inode
file desc tbl
parent process
mm
files
file desc tbl
child process

60
What Happens During an exit
 A task terminates by executing the exit( ) system call.
 A process may invoke exit ( ) as follows :
 explicitly : the startup routine calls exit ( ) when the
program returns from the main function.
 implicitly : the kernel may invoke exit ( ) internally
for a process on receipt of an uncaught signal.
 An exiting process enters the zombie state.
 An exiting process relinquishes its resources.
 The idle task task [ 0 ] with pid = 0 can not be killed

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During a process exit the following takes place :
 All task related memory is freed :
 All the open files of the task are closed by setting the file
descriptor array to NULL
 The task state is set to TASK_ZOMBIE.
 All the closest relatives are informed of the death of the
current task. It takes care of the following :
 for all the child tasks of the dying task, set parent to the
child_reaper ( the init process).

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 the parent task is notified of the death, the exit_signal of
the dying task(as stored in the task_struct of the dying
task) is send to the parent
 if the parent has issued a wait for the child (about to die),
then wake up the parent
Finally the scheduler is invoked to schedule another task.
During exit the dying process releases the memory it had
acquired accept for the task structure and its entries in the task
table and the pid hash table.

63
The wait system call
 This system call must be executed by the parent process for
every child in order to ensure that the child process does not
eat up the system resources as Zombies.
 During execution of wait either of the following is possible :
 the parent finds that there is a child that has already
finished execution, i.e., there is a child in zombie state
OR
 all the children are currently executing.
 In the first case the parent returns immediately releasing the
system resources held by this zombie child

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 In the latter case the parent goes in an interruptible sleep and
would be woken up when the child exits or some other signal is
delivered to this parent process.
Release of system resources of the child implies the following :
 the slot occupied by the process in the task table is released
and marked as free
 the task entry is removed from the pid hash table
 remove the child process from the run queue
 remove the child process from its siblings list (this will
effect the older and younger siblings maintained by both the
immediate siblings of this child process)
 the task structure held by the process itself is released

65
What Happens During an exec
 During an exec a new executable has to be loaded in place of
the old one on the same process.
 This happens by first flushing out the old executable. This
includes :
 A private copy of the signal table is created. (During a
fork – creation of a process, the child shares the signal
table of the parent.
 The unhandled signals of the old executing program are
cleared up.
 The signal table is cleared up.

66
 The old mmap stuff is released. The virtual memory
maps of the old executable are set to NULL and the
page tables are cleared up.
 Those file descriptors in the file descriptor table are
cleared for which the process has set the close-on-exec
flag (This can be done through the fcntl system call).
 The new executable is then loaded. For this the page tables
are updated for the process and the new virtual memory
map is generated.

67
Processing During System Startup
 The kernel goes through the various initializations required
for the different parts of the kernel.
 Then when the kernel is ready to ‘move to user mode’ it
starts the init process (details covered later) which has a pid =
1.
 Then for pid = 0, the kernel executes the idle task.
 This is now the Process number 0, the idle task, which
keeps running in an infinite loop.
 Whenever there’s nothing else to do, the scheduler will
run this idle process.

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 The ‘init process’ execution starts as follows :
 starts the following daemon processes :
• bdflush
• kpiod
• kswapd
 opens /dev/console in O_RDWR mode. As a result of
this fd = 0 (stdin) gets associated with the process.
 twice it dup()s fd = 0. Thus fd = 1 (stdout) and fd = 2
(stderr) also get associated with the process.
 finally the /sbin/init program is exec( )ed.

69
 Now that the init program has been
exec()ed, the kernel has no direct
control on the program flow.
 Now the kernel proceeds with
providing scheduling services
amongst the others to the alive
processes.
 The flow of control can now be seen
from the ‘process relationships fig’
 For each terminal to be activated, init
fork( )s a copy of itself.
 Each of these children exec( ) the
getty program.

70
 After a user logs on to one of the terminals, the getty exec( )s
the login program.
 The login then fork( )s and exec( )s the /bin/sh
 Now when a user command is entered, the sh first fork( )s a
copy of itself and then exec( )s the program corresponding to
the user command.

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init
/bin/sh
login
gettygettygetty
initinitinit
fork fork
fork
exec exec
exec
fork and exec
exec

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Exercises :
1) Write a program that behaves as follows :
The program maintains a file “login” which contains the
following information in the specified format -
usr1 logged in at hh:mm:ss on DD’ MM YY
usr1 logged out at hh:mm:ss on DD’ MM YY
total time taken is hh:mm:ss
usr2 logged in at hh:mm:ss on DD’ MM YY
usr2 logged out at hh:mm:ss on DD’ MM YY
total time taken is hh:mm:ss
Initially the program waits at the following prompt - login :
Whenever a user enters a name, the program proceeds.
Whatever command the user enters (the user enters the
command specifying the correct path) is executed by the
program and then the program waits for the next user
command. This continues till the user enters exit to exit.

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On exit the log out information is stored in the “login” file
and the program then waits for the next login.
The program itself terminates when the user name is entered
as exit.
1) Implement the system function.
1) Write a program in which a process creates 10 child
processes. Each of the child process is started by passing the
pid of its immediate older sibling – this child process
displays the pid of its immediate older sibling.
All the children should continue to run till the last child is
created.
The parent should wait for all its children to die out.

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