Topic 6 - Process & Job Control 1

Question 1

All processes are handled and managed by:

Answer 1

the kernel through a scheduling services called the scheduler

Question 2

In a single-processor computer, how many processes can be running at a time?

Answer 2

Only one process

Question 3

What are the statuses a process can have at a time?

Answer 3

Either one of three:

1) Running
2) Ready to run
3) Blocked or sleeping

Question 4

What does a Running Process mean?

Answer 4

Is executing for a short slice of time only

Question 5

What does a Ready Process mean?

Answer 5

It could use the CPU immediately if it is allowed to do so.

The scheduler decides which of the ready processes to run next

Question 6

What does a Blocked or Sleeping process mean?

Answer 6

Waiting for an event to occur, e.g., I/O or signal

Question 7

Can you go from:

1) Running to Ready to Run?
2) Running to Blocked / Sleeping?
3) Ready to Run to Running?
4) Ready to Run to Blocked / Sleeping?
5) Blocked / Sleeping to Running?
6) Blocked / Sleeping to Ready to Run?

Answer 7

1) Yes
2) Yes
3) Yes
4) No
5) No
6) Yes

Question 8

How does the system keep track of all the processes?

Answer 8

A process table

Question 9

What is the first thing that happens when a process is created?

Answer 9

A process ID is created, or simply, PID.

Question 10

Within Unix, every object is represented by?

Answer 10

Either a file or a process.

Question 11

What maintains the process table?

Answer 11

Maintained by the kernel

Question 12

What are the characteristics of a process table?

Answer 12

1) Indexed by the PID

2) Contains one entry for each process to store its attributes

Question 13

List the 3 things a scheduler does:

Answer 13

1) Maintains a list of all the processes waiting to execute
2) Chooses one process at a time, and gives it change to run for short time called time slices (typically 10 milliseconds).
3) Saves data related to the interrupted processes so that they can be resumed later

Question 14

What happens when a process needs the kernel to perform a service?

Answer 14

It sends a request (a system call) which will create new a process.

Question 15

System calls provides an __________ between________ and _______.

Answer 15

an essential interface between a process and the operating system

Question 16

What is the rule of process creation?

Answer 16

Every created process is a child process where it was generated from another parent process. (Except the first process during boot up)

Question 17

There are 4 types of System Calls:

Answer 17

1) File-related calls
2) Information-related calls
3) Communication-related calls
4) Process-related calls

Question 18

Define:

1) File-related calls
2) Information-related calls
3) Communication-related calls
4) Process-related calls

Answer 18

1) Opening- closing reading from or writing file
2) Information related calls - getting date / time
3) interprocess communication
4) Creating, executing, stopping a process

Question 19

Some of the system calls that co-ordinate processes include the following. Describe them.
Fork:
Exec:
Wait:
Exit:
Kill:

Answer 19

Fork - creates a copy (child) of current process (parent)
Exec - changes the program that a process is running
Wait - forces a process to pause
Exit - terminate a process
Kill - send a signal to another process

Question 20

What are the two types of commands?

Answer 20

1) Internal Commands

2) External Commands

Question 21

Define Internal Commands

Answer 21

Intercepted directly by the shell (no need to create a new process)

Question 22

Define External commands

Answer 22

Required the shell to run a separate program (need to create a new process)

Question 23

How do you run an external command?

Answer 23

Use the fork system call to create a duplicate of the currently running program. Key point here is currently running program is duplicated.

Question 24

The fork() command returns what?

Answer 24

• Returns the child PID (a positive number) to the parent process
• 0 to the child process
• A negative value, the creation of a child process was unsuccessful

Question 25

How do you identify a child process?

Answer 25

Fork() returns 0

Question 26

The child process uses an exec system call to:

Answer 26

to change itself from a process running the shell into a process running the external command

Question 27

At the same time a child process is made & running, what is the parent process doing?

Answer 27

- identifies itself by a positive value from the return value of the fork - Uses the Wait system call to pause itself until the child is finished the execution

Question 28

Go through the steps when a child process finishes execution.

Answer 28

1) Uses Exit system call a) send a signal to the parent process indicating the it is done b) go to a zombie state waiting for the terminating status to be accepted 2) Parent process receives a wake up signal from child a) Informs kernel to do 2 things; 1) De-allocate the child's used resources 2) Removes the child PID from the process table, killing the child

Question 29

What does Process#0 do?

