Lecture 6: Intro to Signals & Brief Intro to Threads

Question 1

Define: Signal

Answer 1

A mechanism for notifying a process that an event has occurred.

Question 2

Where can a signal originate from?

Answer 2

From within a process (running into a divide-by-zero) or from outside a process (ex. keyboard)

Question 3

What can happen when a process receives a signal?

Answer 3

Can either: take a default action OR use a pre-defined signal handler that is programmer-specified ( signal(par1,par2); ) OR Ignore

Question 4

What handles the sending and receiving of signals?

Answer 4

The OS Kernel:
Sending: OS Kernel updates info for destination process
Receiving: OS Kernel forces target process to handle signal

Question 5

Does each signal have a system-defined default action or does each action have to be defined by the user.

Answer 5

Each signal type has a system-defined default action

Question 6

What must you include first before being able to use the system signal call?

Answer 6

Include signal.h header

Question 7

Define: Thread

Answer 7

A thread is a basic unit of CPU utilization where threads of a process share memory but can execute independently

Question 8

What is wrong with using fork() to create multiple processes?

Answer 8

Process creation is time consuming and resource intensive.

Question 9

What do threads share?

Answer 9

1) Process address space

2) OS State

Question 10

Define: Process address space in terms of what threads share

Answer 10

• text
• data (global variables)
• heap (dynamically allocated data)

Question 11

Define: OS State in terms of what threads share

Answer 11

• open files
• sockets
• locks

Question 12

What do threads uniquely possess?

Answer 12

Their own CPU context:

• Program Counter (PC)
• Stack Pointer (SP)
• Registers & Register State
• Stack

Question 13

List the benefits of threads:

Answer 13

1) Responsiveness
2) Resources Sharing
3) Allocating memory and resources for process creation is costly
4) Context-swtiching is faster

Question 14

Define: Thread Libraries

Answer 14

A thread library provides the programmer with an API for creating and managing threads

Question 15

List 3 main thread libraries:

Answer 15

1) POSIX PThreads
2) Win32
3) Java

Question 16

State potential problem(s) with threads:

Answer 16

Sharing global variables is dangerous because 2 threads may attempt to modify the same variable at the same time. Solution is to lock when in use and unlock when done with variable.