Page 59 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes with the link’s physical implementation but rather with its logical implementation. Here are
several methods for logically implementing a link and the send/receive operations:
• Direct or indirect communication
• Symmetric or asymmetric communication
• Automatic or explicit buffering
• Send by copy or send by reference
• Fixed-sized or variable-sized messages
We look at each of these types of message systems next.
2.5.2 Naming

Processes that want to communicate must have a way to refer to each other.
They can use either direct or indirect communication.
2.5.3 Direct Communication
With direct communication, each process that wants to communicate must explicitly name the
recipient or sender of the communication. In this scheme, the send and receive primitives are
defined as:
• send(P, message) –Send a message to process P.
• receive(Q, message) –Receive a message from process Q.
A communication link in this scheme has the following properties:
• A link is established automatically between every pair of processes that want to
communicate. The processes need to know only each other’s identity to communicate.
• A link is associated with exactly two processes.
• Exactly one link exists between each pair of processes.

This scheme exhibits symmetry in addressing; that is, both the sender and the receiver processes
must name the other to communicate. A variant of this scheme employs asymmetry in addressing.
Only the sender names the recipient; the recipient is not required to name the sender. In this
scheme, the send and receive primitives are defined as follows:

• send(P, message) –Send a message to process P.
receive (id, message)—Receive a message from any process; the variable id is set to the name
of the process with which communication has taken place.

The disadvantage in both symmetric and asymmetric schemes is the limited modularity of the
resulting process definitions. Changing the name of a process may necessitate examining all other
process definitions. All references to the old name must be found, so that they can be modified
to the new name. This situation is not desirable from the viewpoint of separate compilation.
2.5.4 Indirect Communication

With indirect communication, the messages are sent to and received from mailboxes, or ports. A
mailbox can be viewed abstractly as an object into which messages can be placed by processes
and from which messages can be removed. Each mailbox has a unique identification. In this
scheme, a process can communicate with some other process via a number of different mailboxes.
Two processes can communicate only if they share a mailbox. The send and receive primitives
are defined as follows:
• send(A, message) –Send a message to mailbox A.

• receive(A, message) –Receive a message from mailbox A.

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