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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 175
144
PROCESSES AND THREADS
CHAP. 2
inapplicable. The conclusion is that semaphores are too low level and monitors are
not usable except in a few programming languages. Also, none of the primitives
allow information exchange between machines. Something else is needed.
2.3.8 Message Passing
That something else is
message passing
.
This method of interprocess commu-
nication uses two primitives,
send
and
receive
, which, like semaphores and unlike
monitors, are system calls rather than language constructs.
As such, they can easi-
ly be put into library procedures, such as
send(destination, &message);
and
receive(source, &message);
The former call sends a message to a given destination and the latter one receives a
message from a given source (or from
ANY
, if the receiver does not care).
If no
message is available, the receiver can block until one arrives. Alternatively, it can
return immediately with an error code.
Design Issues for Message-Passing Systems
Message-passing systems have many problems and design issues that do not
arise with semaphores or with monitors, especially if the communicating processes
are on different machines connected by a network. For example, messages can be
lost by the network. To guard against lost messages, the sender and receiver can
agree that as soon as a message has been received, the receiver will send back a
special
acknowledgement
message. If the sender has not received the acknowl-
edgement within a certain time interval, it retransmits the message.
Now consider what happens if the message is received correctly, but the ac-
knowledgement back to the sender is lost. The sender will retransmit the message,
so the receiver will get it twice.
It is essential that the receiver be able to distin-
guish a new message from the retransmission of an old one. Usually, this problem
is solved by putting consecutive sequence numbers in each original message.
If
the receiver gets a message bearing the same sequence number as the previous
message, it knows that the message is a duplicate that can be ignored. Successfully
communicating in the face of unreliable message passing is a major part of the
study of computer networks. For more information, see Tanenbaum and Wetherall
(2010).
Message systems also have to deal with the question of how processes are
named, so that the process specified in a
send
or
receive
call is unambiguous.
Authentication
is also an issue in message systems: how can the client tell that it
is communicating with the real file server, and not with an imposter?
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