Operating Systems 2021F Lecture 9

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Video from the lecture given on October 7, 2021 is now available:

Video is also available through Brightspace (Resources->Class zoom meetings->Cloud Recordings tab)


Lecture 9

* foreground & background processes
* signals
* process hierarchy & orphans
* process lifecycle
  - key system calls

"not yet finalized" - feature complete, but may have bugs
 - finalized after I've made solutions
 - go ahead and work on it, but if something seems too hard
   please ask!

What is a process's lifecycle?
 - process is born: fork/clone
    - has to start with an existing process
    - one who did the fork/clone - the parent
    - new process - the child
    - all processes have a parent except pid 1
       - "init", now part of systemd
       - 1 is started when the system is booted
       - when 1 stops, the system shuts down
       - the kernel has to start 1 explicitly,
         hardcoded in the kernel
    - note there is always a process hierarchy
       - like a family tree
       - pstree shows the hierarchy
 - thus, a process starts always as a copy of another process
 - to run a new program, use execve
    - but this replaces the code in the execve'ing process
    - it "commits suicide", throws away its code in
      favor of another
 - what happens when a process that has children terminates?
    - another process has to become its parent,
      it must be adopted
    - traditionally this was pid 1, but nowadays
      systemd has master processes for each user
 - maintaining the hierarchy is essential because that's what
   determines who gets the return value when a process

When a process terminates
 - it returns a value from main (0 if all is well)
 - kernel holds on to the return value
 - kernel sends SIGCHLD to parent
 - when parent process calls wait, kernel gives it
   the return value returned by child

What happens if the parent DOESN'T call wait?
 - kernel has to retain information
   - must maintain an entry in the list of processes
     (process table)
 - this entry in the process table will stay no matter
   what signal is sent to the associated PID
     - the process has already terminated
     - this is known as a "zombie" process
       (actually denoted as a "Z" state)
 - only way to get rid of a zombie process is
   to kill the parent
     - because then a new parent will take over
       and can call wait
 - there are a finite number of processes that can be
   created (16K traditionally, but more now)
    - so each zombie represents a process that
      can't be created
    - generally not an issue unless situation is pathological

Why doesn't the kernel just take care of this?
 - kernel tries to implement mechanism more than policy
 - and maybe the return value of a process is important
    - kernel doesn't want to decide this sort of matter
    - responsibility of userspace (i.e., init/systemd)

A process can be in many states
 - ps will show you them
 - most common ones are R (running) S (sleeping)
 - Z is for defunct (zombie) processes

Signals are small messages sent to processes
 - can be sent by kernel or another process
 - processes register handlers for each signal
   - C library defines default handlers, most cause
     the process to terminate

When a parent process is killed, the child will normally keep running
 - unless it is part of a session that has ended,
   then all processes in the session are terminated
 - this is how things are cleaned up when you logout
    - but you can defeat this easily

When you type & at the end of a command in bash,
you tell bash to run it in the background
 - by default, bash will call wait immediately after
   creating a child process to run a command
 - this wait will block, i.e., bash will actually wait and
   do nothing else until the child has terminated
 - but, with &, bash will just go and print a prompt
   - child will continue running
   - if you haven't redirected standard out and error,
     output will be mixed with bash
 - background processes with standard out & error (and input,
   if it takes input) redirected can continue running
   even if you log out

Redirecting files
 >   redirect standard out (fd 1)
 <   redirect standard in  (fd 0)
 2>  redirect standard error (fd 2)
 >&  redirect standard out and error (1 & 2)

 #>  redirect file descriptor #

>& and & are completely different, sorry

Note redirecting standard in/out/error is essential for
running code in the background
 - unless it never uses standard in/out/error

tail gives you the last few lines of a file
 - tail -f "follows the file" keeps updating
   - useful for watching logs

Ctrl-D is end of file
 - can use to exit programs that are expecting input on standard in, as we're saying there is no more input to be had

A process that is running in the background and doesn't terminate when you log out is known as a "daemon" in UNIX
 - typically is a child of init/systemd
 - normally provides some sort of service
 - this is why background processes on UNIX end with a "d"

You can start new daemons easily, just run something
in the background and make sure to redirect its standard out & error
 - and for proper usage, create it as a grandchild and have the child terminate, instant orphan => becomes child of init/systemd

You can always kill a daemon by sending it a signal
 - you may need to be root

But if the daemon was started by systemd this may be futile
 - systemd may just restart it (respawn the daemon)
 - to truly shut it down, you have to tell systemd to stop it

Why do you have to be root to kill most daemons?
 - because users can normally only send signals to their
   own processes (otherwise they are ignored)
 - only root can send signals to any process

Any user can normally start a daemon
 - privileges will be limited to what privileges the starting
   user has
 - but on UNIX that is quite enough to start up a network server
   - but not on all ports, so not a true web or ssh server

So can multiple users be logged in at the same time?
 - YES
 - entire point of UNIX, it was for users sharing a computer
 - nowadays we more use one user per computer, and
   different "users" are just for background processes,
   i.e., a database will run as one user, web server as another
 - "root" is just a user, just one with all the privileges
    - kernel defers to its requests, but kernel does
      the actions
    - many security configurations nowadays limit
      the power of the root user (e.g., SELinux)
    - like "admin" on Windows

Process lifecycle cont
 - started with fork/clone
 - runs a program binary with execve
    (in same process)
 - terminate with exit, send return value to parent
   (normal termination)
 - terminate on receiving a signal
    - say SIGTERM

Signals MOSTLY have handlers
 - except for two, SIGSTOP and SIGKILL
 - SIGSTOP pauses execution of a process
 - SIGKILL (-9) forcibly terminates a process
   "force quit"
 - otherwise, processes can choose to ignore or do
   almost anything when they receive other signals
 - think of signal handlers as primitive event handlers

You send signals with the kill system call
 - defaults to SIGTERM, request to terminate process
 - kill -SIGKILL  or kill -9, only use it if
   process doesn't respond to SIGTERM (it can ignore it)
 - trouble with SIGKILL is process doesn't run
   a handle so it can't clean up, could result in loss
   of data (i.e., an editor might do an emergency save on
   SIGTERM but can't do that on SIGKILL)

Ctrl-C by default sends SIGINT, an interrupt signal
 - normally just terminates the process too,
   but process can ignore

What happens if we call wait and
 - there are no children at all?
   => wait returns immediately, returns -1
 - there is a child that has a return value
   => wait returns immediately with the return value of the
 - multiple children have terminated?
   => wait gets the return value for just one,
      you can tell which by the return PID
 - no child has terminated, but one or more children are
   => wait blocks, process will sleep until a child exits,
      then wait will return with child's exit status
When a signal handler runs, does it run in another
process or thread?
 - NO, runs in "the main thread", same as rest of code
 - when a process receives a signal, control
   immediately jumps to the signal handler
   - when signal handler exits, process execution
     continues as before
 - process could have been doing anything,
   so signal handler has to be careful what it does,
   otherwise it could corrupt things
 - process can say "don't send me signals now",
   can ignore any signal except SIGSTOP and SIGKILL
   for as long as it wants

Signals are a VERY PRIMITIVE form of concurrency
 - kinda dangerous
 - normally signal handlers should do as little as possible
   so they don't mess things up
   - definitely avoid doing any system calls if you
     don't have to, let the main execution take care
     of that