ENTERPROCESS COMMUNICATION AND SYNCHRONIZATION

     

ENTERPROCESS COMMUNICATION AND SYNCHRONIZATION

·         In multi programming environment multiple process co-exit . a single  program may be broken into number of processes.

·         The process are classified into two categories : independent processes and cooperating processes.

·         An independent process is a standalone process that does not share any data with any other process. It cannot affect or be affected by the other processes executing  in the system. In other words, the modification made to an independent process does not affect the functioning of other process.

·         A cooperating processes is a process that shares data with other processes in a system it can affect or be affectedly the other processes executing in the system

·         Cooperating processes can communicate in a shred memory environment .

·         The various reasons for using cooperating processes are:

1.       Information sharing : when several  users want to access to these type of reasourses

2.       Computational speed up: a task can be broken into various subtak so that each of them can run in parallel and this gives faster computation results.

3.       To support modularity: when a system is to be created in module fashion by dividing in into small functional units, co-operating processes are required. Cooperation processes establishes communication between the different modulus.

 

INTERPROCESSES COMMUNICATION

·         Inter-process communication (IPC) is a facility provided by an operating system via which cooperating processes can communicate with each other.

·         IPC facility allows the processes to cooperate and synchronize their action without sharing the sme addresses space

·         IPC is particularly useful in a distributed environment where the communication processes may reside on different computers connected with a network. An example is chat program used in the world wide web

·         There several different methods for establishing interprocess communication. Some of these methods are:

Ø Message Passing

Ø Shared Memory

Ø Signals

Ø Shared Files ,I.E. Pipes

Ø Dynamic Data Exchange (DDE)

Ø Object Linking And Embedding(OLE)

MESSAGE PASSING MODEL

·         A message is a collection of information that may be exchanged between a sending and receiving processes.

·         A message may contain data, execution commands, or even some code to be transmitted between two or more process.

·         A message format is flexible and negotiable by each sender- reciver pair.

·         A message Is characterized by its type , length ,sender and receiver IDS and a data field.

MESSAGE FORMAT

·         The format of a message depends upon two factors:

1.       The objective of the message facilty.

2.       Whether the facility runs on a single computer or on a distributed system

·         In some operating system short, fixed length messages are preferred in order to minimize processing and storage overhead.

·         In case , a large amount of data is to be sent , the data is placed in a file and the message then simply reference that file

       

·         The header has a fixed format within a given operating system and contains the information about the message.

·         The message body is optional and contains the actual content of the message.

·         The header may contain an identification of the message a length field , and a type field to discriminate among various types of messages.

·         There may also be additional control information such as pointer field so that a linked list of message can be created ,  a sequence number , to keep track of the number and order of messages passed between source and destination and a priority field.

·         Processes generally send and receive message by using send and receive primitives:

                          Send(receiver process, message)

                          Receive (sender process, message)

·         The send and receive calls are normally implemented as operating system calls.

·         The send call sends a message to a give receiver process. The receiver call receives a message from a given sender process.

·         The following four system calls are used for message transfer among processes:

Ø  msgget(): it returns (and possibly creates ) message descriptors to designate a message from queue for in other system calls.

Ø  msgetl(): it has options to set and return parameters associated with a message descriptor. It also has an option to remove descriptors.

Ø  msgsnd(): it senda a message using a message queue.

Ø  msgrev(): it receives using a message queue.

Implementing Issues In Messages

The various implementation issues that arise in interprocess communication using messages are:

1.       naming of the sender and receiver processes : naming conventions used in the send and receive calls provide answer to some key questions:.

·         How does the sender process know the name of the receiver?

·         How does the receiver process know the name of sebder ?

2.       Message delivery protocol : protocol are the set of rules that determines the message data format and actions of processes while sending and receiving messages

3.       Operating system responsibilities : buffering of message, bloicking and waking of processes etc.

 

 

Naming

·         Processes that want to communicate must have a way to refer tro each other processes can name each other directly or indirectly.

·         If the processes use indirect naming , it is know as indirect communication.