Exceptions, Interrupts, and Signals

Question 1

Process

Answer 1

Executing instance of a program. Separate mem address spaces. Resource heavy-weight

Question 2

Thread

Answer 2

Entity within a process that can be schedules for code execution. Each process has at least one thread. Share mem address space, opened files, and more. Resource light-weight

Question 3

Thread Pros and Cons

Answer 3

PROS: Cheaper to create thread. Cheaper task switch. cheaper data sharing
CONS: One bug or attack in thread makes whole process unstable (bc of data sharing).

Question 4

Exceptions

Answer 4

Event that interrupt the normal execution of a program or a system, causes mode switch

Question 5

Types of Exceptions

Answer 5

1. Programmed exception (traps)
2. Anomalous executions (faults)

Question 6

Programmed Exceptions (traps)

Answer 6

‘int 0x80’ (system call), ‘int 3’ (single-step debugging)

Question 7

Anomalous Executions (faults)

Answer 7

Divide by zero, dereference null pointer.

Question 8

How is ‘divide by zero’ handled?

Answer 8

1. CPU executing division triggers exception
2. Mode switch to kernel, divide by zero handler invoked automatically
3. Handler sends SIGFPE to faulty process, mode switch back to user space
4. If process has SIGFPE handler then executes it. Otherwise default handler in libc kills the process

Question 9

Signal Causes

Answer 9

1. “kill -signame pid”
2. “pthread_kill()”, sends signals to specific thread within sender’s process
3. “tgkill(pid, tid, sig)”, send signal to thread ‘tid’ in process ‘pid’
4. “sigqueue(pid, sig, value)”, sends signal and associated value to process ‘pid’

Question 10

When does SIGCHLD happen?

Answer 10

What a process exits, the parent process receives a SIGSHLD.

Question 11

Thread Signal Handler

Answer 11

Handlers are shared among threads of a process, while signal mask is per thread.

Question 12

sigaction()

Answer 12

Used to change default signal handler to be customized.

Question 13

signal()

Answer 13

Like sigaction(). After invocation of your handler, default is restored, so not reliable.

Question 14

Vector

Answer 14

Number in [0,255] used to identify an interrupt or exception. Can be specified by programmer

Question 15

IDT

Answer 15

Interrupt Descriptor Table. Each entry is a descriptor that refers to an interrupt or exception handler.

Question 16

Interrupt/Exception Hander

Answer 16

1. Mode switch to kernel mode in is currently in user mode
2. Save current context
3. Invoke corresponding handler function using IDT and interrupt/exception vector
4. Restore context
5. Mode switch back to user mode in needed

Question 17

fork()

Answer 17

Parent process receives child process id and child receives 0 (but child id isn’t actually 0)

Question 18

exec()

Answer 18

Changes the program that is being executed. All code is overwritten with new program.

Question 19

wait()

Answer 19

Suspends execution of the calling process until one of its children terminate.

Question 20

How does computer HARDWARE respond to a keystroke?

Answer 20

• Interrupt generated by keyboard
• CPU notices interrupt and sets program counter to corresponding interrupt handler

Question 21

Zombie

Answer 21

Occurs once a child process exits. Occupies important resources. Cleaned up by:
- parent waiting w wait()
- parent explicitly ignores SIGCHLD by setting handler to SIG_IGN
- if parent exits, zombie gets reaped by init()

Question 22

How does computer SOFTWARE-KERNEL respond to a keystroke?

Answer 22

• interrupt handler executes the device driver for the keyboard
• handler then reads character from keyboard buffer to kernel-space buffer

Question 23

How does computer SOFTWARE-USER/KERNEL respond to a keystroke?

Answer 23

• process that waits for input is waken up
• kernel copies character from the kernel space to the user-space buffer