semaphore

Question

Which of the following regarding semaphores are true?

(a) Semaphores always guarantee Mutual Exclusion but fail to guarantee progress

(b) Under certain situations may fail to guarantee Mutual exclusion

(c) Improper usage of semaphore operations may lead to deadlock

(d) Both (a) & (c) are true

Comments

my doubt is in the counting semaphore more then one one process can be in critical section then how its guarantee mutual exclusion and how semaphore fails to guarantee progress

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in question it is not mention anything i.e it is binary or counting semaphore so it will depend on implementation.sometimes semaphore does not provide ME and sometimes it will lead to deadlock all this things are totally depend on situation as well as implementation.

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@kaushal I also have the same doubt. Did you get any proper explanation?

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Do semaphore always ensure mutual exclusion?

What if they are arranged or used in such a way that they don’t?

Answer 1

various Synchronization mechanism fails because they were not atomic ..solution like lock variable and turn variable tends to fail as they were Unatomic.

Semaphore are provision given by Operating System in which the Semaphore Operations are always done ATOMICALLY ..

so no variable can be prempted during performing UP and DOWN ..so they will always GIVES MUTX

But they cant assure Progress....

Improper implementation of Semaphore leads to deadlock ..

Say we have P1 and P2 ..2 semaphore variable are S and K so 

P1	P2
Down(S)	Down(K)
Down(K)	Down(S)
code	Code

 here process goes to deadlock ...if they perform Down(S) and Down(K) at the same time.\

SO most appropriate answer is D

Comments

what about B?

counting semaphores are not provide ME

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@Shaik Masthan & @  kaushal dewangan

when it comes to the case of resources like Data Structure or Hardware resources ..then we always go for BINARY semaphore because say if i am using Speaker as hardware then only one process should access the speaker at given movement ...

But we use COUNTING semaphore in cases of threads where multiplte threads can be present in same address space..

In short ...Counting semaphore is used in cases ..where resources can be shared ...

Say for ex:

There are 100 process which want GPU ..but GPU can provide service to max 20  process at the same time ..in such cases we can have COUNTING SEMAPHORE AS 20 ..so any 20 processes are allowed to use the GPU while other will go to sleep ...so out of 100 process we allow only 20 ..its a king of MUTX ...only ..

 

 

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@SHUBHAM SHASTRI

in the question they didn't specify any thing.

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a process can not got preempted inside of entry section and exit section but they can got preempted when they are in  critical section .many problems i have solved in which preemption was happening when a process was in the critical section .

in case of counting semaphore,

one and more process can enter into the critical section so mutual exclusion is not there  ,that's why answer should be B and C .

improper use of semaphore may lead in deadlock .ex - dinning philosopher problem