Processes 1: Basics

Overview

This lecture introduces the process control unit. We discuss the goals of process control and the basic system calls used to create and manage processes.

Textbook readings

About process control

• Advantages of processes
  • Strong modularity
  • Isolation
• Coordinate multiple processes to accomplish complex tasks
  • As opposed to one enormous program
  • Take advantage of more computing resources
  • When one process is paused (e.g., reading from storage), run another
  • Reusable components
• Requires one process coordinate the operation of others
  • Prototypical coordination program: the shell

Shell question 1

• Produce a lowercase version of file.txt
  • “Hello! hello hello” ⟶ “hello! hello hello”
  • “Hello hi there HI” ⟶ “hello hi there hi”
• Program toolbox
  • cat FILE: Write contents of FILE to standard output
  • sleep N: Exit after N seconds
  • sort: Sort lines
  • tr A-Z a-z: Change uppercase characters to lowercase
  • tr -cs a-z "\n": Change non-lowercase characters to newline
  • uniq: Remove duplicate adjacent lines
  • wc -l: Count lines
• Coordination toolbox
  • PROG < FILE: Read standard input of PROG from FILE
  • PROG > FILE: Write standard output of PROG to FILE
  • PROG1 | PROG2: Send standard output of PROG1 to standard input of PROG2

Shell question 2

• How many different ‘words’ are used in file.txt?
  • “Hello! hello hello” ⟶ 1
  • “Hello hi there HI” ⟶ 3
• Program toolbox
  • cat FILE: Write contents of FILE to standard output
  • sleep N: Exit after N seconds
  • sort: Sort lines
  • tr A-Z a-z: Change uppercase characters to lowercase
  • tr -cs a-z "\n": Change non-lowercase characters to newline
  • uniq: Remove duplicate adjacent lines
  • wc -l: Count lines
• Coordination toolbox
  • PROG < FILE: Read standard input of PROG from FILE
  • PROG > FILE: Write standard output of PROG to FILE
  • PROG1 | PROG2: Send standard output of PROG1 to standard input of PROG2

Process control system calls

• What are the tasks of a shell?
• Create a process: fork
• Run a program: exec family
• Exit a process: _exit
• Stop a process: kill
• Await completion: waitpid
• Shell code depends on system call API

Process = program image + identity + environment

• Program image: unprivileged, contents of memory
  • Code, data, (initially empty) stack and heap
  • Command line arguments (argc, argv)
• Identity: kernel, managed by kernel
  • Process ID
  • Process relationships (parent process ID)
  • Ownership, timing, etc.
• Environment: kernel, managed by system calls
  • File descriptor table: open file descriptors, file positions

Process hierarchy

• Every process has a parent process
  • getpid system call: Return current process ID
  • getppid system call: Return parent process ID
  • fork creates a new child process
• Root of process hierarchy is process with ID 1

Question

• Fill in the ???s with the most complete assertion you can relating the ps! (Assume fork does not fail.)

pid_t p1 = getpid();
pid_t p2 = getppid();
pid_t p3 = fork();
pid_t p4 = getpid();
pid_t p5 = getppid();
assert(???);

Some answers

• assert(p1 > 0 && p2 > 0 && p4 > 0 && p5 > 0): all process PIDs are >0
• assert(p3 >= 0): fork did not fail (it’s >0 in parent, 0 in child)
• assert(p1 != p2 && p4 != p5): a process’s PID ≠ its PPID
• assert(p4 != p2): new PID ≠ original PPID
• assert(p1 != p3 && p2 != p3): child PID ≠ parent or grandparent PID
• assert(p3 != 0 ? p1 == p4 : p1 == p5)
  • In parent (p3 != 0), original PID == new PID
  • In child (p3 == 0), original PID == new PPID
• assert(p3 != 0 ? p2 == p5 : p2 != p5)
  • In parent (p3 != 0), original PPID == new PPID
  • In child (p3 == 0), original PPID ≠ new PPID

fork: Which runs first?

• forkorder.cc

fork vs. exec

• fork
  • Cloned program image
  • New identity
  • Cloned environment
• exec (e.g., execvp)
  • New program image
  • Unchanged identity
  • Unchanged environment

Echo programs

• myecho.cc
• execmyecho.cc
• forkmyecho.cc

waitmyecho.cc