CS 3733 Operating Systems

Reading: Chapter 2.1 of Tanenbaum and 2.1, 2.3, 2.4 and 2.5 of PUP

• What is a process?

  • Process model:

    Process contains program code, stack, space for variables and a program counter.

  • Three ways of looking at execution: (Tanenbaum Figure 2-1)
    • Threads of execution from viewpoint of the processor.
    • Execution flow from viewpoint of process.
    • Time execution showing which process is active (Gantt chart).

  • What information must an OS keep about a process?

  • Relationships between processes
    • Independent:
      • Can affect or be affected by other processes.
      • State not shared by other processes.
      • Execution is reproducible.
      • Execution can be stopped and restarted without causing ill effects.
    • Cooperating:
      • State may be shared.
      • Result of execution can not be predicted in advance.
      • Nondeterministic.

  • Process state:
    • New: starting up
    • Running: instructions being executed
    • Waiting: waiting for an event such as I/O
    • Ready: waiting to be assigned to a processor
    • Halted: finished

• Operations on Processes

  • Creation
    • A process may create other processes.
    • The creating process is called the parent
    • The created process is called the child
    • Resources of the parent may be shared among children or the children may get completely new resources.
    • Parent might execute concurrently or might wait for all children to complete.
  • Destruction
  • Changing State of Process

• Processes in Unix

  • Child processes are created with the fork system call.
  • The new process has a copy of the address space of parent.
  • Both processes continue execution after the fork
  • The fork returns 0 to the child and the Child's PID to the parent.

• Code Examples

  • Example 2.3 PUP: What is the value of x for each process?
  • Example 2.4 PUP: Parent and child print different messages.
  • Example 2.5 PUP: A chain of processes, the parent always breaks.
  • Example 2.6 PUP: A fan of processes, the child breaks.
  • Example 2.7 PUP: A tree of processes, only break when error.

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  SKILL: Understand the basics of process creation.

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  Revision Date: 8/26/97

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