50 Operating System MCQs with Answers

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Question 1

Three concurrent processes X, Y, and Z execute three different code segments that access and update certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, b and c; process Y executes the P operation on semaphores b, c and d; process Z executes the P operation on semaphores c, d, and a before entering the respective code segments. After completing the execution of its code segment, each process invokes the V operation (i.e., signal) on its three semaphores. All semaphores are binary semaphores initialized to one. Which one of the following represents a deadlockfree order of invoking the P operations by the processes? (GATE CS 2013)

  • X: P(a)P(b)P(c) Y:P(b)P(c)P(d) Z:P(c)P(d)P(a)

  • X: P(b)P(a)P(c) Y:P(b)P(c)P(d) Z:P(a)P(c)P(d)

  • X: P(b)P(a)P(c) Y:P(c)P(b)P(d) Z:P(a)P(c)P(d)

  • X: P(a)P(b)P(c) Y:P(c)P(b)P(d) Z:P(c)P(d)P(a)

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Question 2

Processes P

1

and P

2

use critical_flag in the following routine to achieve mutual exclusion. Assume that critical_flag is initialized to FALSE in the main program.


get_exclusive_access ( ) { if (critical _flag == FALSE) { critical_flag = TRUE ; critical_region () ; critical_flag = FALSE; } }

Consider the following statements.


i. It is possible for both P

1

and P

2

to access critical_region concurrently.


ii. This may lead to a deadlock.


Which of the following holds?

  • (i) is false and (ii) is true

  • Both (i) and (ii) are false

  • (i) is true and (ii) is false

  • Both (i) and (ii) are true

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Question 3

A system has n resources R0,...,Rn-1,and k processes P0,....Pk-1 .The implementation of the resource request logic of each process Pi is as follows:  

 if (i % 2 == 0) {
      if (i < n) request Ri
      if (i+2 < n) request Ri+2
}
else {
      if (i < n) request Rn-i
      if (i+2 < n) request Rn-i-2
}

In which one of the following situations is a deadlock possible?

  • n=40, k=26

  • n=21, k=12

  • n=20, k=10

  • n=41, k=19

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Question 4

Consider a system with 4 types of resources R1 (3 units), R2 (2 units), R3 (3 units), R4 (2 units). A non-preemptive resource allocation policy is used. At any given instance, a request is not entertained if it cannot be completely satisfied. Three processes P1, P2, P3 request the sources as follows if executed independently. 

Process P1: 
t=0: requests 2 units of R2 
t=1: requests 1 unit of R3 
t=3: requests 2 units of R1 
t=5: releases 1 unit of R2    
        and 1 unit of R1. 
t=7: releases 1 unit of R3 
t=8: requests 2 units of R4 
t=10: Finishes

Process P2: 
t=0: requests 2 units of R3 
t=2: requests 1 unit of R4 
t=4: requests 1 unit of R1 
t=6: releases 1 unit of R3 
t=8: Finishes

Process P3: 
t=0: requests 1 unit of R4 
t=2: requests 2 units of R1 
t=5: releases 2 units of R1 
t=7: requests 1 unit of R2 
t=8: requests 1 unit of R3 
t=9: Finishes

Which one of the following statements is TRUE if all three processes run concurrently starting at time t=0?

 

  • All processes will finish without any deadlock

  • Only P1 and P2 will be in deadlock.

  • Only P1 and P3 will be in a deadlock.

  • All three processes will be in deadlock

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Question 5

Which of the following is NOT true of deadlock prevention and deadlock avoidance schemes?

  • In deadlock prevention, the request for resources is always granted if the resulting state is safe

  • In deadlock avoidance, the request for resources is always granted if the result state is safe

  • Deadlock avoidance is less restrictive than deadlock prevention

  • Deadlock avoidance requires knowledge of resource requirements a priori

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Question 6

In Unix, files can be protected by assigning each one a 9-bit mode called rights bits. Now, consider the following two statements: I. A mode of 641 (octal) means that the owner can read and write the file, other members of the owner’s group can read it, and users can execute only. II. A mode of 100 (octal) allows the owner to execute the file, but prohibits all other access. Which of the following options is correct with reference to above statements ?
  • Only I is correct.
  • Only II is correct.
  • Both I and II are correct.
  • Both I and II are incorrect.
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Question 7

Which of the following commands or sequences of commands will rename a file x to file y in a Unix system? I. mv y, x II. mv x, y III. cp y, x (rm x) IV. cp x, y (rm x)
  • II and III
  • II and IV
  • l and III
  • II only
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Question 8

The shell command 

find -name passwd -print

is executed in /etc directory of a computer system running Unix. Which of the following shell commands will give the same information as the above command when executed in the same directory?  

  • ls passwd

  • cat passwd

  • grep name passwd

  • grep print passwd

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Question 9

Consider the following statements : 

(a) UNIX provides three types of permissions 
* Read 
* Write 
* Execute 

(b) UNIX provides three sets of permissions 
* permission for owner 
* permission for group 
* permission for others 

Which of the above statement/s is/are true?

  • only (a)

  • only (b)

  • Both (a) and (b)

  • Neither (a) nor (b)

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Question 10

The linux command “mknod myfifo b 4 16”

  • Will create a character device if the user is root

  • Will create a named pipe FIFO if the user is root

  • Will create a block device if the user is root

  • None of the above

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