Signals

Lab exercises for February 12th

The goal for this assignment are:

Working with system tools: ps, top -u <user>
Understanding process state: sleeping, running, stopped, terminated
Working with additional system calls: sigprocmask, tcgetpgrp, tcsetpgrp, setpgid

We will use the same repository as last week: Labs Repo. Do a git pull to get the basecode.

1. Infinite

Write a program, infinite.c, that calls pause() in an infinite loop. When you run ps w, the status of infinite should be S.

$ ./infinite &
[1] 2320
$ ps w
  PID TTY      STAT   TIME COMMAND
   10 pts/0    Ss     0:00 -bash
  134 pts/2    Ss+    0:00 -bash
  347 pts/3    Ss+    0:00 -bash
  743 pts/1    Ss+    0:00 -bash
 2320 pts/0    S      0:00 ./infinite
 2321 pts/0    R+     0:00 ps w

Use your program to see how job control works in bash. Use the tools, top H and ps w to see how suspending (job status: T), foregrounding (job status: S+), and backgrounding (job status: S) change the state of the process.

$ ./infinite
^Z
[1]+  Stopped                 ./infinite
$ bg
[1]+ ./infinite &
$ ./infinite &
[2] 2302
$ ./infinite &
[3] 2303
$ ./infinite &
[4] 2304
$ ./infinite &
[5] 2305
$ ./infinite &
[6] 2306
$ jobs
[1]   Running                 ./infinite &
[2]   Running                 ./infinite &
[3]   Running                 ./infinite &
[4]   Running                 ./infinite &
[5]-  Running                 ./infinite &
[6]+  Running                 ./infinite &
$ kill %2
$
[2]   Terminated              ./infinite
$ jobs
[1]   Running                 ./infinite &
[3]   Running                 ./infinite &
[4]   Running                 ./infinite &
[5]-  Running                 ./infinite &
[6]+  Running                 ./infinite &
$ fg %5
./infinite

2. Child Monitor

Write a program, monitor.c, that forks a child in their own process group. The parent process should then register a signal handler on SIGCHLD to monitor the child’s process state. When the user quits monitor, the parent should send a terminate signal so that they child also quits.

alinen@sutekh:~/cs355/os-devel/labs/03$ ./monitor
Monitoring child process 6242.
quit

6242 received signal Terminated

Requirements/Hints:

Make sure the child has a different process ID from the parent.
Create a child that loops forever so it is convenient to change its process status.
From another terminal, use the command: kill -STOP <pid> to stop the process.
From another terminal, use the command: kill -CONT <pid> to continue the process.
From another terminal, use the command: kill -TERM <pid> to terminate the process.
Try sending other signals, such as kill -ALRM <pid>, and see what happens to the process. Some signals, such as SIGTTOU will cause the process to stop; others will cause the process to quit.
Typing quit should quit the parent process and kill the child
Inside your signal handler, use a non-blocking waitpid call, configured to also return if the child process stops or continues (see the man page for details).

Use strace to better understand the signaling behavior, e.g.

$ strace -e 'trace=!all' -D -I 3 -f ./monitor
strace: Process 6254 attached
Monitoring child process 6254.
[pid  6254] --- SIGSTOP {si_signo=SIGSTOP, si_code=SI_USER, si_pid=743, si_uid=1000} ---
[pid  6254] --- stopped by SIGSTOP ---
[pid  6253] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_STOPPED, si_pid=6254, si_uid=1000, si_status=SIGSTOP, si_utime=0, si_stime=0} ---

6254 stopped.

3. Foreground/Background

Implement a program, fgbg.c, that forks a child process in the background and allows the user to switch the child between the foreground and background mode using the commands, fg, bg, and control-Z.

$ ./fgbg
[1] Created process 6982 in the background
$ fg
^Zchild stopped.
$ bg
child continued.
$ fg
^Zchild stopped.
$ quit
child received signal Terminated

Requirements/Hints:

Make sure the child has a different process ID from the parent.
To mimic the logic you will need in your job shell, have the child involve infinite, e.g. call execlp("./infinite", "./infinite", (char*) NULL);
Typing Control-Z should suspend the foreground process.
Typing Control-C should exit the foreground process. If the foreground process is the parent, the child should also be exited.
Typing bg should move the child into the background
Typing fg should move the child into the foreground
Typing quit should quit the parent process and exit the child
Inside a signal handler, use a non-blocking waitpid call, configured to also return if the child process stops or continues (see the man page for details).
Whenever the child is stopped or exited, make sure to return the terminal to the parent.
Configure the parent to ignore SIGTTOU and SIGTTIN signals so it can modify ownership of the terminal, even when it is running in the background.
Use top -u <user> or ps w to monitor the terminal and process status of both the parent and child