csci4061/lab06-code/QUESTIONS.txt

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                            LAB 06 QUESTIONS
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- Name: Michael Zhang
- NetID: zhan4854

Answer the questions below according to the lab specification. Write
your answers directly in this text file and submit it to complete Lab01.


PROBLEM 1 `birth_death.c'
=========================

A
~

  Compile `circle_of_life.c' to the program `circle_of_life' and run
  it. Examine the results and feel free to terminate execution
  early. Examine the source code if desired though it is merely a
  print/sleep loop.

  Compile `birth_death.c' to the program `birth_death'. This program is
  invoked with two arguments, another program name and a "lifetime"
  which is an integer number of seconds. Run it like
  ,----
  | $> ./birth_death ./circle_of_life 4
  `----
  and show the output below.

Nants ingonyama bagithi baba
Sithi uhm ingonyama
Nants ingonyama bagithi baba
Sithi uhm ingonyama
kill result: 0
child process 13697 terminated with signal 2


B
~

  Examine the source code for `birth_death.c' and determine the system
  call the parent program (`birth_death') uses to send signals to the
  child program. Paste this line below and explain which signal is being
  sent.

int result = kill(pid,SIGINT)

This line invokes the kill system call and tells the child to kill itself. 


C
~

  `birth_death.c' waits for a child to finish then outputs what signal
  caused it to be terminated if that was the cause of death. Paste the
  lines of code which determine if a child was terminated due to a
  signal below and mention the macros used for this purpose.

 32   if(WIFSIGNALED(status)){                   // check if a signal ended the child                                                                                                                                          
 33     printf("child process %d terminated with signal %d\n",                                                                                                                                                                 
 34            pid,WTERMSIG(status));                                                                                                                                                                                          
 35   }

Macros used were WIFSIGNALED to check if the child was terminated by a signal,
and WTERMSIG which retruns the actual signal.


D
~

  Compile the program `no_interruptions.c' and run it with
  `birth_death'. Show your results below.

  Note that you may need to send signals to `no_interruptions' to
  forcibly end it. The `pkill' command is useful for this as in
  ,----
  | pkill no_inter        # send TERM signal to proc name matching "no_inter"
  | pkill -KILL no_inter  # send KILL signal to proc name matching "no_inter"
  `----

Ma-na na-na!
Ma-na na-na!
Ma-na na-na!
Ma-na na-na!

kill result: 0
No SIGINT-erruptions allowed.
Ma-na na-na!
Ma-na na-na!
Ma-na na-na!
Ma-na na-na!

Try to SIGTERM me? Piss off!
Ma-na na-na!
Ma-na na-na!
Ma-na na-na!


E
~

  Examine the `no_interruptions.c' code and describe how it is able to
  avoid being killed when receiving the interrupt and TERM signals. Show
  the lines of code used to accomplish this signal handling.

It uses signal handlers to catch the signals and handle them by not terminating
itself, contrary to what the signal is telling it to do. The only signal it
can't avoid is SIGKILL (and SIGSTOP but not used here) which forcefully
terminates it.

 28   // Set handling functions for programs                                                                                                                                                                                   
 29   signal(SIGINT, handle_SIGINT);                                                                                                                                                                                           
 30   signal(SIGTERM, handle_SIGTERM); 


PROBLEM 2 `start_stop.c'
========================

A
~

  Compile `start_stop.c' to the program `start_stop'. This program is
  invoked identically to `birth_death' with two arguments, another
  program name and an "interval" which is an integer number of
  seconds. Run it like
  ,----
  | $> ./start_stop ./circle_of_life 3
  `----
  and show the output below. You may terminate the program early once
  you see the pattern.

Nants ingonyama bagithi baba
Sithi uhm ingonyama
Nants ingonyama bagithi baba
SIGSTOP result: 0
SIGCONT result: 0
Sithi uhm ingonyama
Ingonyama
Siyo Nqoba
SIGSTOP result: 0
SIGCONT result: 0
Ingonyama Ingonyama nengw' enamabala 
From the day we arrive on the planet
And blinking, step into the Sun


B
~

  Describe at a high level the effect that `start_stop' has on a child
  process it runs

start_stop pauses and resumes the execution of the child process periodically
(more precisely, based on the interval passed through the next argument), so in
this case, it would let the child run for 3 seconds, then pause for 3 seconds,
and then resume the loop until the child finished executing.

C
~

  Examine the source code of `start_stop.c' and determine the types of
  signals it uses to alter behavior of its child process and paste the
  corresponding lines of code below.

It uses the SIGSTOP signall with kill(), which stops the program that's
executing but its behavior is more like pause. It resumes execution with SIGCONT
afterwards. The relevant lines of code are:

 25     result = kill(pid,SIGSTOP);
 28     result = kill(pid,SIGCONT);

D
~

  The program `no_interruptions.c' establishes signal handlers for some
  signals allowing it to ignore signals that would normally terminate a
  process. Investigate whether it is possible to similarly ignore the
  signal used by `start_stop' to pause its child process. Hint: Consider
  the provided program `cant_catch_sigstop.c' and run it with
  `start_stop'.

According to the manpage for SIGNAL(2), SIGSTOP cannot be caught by the program:

The signals SIGKILL and SIGSTOP cannot be caught or ignored.

As expected, the output of cant_catch_sigstop is:

I'm awake, I'm awake!
I'm awake, I'm awake!
SIGSTOP result: 0
SIGCONT result: 0
I'm awake, I'm awake!
I'm awake, I'm awake!

and the signal has not been caught.