Processes & Signals

Question 1

What’s a process?

Answer 1

1. An implementation of the abstract machine concept
2. A running instance of an executable, invoked by a user
   – Can have multiple independent processes of the same executable
3. A schedulable entity, on the CPU

Also called Task or Job sometimes

Question 2

How does the Process address space looks like?

Answer 2

מכתובות נמוכות לגבוהות:
text segment אחריו data segment אחר כך אולי עוד כל מיני סגמנטים ואז הערימה ובסוף בכתובות הגבוהות המחסנית.
המחסנית גדלה כלפי מטה והערימה כלפי מעלה.

Question 3

Can a process access its own PCB?

Answer 3

No. because the pcb is in kernel space.

Question 4

What happens to a process after it dies?

Answer 4

It becomes a zombie

Question 5

What syscall a parent uses to clear zombie children from the system?

Answer 5

wait

Question 6

What flag is needed for wait to not block the parent?

Answer 6

WNOHANG

Question 7

What are the steps of fork?

Answer 7

1. fork() initializes a new PCB
2. Creates a complete copy of parents space in the child.
3. Return twice:
   – At the parent, with pid>0
   – At the child, with pid=0

Question 8

What are the steps of exec invokation?

Answer 8

– Stops the execution of the invoking process
– Loads the executable ‘programPath’
– Starts ‘programPath’, with ‘argv’ as its argv
– Never returns (unless fails)
– Replaces the new process; doesn’t create a new process
• In particular, PID and PPID are the same before/after exec*()

Question 9

Who wait()-s upon an “orphan” process?

Answer 9

In linux, Init.
If a parent process terminates, then its zombie children (if any) are
adopted by init

Question 10

What are signals & signal handlers?

Answer 10

Signal = notification “sent” to a process
– To asynchronously notify it that some event occurred
• Upon receiving a signal
– The process stops whatever it is doing & handles it
• Default signal handling action
– Either die or ignore (depends on the type of the signal)

Question 11

What does the kill -s n pid utility does?

Answer 11

sends signal number n to process pid.

Question 12

What 2 signals the process can’t ignore?

Answer 12

– SIGKILL = terminate the receiving process

– SIGSTOP = suspend the receiving process

Question 13

Explain the difference between signal and interrupt by following criteria:

1. When do they occur?
2. Who (i) triggers them and (ii) defines their semantics
3. Who handles them, who blocks them

Answer 13

1. – Both can be asynchronous
   – But signals are invoked only when returning from kernel to user, and
   interrupts can really fire anytime
2. – Interrupts: hardware – devices (async) or cores (sync)
   – Signals: software (OS) – HW is unaware
3. – Interrupts: OS
   – Signals: processes

Question 14

How is blocking signals implemented?

Answer 14

– The PCB maintains a set of currently blocked signals
– Which can be manipulated by users via the following syscall
• int sigprocmask(int how, const sigset_t *set, sigset_t *oldset)
– ‘how’ = SIG_BLOCK (+=), SIG_UNBLOCK (-=), SIG_SETMASK (=)
– ‘set’ can be maipulated with sigset ops sigetmptyset(sigset_t *set),
sigfillset(sigset_t *), sigaddset(sigset_t *set, int signum),
sigismember(sigset_t *set, int signum)