Chap 7Lesson09EmsysNewPVSem

8/3/2019 Chap 7Lesson09EmsysNewPVSem

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2008 Chapter-7 L9: "Embedded Systems - Architecture, Programmingand Design" , Raj Kamal, Publs.: McGraw-Hill, Inc.

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Inter-Process Communication and

Synchronization of Processes, Threadsand Tasks:

Lesson-9: P and V SEMAPHORES


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P and V SEMAPHORES


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P and V semaphoresP and V semaphores

An efficient synchronisation mechanism

POSIX 1003.1.b, an IEEE standard. POSIXfor portable OS interfaces in Unix.

P and V semaphores

represents the byintegers in place of binary or unsigned

integers


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P and V semaphoreP and V semaphore VariableVariabless The semaphore, apart from initialization, is

accessed only through two standard atomicoperationsP and V

P (for wait operation) derived from a

Dutch word Proberen, which means 'totest'.

V (for signal passing operation) derived

from the word 'Verhogen' which means 'to

increment'.


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P semaphore function signals that the task

requires a resource and if not availablewaits for it.

V semaphore function signals which the

task passes to the OS that the resource is

now free for the other users.

P and V Functions forP and V Functions for Semaphore


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1. /* Decrease the semaphore variable*/

sem_1 = sem_1 -1;

2. /* If sem_1 is less than 0, send a message toOS by calling a function waitCallToOS.Control of the process transfers to OS,

because less than 0 means that some otherprocess has already executed P function onsem_1. Whenever there is return for the OS,it will be to step 1. */

if (sem_1 < 0){waitCallToOS (sem_1);}

P FunctionP Function

P (&Sem1)P (&Sem1)


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3. /* Increase the semaphore variable*/

sem_2 = sem_2 + 1;

4. /* If sem_2 is less or equal to 0, send amessage to OS by calling a functionsignalCallToOS. Control of the processtransfers to OS, because < or = 0 means thatsome other process is already executed Pfunction on sem_2. Whenever there isreturn for the OS, it will be to step 3. */

if (sem_2 < = 0){signalCallToOS (sem_2);}

V FunctionV Function

V (&Sem2)V (&Sem2)


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P and V SEMAPHORE FUNCTIONS

WITH SIGNALING ORNOTIFICATION PROPERTY


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P and V SEMAPHORE FUNCTIONS WITH

SIGNAL OR NOTIFICATION PROPERTYProcess 1 (Task 1):

while (true) {

/* Codes */

.

V (&sem_s);/* Continue Process 1 if sem_s is not equal to

0 or not less than 0. It means that no process

is executing at present. */.

};


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P and V SEMAPHORE FUNCTIONS WITH

SIGNAL OR NOTIFICATION PROPERTYProcess 2 (Task 2):

while (true) {

/* Codes */

.

P (&sem_s);/* The following codes will execute only

when sem_s is not less than 0. */

.

};


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P and V SEMAPHORE FUNCTIONSWITH MUTEX PROPERTY


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Use of P and V semaphores at the tasks I and J andUse of P and V semaphores at the tasks I and J and

actions by scheduleractions by scheduler


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Program counter assignments (context

Switch) to the process or function when using

P and V semaphores


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P and V Semaphore functions with mutex

property

Wait for starting Critical SectionProcess 1 (Task 1):

while (true) {

/* Codes before a critical region*/

.

/* Enter Process 1 Critical region codes*/P (&sem_m);

/* The following codes will execute only

when sem_m is not less than 0. */

.


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property

running end exiting Critical SectionProcess 1 (Task 1):

.

/* Exit Process 1 critical region codes */

V (&sem_m);

/* Continue Process 1 if sem_m is not equal to0 or not less than 0. It means that no processis executing at present. */

.};


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propertyWait for starting other Critical

Section

Process 2 (Task 2):

while (true) {/* Codes before a critical region*/

.

/* Enter Process 2 Critical region codes*/

P (&sem_m);

/* The following codes will execute onlywhen sem_m is not less than 0. */

.


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property

running end exiting the CriticalSection

Process 2 (Task 2):

./* Exit Process 2 critical region codes */

V (&sem_m);


.

};


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WITH COUNTING SEMAPHOREPROPERTY


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P and V Semaphore functions with Count

property

producer Wait for empty place if = 0Process c (Task c):

while (true) {/* sem_c1 represent number of

empty places*/

/* Codes before a producer region*/

.

./* Exit Process 3 region codes */

P (&sem_c1);

/* Continue Process c if sem_c1 is not equalto 0 or not less than 0. */

.


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FOR PRODUCER-CONSUMER

PROBLEM (BOUNDED BUFFER

PROBLEM)


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ProducerProducer--Consumer and Bounded BufferConsumer and Bounded Buffer

ProblemsProblemsConsider three examples:

(i) a task transmits bytes to an I/O streamfor filling the available places at the stream;

(ii) a process 'writes'an I/O stream to a

printer-buffer; and

(iii) a task of producing chocolates is being

performed.


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ProducerProducer--Consumer and Bounded BufferConsumer and Bounded Buffer

ProblemsProblems Example (i) another task reads the I/O-

stream bytes from the filled places andcreates empty places.

Example (ii), from the print buffer an I/Ostream prints after a buffer-read and after

printing, more empty places are created.

Example (iii), a consumer is consuming the

chocolates produced and more empty places(to stock the produced chocolates) arecreated.


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Bounded Buffer ProblemBounded Buffer Problem

Example (ii) The task cannot write from

the memory to print buffer if there are noempty places at the print-buffer.


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ProducerProducer

--Consumer ProblemConsumer Problem

Example (iii) The producer cannot

produce chocolates if there are no emptyplaces at the consumer end.


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P and V Semaphore functions with producer-

consumer problem solutionProcess 3 (Task 3):

while (true) {

/* Codes before a producer region. sem_c2number of empty places created by process4 */

.

/* Enter Process 3 Producing region codes*/

P (&sem_c2);/* The following codes will execute only

when sem_c2 is not less than 0. */

.


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.

/* Exit Process 3 region codes */

V (&sem_c1);

/* Continue Process 3 if sem_c1 is not equalto 0 or not less than 0. */

.

};


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while (true) {/* Codes before a producer

region. sem_c1 number of filled placescreated by process 3 */

.

/* Enter Process 4 Consuming region codes*/

P (&sem_c1);

/* The following codes will execute onlywhen sem_c1 is not less than 0. */

.


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consumer problem solutionProcess 4 (Task 4):.

/* Exit Process 4 region codes */V (&sem_c2);


.};


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SummarySummary


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We learnt P and V semaphore functions are POSIXP and V semaphore functions are POSIX

100.3b IEEE accepted standard for the100.3b IEEE accepted standard for the

semaphoresemaphore IPCsIPCs..

These can be used as event signaling, asThese can be used as event signaling, as

mutex, as counting semaphore and asmutex, as counting semaphore and assemaphores for the bounded buffer problemsemaphores for the bounded buffer problem

solution (producersolution (producer

consumer problem)consumer problem)


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End of Lesson 9 of Chapter 7End of Lesson 9 of Chapter 7

Chap 7Lesson09EmsysNewPVSem

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