Operating Systems

Question 1

Interface

Answer 1

Describes what a computer program does and how it interacts with the rest of programs

There is nothing more fundamental in computer science than Interfaces.

Question 2

Abstraction

Answer 2

An interface to application programmers that separates policy—what the interface commits to accomplishing—​from mechanism—how the interface is implemented.

Question 3

Who is responsible for intra-process communication?

Answer 3

The software developer (You)

Question 4

Who is responsible for inter-process communication?

Answer 4

The Operating System

Question 5

Are Thread stacks tipically private (can only be seen by the owner Thread) or public (can be seen by all Threads in a process?

Answer 5

Private

Question 6

Mechanisms of Thread intra-process communication

Answer 6

Shared memory
Share open file handles
Static and dynamically-allocated global variables

Question 7

Mechanisms of Thread inter-process communication

Answer 7

Shared files 
Sockets
Exit codes
Signals
Pipes
Shared memory

Question 8

Can a Thread from one Process contact a Thread from another Process to request/send resources?

Answer 8

Only with mutual consent from both Processes, meaning a mechanism for the communication to happen (i.e. sockets, shared files…) was previously set .

Question 9

The only thing that can ‘run’ on a system

Answer 9

Thread

Question 10

When is a Process considered running?

Answer 10

When at least one of its Threads is running

Question 11

Who are the primary users of the kernel?

Answer 11

Processes

Question 12

What do processes contain?

Answer 12

One or more threads
An address space
Zero or more open file handles representing files.

Question 13

Who populates /proc entries?

Answer 13

The kernel. It uses /proc to export infomation

Question 14

Pseudo filesystem similar to /proc that can modify system behaviour while running

Answer 14

/sys

Question 15

Who protects Processes from each other??

Answer 15

The OS

Question 16

File Descriptor

Answer 16

An int index returned by open() that points to a position in the File Table array

Question 17

File Handle

Answer 17

Contains FIle Mode and offset (position in File where the next read or write is going to be done).

Question 18

File Object

Answer 18

Hold file state, other than the state held by File Handle.

Question 19

How is a new process created?

Answer 19

By calling fork() (or clone() a more flexible primitive)

Question 20

Used to abstract and multiplex the CPU

Answer 20

Threads

Question 21

The fastest part of your computer system

Answer 21

The CPU (cache memory is a lot faster than main memory)

Question 22

Why is a ‘race condition’ bad for program behaviour?

Answer 22

Races can be won by any of its competitors. In this case is Threads. We don’t want the outcome of a program to change depending on which Thread, or instructions within the Thread, ‘wins the race’

Question 23

Atomicity

Answer 23

An atomic transaction is an indivisible and irreducible series of database operations such that either all occurs, or nothing occurs.

Question 24

2 most important things about concurrency programming

Answer 24

• Coordination

- Correctness

Question 25

Critical Section - Requirements

Answer 25

Mutual Exclusion: this is the most basic property. Only one thread should be executing in the critical section at one time.

Progress: all threads should eventually be able to proceed through the critical section.

Performance: we want to keep critical sections as small as possible without sacrificing correctness.

Question 26

Critical Section

Answer 26

A set of instructions that must have atomic behaviour with respect to other threads executing (other Threads can’t observe intermediate values), typically to access a shared resource

Question 27

Command to read ELF files

Answer 27

readelf

Question 28

exec() steps, including argument passing

Answer 28

• Arguments saved by the kernel
• Address space wiped out (often saved to be swaped-in in case of exec failure)
• The wanted program file is located and loaded into the Address Space
• Arguments are retrieved by the kernel and loaded into the stack
• Process starts running, arguments can be used by the main function (argc argv)

Question 29

New Process creation process

Answer 29

• The parent process creates a pipe object by calling pipe(). The File Table now has File Descriptors for both the reading and writing ends of the pipe.
• The parent Process calls fork(), making an exact copy of itself, the child.
• The parent closes its copy of the read end and the child closes its copy of the write end.
• The child tipically calls exec() to load a new program file.

Question 30

Full process for a new program to run (short steps)

Answer 30

• pipe(): A pipe is created
• fork(): The calling Process is copied, File Descriptors shared between parent and child
• Close pipe ends: parent/read, child/write
• exec(): Child Address Space is wiped. The wanted program is located and loaded into the Address Space

Question 31

What gets copied into the child Process when fork() is called?

Answer 31

• The calling Thread
• The contents of the Address Space
• The File Descriptors