Interprocess Communication (IPC)

IPC provides a mechanism to permit the processes to communicate and to synchronize their actions without sharing same address space. We talk about in this section the several message passing techniques and the issues related to them.

Message Passing System can be described as follows

The function of the message system is to permit the processes to communicate without the requirement to resort to the shared data. Messages sent by the process might be of either of fixed or the size is variable.  If  processes  P  and  Q  desires  to  communicate,  the  communication  link  should  prevail between them and they should send messages to and receive messages from each other through this link. Here are several techniques for logically implementing a link and the send and receive options are explained as follows:

1)   Direct or indirect communication

2)   The symmetric and the asymmetric communication

3)   Automatic or explicit buffering

4)   Send by copy or send by reference

5)   Fixed size or variable size messages

We now glance at different types of the message systems used for the IPC.

Direct Communication is defined as follows

With direct communication, each process which wants to communicate should explicitly name the recipient or the sender of the communication. The send and receive primitives are given as follows:

1)   Send (P, message) - send a message to process P

2)   Receive (Q, message) - receive a message from process Q.

A communication link in this scheme has the below written properties:

1)   A link is established automatically amongst every pair of the processes which want to communicate.  The processes require knowing only each other's identity to communicate

2)   A link is related to exactly two processes.

3)   Exactly one link exists among each pair of processes.

Unlike  this  symmetric  addressing  scheme,  the  variant  of  this  scheme  employs asymmetric addressing, in which the recipient is not needed to name the sender.

4)   Send (P, message) - send the message to process P

5)   Receive (id, message) - receive the message from any process; the variable id is set to the name of the process with which the communication has taken place.

Indirect Communication can be explained as follows

With the indirect communication, the messages can be sent to and received from the mailboxes. Now, two processes can communicate only if they share the mailbox. The send and receive primitives are defined as stated below:

1)   Send(A, message) - send the message to mailbox A.

2)   Receive(A, message) - receive the message from mailbox A.

A communication link in the scheme has following below written properties:

1)   A link is created between a pair of processes only if both members have a shared mailbox.

2)    A link is associated with more than two processes.

3)   Various links might exist between each pair of communicating processes, with each link corresponding to one mailbox.

Synchronization can is explained below

Communication between the processes takes place by calls to send and receive primitives (i.e., functions). Message passing may be either blocking or non-blocking also called as synchronous and asynchronous.

1)   Blocking send The sending process is blocked or stopped until the receiving process or the mailbox receives message.

2)   Non-blocking  send  The  sending  process  sends  the  message  and  resumes or begins operation.

3)   Blocking receive The receiver blocks until the message is available to us.

4)   Non-blocking receiver The receiver receives either a valid message or a null message.

Buffering can be described as follows

Whether the communication occurring is indirect or direct, messages exchanged by the processes reside in the temporary queue. This queue can be implemented in the three methods:

1)   Zero Capacity The queue has maximum length zero, hence the link cannot have

any messages waiting in it. In this case the sender must block until the message has been received.

2)   Bounded Capacity This queue has finite length n; thus at most n messages can

reside in it. If the queue is not full when a new message is sent, the later is placed in the queue and the sender resumes operation. If the queue is full, the sender blocks until space is available.

3)   Unbounded  Capacity  The  queue  has  never-ending  length;  hence  the  sender  never blocks.

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