1. Subject Name: OPERATING SYSTEMS Subject Code: 10EC65
Prepared By: Kala H S and Remya R
Department: ECE
Date:
9/21/2018

2. Unit 8
MESSAGE PASSING
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3. TOPICS TO BE COVERED Overview of message passing
Implementing message passing
Mailboxes
Inter process communication in UNIX
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4. Overview of Message Passing
Ways in which processes interact with one another
Control synchronization
Data access synchronization
Signals
Message passing
Differentiation between exchange of signals & message passing
Signals have standard syntax & semantics
Are used to implement pre-defined interactions between processes
Messages does not have standard syntax & semantics
It is up to the receiver to deduce its meaning
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5. An example of message passing
Send makes a system call send with pj and the message as parameters
At a send call by pi, the kernel checks if process pj is blocked on a receive call, in which case it copies the message into msg_area and activates pj.
Processes may exist in the same computer or in different computers connected to a network
Uses of message passing:
Client-server paradigm
Backbone of higher-level communication protocols
Parallel and distributed programs
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6. Overview of Message Passing (continued)
Two important issues in message passing are:
Naming of processes
Names may be explicitly indicated or deduced by the kernel in some manner
Delivery of messages
Whether sender should be blocked until delivery
What the order is in which messages are delivered to a destination process
How exceptional conditions are handled
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7. Direct and Indirect Naming
In direct naming, sender and receiver processes mention each other’s name
In symmetric naming, both sender and receiver processes specify each other’s name
In asymmetric naming, receiver does not name process from which it wishes to receive a message; kernel gives it a message sent to it by some process
In indirect naming, processes do not mention each other’s name in send and receive statements
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8. Blocking and Non-blocking Sends
A blocking send blocks sender process until message is delivered to destination process
Synchronous message passing
Simplifies design of concurrent processes
A nonblocking send call permits sender to continue its operation after send call
Asynchronous message passing
Enhances concurrency between sender and receiver
In both cases, receive is typically blocking
Kernel performs message buffering pending delivery
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9. Exceptional Conditions in Message Passing
To facilitate handling exceptional conditions, send and receive take two additional parameters:
Flags indicate how exceptions should be handled
status_area is for storing code concerning outcome
Some exceptional conditions:
Destination process mentioned in send doesn’t exist
Source process does not exist (symmetric naming)
send cannot be processed (out of buffer memory)
No message exists for process when it makes receive
A set of processes become deadlocked when one is blocked on a receive
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10. Implementing Message Passing
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11. Delivery of Interprocess Messages
Remember that:
An event control block (ECB) has three fields:
Description of the anticipated event
Id of the process that awaits the event
An ECB pointer for forming ECB lists
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12. 9/21/2018

13. Mailboxes Mailbox: repository for interprocess messages
Has a unique name
Owner is typically the process that created it
Indirect naming
Only owner process can receive messages
Any process that knows name of a mailbox can send messages to it
Kernel may provide fixed set of mailbox names, or it may permit user processes to assign the names
Varying levels of confidentiality
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14. Mailboxes (continued)
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15. Mailboxes (continued)
Kernel may require a process to explicitly “connect” to a mailbox before starting to use it, and to “disconnect” when it finishes using it
May permit owner to destroy it, transfer ownership, etc.
Use of a mailbox has following advantages:
Anonymity of receiver
Classification of messages
Through use of separate mailboxes for different classes
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16. Example: Use of Mailboxes
An airline reservation system:
A set of booking processes
Server wishes to process cancellations before bookings and queries after both of them
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17. Higher-Level Protocols Using Message Passing
Several protocols use message passing paradigm to provide diverse services:
Simple mail transfer protocol (SMTP) delivers electronic mail
Remote procedure call (RPC) is a programming language facility for distributed computing
Invokes a part of a program that is located on a different computer
Parallel virtual machine (PVM) and message passing interface (MPI) are message passing standards for parallel programming
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18. The Simple Mail Transfer Protocol (SMTP)
SMTP is used to deliver electronic mail to one or more users reliably and efficiently
Uses asymmetric naming
Can deliver mail across a number of interprocess communication environments (IPCEs )
It is an applications layer protocol: uses TCP / IP
SMTP consists of several simple commands:
MAIL, RCPT, DATA
Typically used with a protocol that provides a mailbox
Internet Message Access Protocol (IMAP)
Post Office Protocol (POP)
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19. Remote Procedure Calls
Remote procedure call (RPC): used to invoke a part of a program located in a different computer
Semantics resemble those of a procedure call
call <proc_id> (<message>);
<message> is a list of parameters
Stubs perform marshaling of parameters and convert them to/from machine independent representations
SunRPC and OSF/DCE standards, Java RMI
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20. Message Passing Standards for Parallel Programming
Parallel program: set of tasks that can be performed in parallel
Executed on a heterogeneous set of computers or on a massively parallel processor (MPP)
Parallel virtual machine (PVM) and message passing interface (MPI) are standards used in coding message passing libraries; provide:
Point-to-point communication between processes
Barrier synchronization between processes
Global operations for scattering (gathering) disjoint portions of data in a message to (from) different processes
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21. Message Passing in Unix
Three interprocess communication facilities:
Pipe:
Data transfer facility
Unnamed pipes can be used only by processes that belong to the same process tree
Message queue: analogous to a mailbox
Used by processes within “Unix system domain”
Access permissions indicate which processes can send or receive messages
Socket: one end of a communication path
Can be used for setting up communication paths between processes within the Unix system domain and within certain Internet domains
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22. Message Passing in Unix (continued)
One common feature: processes can communicate without knowing each other’s identities
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23. Message Passing in Unix: Pipes
FIFO mechanism for data transfer between processes called reader and writer processes
Usually implemented in file system
But, data put into a pipe can be read only once
Removed from pipe when it is read by a process
Two kinds of pipes: named and unnamed
Created through the system call pipe
A named pipe has an entry in a directory
Like a file, but size is limited and kernel treats it as a queue
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24. Message Passing in Unix: Message Queues
Analogous to a mailbox
Created and owned by one process
Creator specifies access permissions for send/receive
Size specified at the time of its creation
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25. Message Passing in Unix: Sockets
A socket is one end of a communication path
Can be used for interprocess communication within the Unix system domain and in the Internet domain
Server can set up communication paths with many clients simultaneously
Typically, after connect call, server forks a new process to handle the new connection
Leaves original socket created by server process free to accept more connections
Indirect naming: address (domain) used instead of process ids
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26. Acknowledgement : MY SINCERE THANKS TO By Kala H S and Remya R
THE AUTHOR D M DHAMDHERE. BECAUSE THE ABOVE PRESENTATION MATERIALS ARE HEAVILY BORROWED FROM HIS TEXTBOOK “OPERATING SYSTEMS – A CONCEPT BASED APPROACH” 2ND EDITION, PUBLISHER TATA MCGRAW HILL By
Kala H S and Remya R
Assistant Professor
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