Inter Process Communication

• Mechanism for processes to communicate and to synchronize their actions
• Message system – processes communicate with each other without resorting to shared variables
• IPC facility provides two operations:
  -send(message) – message size fixed or variable
  -receive(message)
• If P and Q wish to communicate, they need to:
  -establish a communication link between them
  -exchange messages via send/receive
• Implementation of communication link
  -physical (e.g., shared memory, hardware bus)
  -logical (e.g., logical properties)

Direct Communication

• Processes must name each other explicitly:
  -send (P, message) – send a message to process P
  -receive(Q, message) – receive a message from process Q
• Properties of communication link
  -Links are established automatically.
  - A link is associated with exactly one pair of communicating
  processes.
  - Between each pair there exists exactly one link.
  - The link may be unidirectional, but is usually bi-directional.
• Asymmetric variant
  - receive(id, message) – receive a message from any
  process, pid stored in id

Indirect Communication

• Messages are directed and received from mailboxes (also referred to as ports).
  - Each mailbox has a unique id.
  -Processes can communicate only if they share a mailbox.
• Properties of communication link
  - Link established only if processes share a common mailbox
  - A link may be associated with many processes.
  - Each pair of processes may share several communication links.

-Link may be unidirectional or bi-directional.

• Operations
  -create a new mailbox
  - send and receive messages through mailbox
  -destroy a mailbox
• Primitives are defined as:
  send(A, message) – send a message to mailbox A

receive(A, message) – receive a message from mailbox A

• Mailbox sharing

-P1, P2, and P3 share mailbox A.

-P1, sends; P2 and P3 receive.

- Who gets the message?

• Solutions

-Allow a link to be associated with at most two processes.

-Allow only one process at a time to execute a receive operation.

-Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.

Synchronization

• Message passing may be either blocking or non-blocking.
• Blocking is considered synchronous
• Non-blocking is considered asynchronous
• send and receive primitives may be either blocking or non-blocking.