Processes

Program vs. Process

• A program is a set of instructions for a computer
• A process is a running instance of a program
  • Code plus variables in memory plus open files plus some IDs
• If files are nouns, processes are verbs
• Tools to manage processes were invented when most users only had a single terminal
  • But are still useful for working with remote/cloud machines

Viewing Processes

• Use ps -a -l to see currently running processes in terminal
  • UID: numeric ID of the user that the process belongs to
  • PID: process's unique ID
  • PPID: ID of the process's parent (i.e., the process that created it)
  • CMD: the command the process is running

ps -a -l

  UID   PID  PPID        F CPU PRI NI        SZ    RSS       TTY       TIME CMD
    0 13215 83470     4106   0  31  0 408655632   9504   ttys001    0:00.02 login -pfl gvwilson /
  501 13216 13215     4006   0  31  0 408795632   5424   ttys001    0:00.04 -bash
  501 13569 13216     4046   0  31  0 408895008  20864   ttys001    0:00.10 python -m http.server
    0 13577 13216     4106   0  31  0 408766128   1888   ttys001    0:00.01 ps -a -l

• Use ps -a -x to see (almost) all processes running on computer
  • ps -a -x | wc tells me there are 655 processes running on my laptop right now

Exercise

1. What does the top command do? What does top -o cpu do?

2. What does the pgrep command do?

Parent and Child Processes

• Every process is created by another process
  • Except the first, which is started automatically when the operating system boots up
• Refer to child process and parent process
• echo $$ shows process ID of current process
  • $$ shortcut for current process's ID because it's used so often
• echo $PPID (parent process ID) to get parent
• pstree $$ to see process tree

Signals

• Can send a signal to a process
  • "Something extraordinary happened, please deal with it immediately"
• Table shows what happened

• Create a callback function to act as a signal handler

import signal
import sys

COUNT = 0

def handler(sig, frame):
    global COUNT
    COUNT += 1
    print(f"interrupt {COUNT}")
    if COUNT >= 3:
        sys.exit(0)

signal.signal(signal.SIGINT, handler)
print("use Ctrl-C three times")
while True:
    signal.pause()

python src/catch_interrupt.py
use Ctrl-C three times
^Cinterrupt 1
^Cinterrupt 2
^Cinterrupt 3

• ^C shows where user typed Ctrl-C

Background Processes

• Can run a process in the background
  • Only difference is that it isn't connected to the keyboard (stdin)
  • Can still print to the screen (stdout and stderr)

import time

for i in range(3):
    print(f"loop {i}")
    time.sleep(1)
print("loop finished")

python src/show_timer.py &
ls site

$ src/show_timer.sh
birds.csv       cert_authority.srl  sandbox         server.pem      species.csv
cert_authority.key  motto.json      server.csr      server_first_cert.pem   yukon.db
cert_authority.pem  motto.txt       server.key      server_first_key.pem
loop 0
$ loop 1
loop 2
loop finished

• & at end of command means "run in the background"
• So ls command executes immediately
• But show_timer.py keeps running until it finishes
  • Or needs keyboard input
• Can also start process and then suspend it with Ctrl-Z
  • Sends SIGSTOP instead of SIGINT
• Use jobs to see all suspended processes
• Then bg %num to resume in the background
• Or fg %num to foreground the process to resume its execution

$ python src/show_timer.py
loop 0
^Z
[1]+  Stopped                 python src/show_timer.py
$ jobs
[1]+  Stopped                 python src/show_timer.py
$ bg
[1]+ python src/show_timer.py &
loop 1
$ loop 2
loop finished
[1]+  Done                    python src/show_timer.py

• Note that input and output are mixed together

Killing Processes

• Use kill to send a signal to a process

$ python src/show_timer.py
loop 0
^Z
[1]+  Stopped                 python src/show_timer.py
$ kill %1
[1]+  Terminated: 15          python src/show_timer.py

• By default, kill sends SIGTERM (terminate process)
• Variations:
  • Give a process ID: kill 1234
  • Send a different signal: kill -s INT %1

$ python src/show_timer.py
loop 0
^Z
[1]+  Stopped                 python src/show_timer.py
$ kill -s INT %1
[1]+  Stopped                 python src/show_timer.py
$ fg
python src/show_timer.py
Traceback (most recent call last):
  File "/tut/sys/src/show_timer.py", line 5, in <module>
    time.sleep(1)
KeyboardInterrupt

Fork

• Fork creates a duplicate of a process
  • Creator is parent, gets process ID of child as return value
  • Child gets 0 as return value (but has something else as its process ID)

import os

print(f"starting {os.getpid()}")
pid = os.fork()
if pid == 0:
    print(f"child got {pid} is {os.getpid()}")
else:
    print(f"parent got {pid} is {os.getpid()}")

starting 41618
parent got 41619 is 41618
child got 0 is 41619

Unpredictability

• Output shown above comes from running the program interactively
• When run as python fork.py > temp.out, the "starting" line may be duplicated
  • Programs don't write directly to the screen
  • Instead, they send text to the operating system for display
  • The operating system buffers output (and input)
  • So the "starting" message may be sitting in a buffer when fork happens
  • In which case both parent and child send it to the operating system to print
• OS decides how much to buffer and when to actually display it
• Its decision can be affected by what else it is doing
• So running the same program several times can produce different outputs
  • Because your program is only part of a larger sequence of operations
• Dealing with issues like these is part of what distinguishes systems programming from "regular" programming

Flushing I/O

• Can force OS to do I/O right now by flushing its buffers

import os
import sys

print(f"starting {os.getpid()}")
sys.stdout.flush()
pid = os.fork()
if pid == 0:
    print(f"child got {pid} is {os.getpid()}")
else:
    print(f"parent got {pid} is {os.getpid()}")

starting 41536
parent got 41537 is 41536
child got 0 is 41537

Exec

• The exec family of functions in os execute a new program inside the calling process
  • Replace existing program and start a new one
  • One of the reasons we need to distinguish "process" from "program"
• Use fork/exec to create a new process and then run a program in it

import os
import sys

print(f"starting {os.getpid()}")
sys.stdout.flush()
pid = os.fork()
if pid == 0:
    os.execl("/bin/echo", "echo", f"child echoing {pid} from {os.getpid()}")
else:
    print(f"parent got {pid} is {os.getpid()}")

starting 46713
parent got 46714 is 46713
child echoing 0 from 46714

Exercise

1. What are the differences between os.execl, os.execlp, and os.execv? When and why would you use each?