Lecture 05

Question 1

What is a ‘signal’?

Answer 1

A mechanism to notify a process that a particular event has occurred.

Question 2

A signal is:
A - Synchronous
B - Asynchronous

Answer 2

B - Asynchronous

Question 3

What does a software interrupt cause?

Answer 3

An immediate jump to handler code.

Question 4

What happens after an interrupt handler finishes?

Answer 4

Execution resumes at the original point.

Question 5

Are functions async-signal-safe?

Answer 5

No

Question 6

A signal is identified using what?

Answer 6

A unique integer value.

Question 7

What are four default actions that a program may take when it encounters a signal?

Answer 7

• Terminate
• Terminate and dump memory contents into a ‘core’ file
• Stop
• Ignore

Question 8

What function is used to send a signal to the current process?

Answer 8

raise

Question 9

What is the function prototype for the ‘raise’ function?

Answer 9

int raise(int sig);

Question 10

What function is used to send a signal to another process?

Answer 10

kill

Question 11

What is the function prototype for the ‘kill’ function?

Answer 11

int kill(pid_t pid, int sig);

Question 12

What signal is sent when you press Ctrl-C on a process?

Answer 12

SIGINT (interrupt)

Question 13

What signal is sent when you press Ctrl-Z on a process?

Answer 13

SIGTSTP (Stop that can be handled)

Question 14

What signal stops a process without giving it a chance to handle it?

Answer 14

SIGSTOP

Question 15

What function is used to register a signal handler?

Answer 15

signal

Question 16

What is the function prototype for ‘signal’?

Answer 16

sighandler_t signal(int signum, sighandlet_t handler);

Question 17

How many handlers can exist per signal?

Answer 17

One

Question 18

What happens when a new handler is added for a signal which already has one?

Answer 18

The old handler is overwritten with the new one.

Question 19

Is it possible to add a handler for SIGSTOP and SIGKILL?

Answer 19

No

Question 20

What is the argument given to a signal handler function?

Answer 20

The signal that was raised.

Question 21

What single line of code will make a program ignore a Ctrl-C interrupt?

Answer 21

signal(SIGINT,SIG_IGN)

Question 22

What key presses send a SIGINT signal to a process?

Answer 22

Ctrl-C

Question 23

What key presses send a SIGQUIT signal to a process?

Answer 23

Ctrl-/

Question 24

What key presses send a SIGTSTP signal to a process?

Answer 24

Ctrl-Z

Question 25

What does the alarm function do?

Answer 25

Sets a timer for a specified number of seconds. When the timer expires, SIGALRM is raised.

Question 26

What is the default action of ‘SIGALRM’?

Answer 26

Terminate the process

Question 27

What will this C code do?

void sigint_h(int signum){
   printf("Caught SIGINT\n");
   signal(SIGINT,SIG_DFL);
}

int main(void){
    signal(SIGINT, sigint_h);
    while(1);
}

Answer 27

The process will do nothing in an endless loop.
The first time the user hits Ctrl-C, it will print ‘Caught SIGINT’ and continue.
The second time, it will use the default handler and exit.