Answer 29

- Initializes many data structures needed by the kernel. | - At the end of the boot procedure, process#0 forks process#1 and then disappears by terminating itself

Question 30

How is Process#0 created?

Answer 30

kernel creates it by hand without forking

Question 31

What is another name for Process#1?

Answer 31

the init process

Question 32

What does Process#1 / the init process do?

Answer 32

Carries out the rest of the steps that are necessary to set up the kernel and finishes the boot process (in doing so, many processes are created by init).

Question 33

What is the only living ancestor of all other processes in the system?

Answer 33

The init process (process#1)

Question 34

Describe process#1 besides what it does.

Answer 34

- Is the first process in the process table - stays there until the system is shut down - Is the only living ancestor of all other processes in the system

Question 35

What happens when a parent process unexpectedly dies? Does the child die prematurely? Do we have a rogue child process that lives on forever without a parent to kill it and keeps resources hostage?

Answer 35

- Called an orphan - The child will keep executing until its done. - Process#1 automatically adopts all orphan process and receives the control from the child process when its finished executing it - Informs the kernel to de=allocate the child used resources and kill it

Question 36

Define: job

Answer 36

A job refers to all the processes that are necessary to execute an entire command line.

Question 37

How do you run a command in the background?

Answer 37

Use the "&" symbol at the end of the command line.

Question 38

How do you send a signal to a process?

Answer 38

Using the Kill command

Question 39

What is displayed / seen by the user when a background command is given to the shell?

Answer 39

1) The shell will display the job number (job ID) that identifies the command 2) display the Process ID (PID) number for each running process in the background 3) Give you another prompt

Question 40

Describe the Kill command:

Answer 40

A command used to send a signal to a process. | It is a wrapper around the kill() system call, which sends signals to processes on the system.

Question 41

Give examples of Standard signals.

Answer 41

``` SIGHUP SIGINT SIGKILL SIGTERM SIGSTOP SIGCONT ```

Question 42

Define: SIGHUP

Answer 42

hangup - sent to process when you logout or if your terminal is disconnected

Question 43

Define: SIGINT

Answer 43

Interrupt: sent when you press ^C

Question 44

Define: SIGKILL

Answer 44

Kill: immediate termination (Can not be trapped or ignored by a process)

Question 45

Define: SIGTERM

Answer 45

Terminate: request to terminate; (Can be trapped or ignored by a process)

Question 46

Define: SIGSTOP

Answer 46

Stop (suspend): sent when you press ^Z

Question 47

Define: SIGCONT

Answer 47

Continue: resume suspended process; usually sent by the fg or bg commands

Question 48

If no single is specified in the kill command, by default, what happens?

Answer 48

SIGTERM is sent - *Requests* that the process to exit

Question 49

Which processes can be intercepted or even ignored?

Answer 49

All signals except of SIGKILL and SIGSTOP.

Question 50

SIGKILL and SIGSTOP are only seen by the ______

Answer 50

The host system's kernel, providing reliable ways of controlling the execution of processes.

Question 51

What are the requirements for a process to successfully send a signal to another?

Answer 51

Essentially, for a process to successfully send a signal to another, the owner of the signalling process must be the owner of the receiving process or be the super-user

Question 52

Can Ctrl-C interrupt key abort a background process?

Answer 52

No, only a kill command can. Ctrl-C can only abort a foreground process.

Question 53

How do you specify a job ID in the command line?

Answer 53

%4 (where "4" is a hypothetic job ID)

Question 54

What does the command "bg" mean?

Answer 54

Lists background jobs. Without job number means, the most recent stopped job.

Question 55

What does the fg command do?

Answer 55

Transfer a process from the background to the foreground and make it the current process.

Question 56

If a background task sends output to the standard output, or standard error, and you do not redirect it - what happens?

Answer 56

output appears on the screen, even if you are running another job

Question 57

If a background task requests input from the standard input, and you have not redirected the standard input - what happens?

Answer 57

Depends on the shell type: - bourns-style shells: supplies a null string - Suspends the job and waits for you to give it input.

Question 58

A process is a program that is started by:

Answer 58

Either a user or the system