Unix InterProcess Communication

1
Unix Inter-Process Communication

• Art of Unix Programming
• Ch 7

2
Multiprocessing

• Most common program-modularization technique in
  Unix paradigm is to split up large programs into
  multiple cooperating processes
• Unix OS's encourage breaking programs into
  simpler small applications by
• making process-spawning cheap
• providing methods (shellouts, I/O redirection,
  pipes, message-passing, and sockets) that make it
  relatively easy for processes to communciate
• encouraging the use of simple, transparent,
  textual data formats that can be passed through
  pipes and sockets

3
Taxonomy of Unix IPC Methods

• Hand off tasks to specialist programs
• program runs another to accomplish a specialized
  task
• called shelling out to the called program
• Caller and calee do not communicate while
  running, so protocol design not an issue
• except caller may pass in command-line arguments,
  and callee might pass back data by, for example,
  writing or modifying a file that the caller then
  uses

4
Taxonomy of Unix IPC Methods

• Pipes, Redirection and Filters
• Depend on convention that every program has
  initially available to it (at least) two I/O data
  streams standard input and standard output
• numeric file descriptors 0 and 1 respectively
• Many programs can be written as filters, which
  read sequentially from standard input and write
  only to standard output
• All stages in a pipeline run concurrently, each
  stage waits for input on the output of the
  previous one, but no stage has to exit before the
  next can run
• Unidirectional, all data flows one way

5
Taxonomy of Unix IPC Methods

• Slave Processes
• Send data back and forth between master and
  slaves, while both are running at the same time
• Requires some sort of protocol and internal-state
  machines to handle communication between them
• When becomes to complex, time to separate into
  client-server

6
Taxonomy of Unix IPC Methods

• Peer-to-Peer IPC
• All previous have a hierarchy, with one process
  effectively controlling or driving another
• In communications and networking we frequently
  need channels that are peer-to-peer, usually with
  data flowing freely in both directions

7
Peer-to-Peer IPC Methods

• Tempfiles
• oldest IPC method
• files act as drops between cooperating programs
• Signals
• two processes on the same machine can communicate
  by sending signals
• a form of soft interrupt
• processes declare signal handlers

8
Peer-to-Peer IPC Methods

• Sockets
• Developed in BSD lineage of unix as a way to
  encapsulate bi-directional stream communication
• Byte streams are sequenced and reliable
• Of course, sockets can be used for communication
  of processes on different machines
• Shared Memory
• shared memory requires that the processes to
  communicate live on the same machine
• Used instead of sockets when performance is
  critical issue (and processes can be run on the
  same machine).