Lecture Operating systems: A concept-based approach (2/e): Chapter 10 - Dhananjay M. Dhamdhere

Chapter 10 - Message passing. This chapter discusses the semantics of message passing, and OS responsibilities in buffering and delivery of interprocess messages. It also discusses how message passing is employed in higher-level protocols for providing electronic mail facility and in providing intertask communication in parallel or distributed programs.

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 1

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Message passing preliminaries

•  The sender process names the destination process and provides the message

•  The destination process specifies an area in which the message should be put

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 3

Issues in message passing

* Direct and indirect naming

 Direct: Process names are specified in send / receive commands

 Indirect: Process names are inferred by the kernel

* Symmetric and asymmetric naming

 Symmetric: Both sender and receiver processes specify each other’s names

 Asymmetric: Receiving process does not specify name of a sender process

Issues in message passing

* Process executing a receive command is blocked until a message is delivered

* The sender process may or may not be blocked until delivery

 Synchronous message passing: sender is blocked

» Simplifies message passing, saves memory

 Asynchronous message passing: sender is not blocked

* Order in which messages are delivered

* Handling of exceptions like non-existing processes, undeliverable messages

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 5

Issues in message passing

– Buffering of messages

* Kernel builds an interprocess message control block (IMCB)

* IMCB is stored in an appropriate data structure

 The message may be stored in IMCB or separately

– Blocking and unblocking of processes

* May be performed using event control blocks (ECBs)

Interprocess message control block (IMCB)

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 7

Lists of IMCBs pending delivery of messages

Symmetric naming and blocking sends

(a) A process executes send and destination process has not executed receive

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 9

Symmetric naming and blocking sends

Symmetric naming and blocking sends

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 11

Mailbox

•  A mailbox has a name and is a repository of messages

•  Processes connect to a mailbox and send / receive messages from it

– Anonimity of receiver

* A sender process need not know identity of receiver process

– Classification of messages

* A process can use numerous mailboxes, one for each kind of message

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 13

Use of mailboxes

•  An airline reservations process uses three mailboxes  ─ book, enquire, receive

•  This way it can process cancellations followed by bookings followed by queries

Message passing in Unix

    Pipe       Message queue       Socket

Message Passing Dhamdhere: Operating Systems—A Concept­Based Approach, 2 ed Copyright © 2008Slide No: 15

Reservations server in Unix 5.4

•  Cancellation, booking and enquiry messages are assigned types 1, 2 and 3 resp

•  The msgrcv call returns a cancellation message, if present

•  If no cancellation messages exist, it returns a bookings message, if present, or

   an enquiry message

Message passing in Windows

– Local procedure call (LPC) for message passing between

processes located in the same computer

* Provides three types of message passing

 Small and large messages are sent using ports, where a port is like a mailbox

» Small messages are stored in the port, large messages are stored in section objects

 Quick LPC: A server creates a thread for each client This

thread and the client use event pair objects to synchronize