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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 168
SEC. 2.3
INTERPROCESS COMMUNICATION
137
Thread call
Description
Pthread
cond
init
Create a condition variable
Pthread
cond
destroy
Destroy a condition variable
Pthread
cond
wait
Block waiting for a signal
Pthread
cond
signal
Signal another thread and wake it up
Pthread
cond
broadcast
Signal multiple threads and wake all of them
Figure 2-31.
Some of the Pthreads calls relating to condition variables.
As an example of how mutexes and condition variables are used, Fig. 2-32
shows a very simple producer-consumer problem with a single buffer. When the
producer has filled the buffer, it must wait until the consumer empties it before pro-
ducing the next item. Similarly, when the consumer has removed an item, it must
wait until the producer has produced another one. While very simple, this example
illustrates the basic mechanisms.
The statement that puts a thread to sleep should
always check the condition to make sure it is satisfied before continuing, as the
thread might have been awakened due to a UNIX signal or some other reason.
2.3.7 Monitors
With semaphores and mutexes interprocess communication looks easy, right?
Forget it. Look closely at the order of the
down
s before inserting or removing items
from the buffer in Fig. 2-28. Suppose that the two
down
s in the producer’s code
were reversed in order, so
mutex
was decremented before
empty
instead of after it.
If the buffer were completely full, the producer would block, with
mutex
set to 0.
Consequently, the next time the consumer tried to access the buffer, it would do a
down
on
mutex
, now 0, and block too. Both processes would stay blocked forever
and no more work would ever be done. This unfortunate situation is called a dead-
lock. We will study deadlocks in detail in Chap. 6.
This problem is pointed out to show how careful you must be when using sem-
aphores. One subtle error and everything comes to a grinding halt.
It is like pro-
gramming in assembly language, only worse, because the errors are race condi-
tions, deadlocks, and other forms of unpredictable and irreproducible behavior.
To make it easier to write correct programs, Brinch Hansen (1973) and Hoare
(1974) proposed a higher-level synchronization primitive called a
monitor
. Their
proposals differed slightly, as described below. A monitor is a collection of proce-
dures, variables, and data structures that are all grouped together in a special kind
of module or package. Processes may call the procedures in a monitor whenever
they want to, but they cannot directly access the monitor’s internal data structures
from procedures declared outside the monitor.
Figure 2-33 illustrates a monitor
written in an imaginary language, Pidgin Pascal. C cannot be used here because
monitors are a
language
concept and C does not have them.
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