Semaphores (counting and binary) in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Counting Semaphore Behavior

What happens when a counting semaphore initialized to 3 is signaled (V operation) twice after three wait (P) operations have already been performed?

A The semaphore value becomes 5, which is invalid because it exceeds the initial count.

B The semaphore value remains 0, so no additional wait operations can proceed without blocking.

C The semaphore value becomes 2, allowing two more wait operations without blocking.

D The semaphore value becomes 1, allowing one more wait operation without blocking.

💡 Hint

Remember that signaling a counting semaphore increments its value, allowing more processes to enter the critical section.

A counting semaphore initialized to 3 starts with a value of 3. After three wait operations, the value decreases to 0. Two signal operations increment it by 2, resulting in a value of 2. This means two more wait operations can proceed without blocking.

📋 Factual

intermediate

Binary Semaphore Characteristics

Which of the following statements correctly describes a binary semaphore?

A It can only have values 0 or 1 and is used to enforce mutual exclusion.

B It automatically resets to 0 after each signal operation.

C It allows multiple processes to enter the critical section simultaneously.

D It can have any non-negative integer value and is used to count resources.

💡 Hint

Think about the simplest semaphore used for locking.

A binary semaphore only takes values 0 or 1 and is typically used to ensure that only one process accesses a critical section at a time, enforcing mutual exclusion.

🚀 Application

advanced

Semaphore Usage in Producer-Consumer Problem

In a producer-consumer problem using semaphores, which semaphore should the consumer wait on before consuming an item?

A The semaphore counting the number of empty slots in the buffer.

B The semaphore counting the number of full slots in the buffer.

C A binary semaphore used to protect the buffer access.

D A semaphore initialized to zero that counts the number of producers.

💡 Hint

Consumers can only consume if there is at least one item available.

The consumer must wait on the semaphore that counts full slots to ensure there is an item to consume. Waiting on empty slots or a binary semaphore for mutual exclusion does not guarantee availability of items.

🔍 Analysis

advanced

Deadlock Scenario with Semaphores

Which of the following semaphore usage patterns can cause a deadlock?

A A counting semaphore is initialized to a value greater than the number of processes.

B A process signals a semaphore before waiting on it.

C A binary semaphore is used to protect a critical section accessed by one process only.

D Two processes each wait on a semaphore held by the other, causing circular wait.

💡 Hint

Consider what happens when two processes wait indefinitely for each other.

Deadlock occurs when two or more processes wait forever for resources held by each other, creating a circular wait condition. This happens if each process waits on a semaphore the other holds.

❓ Reasoning

expert

Effect of Incorrect Semaphore Initialization

What is the most likely outcome if a binary semaphore used for mutual exclusion is incorrectly initialized to 2 instead of 1?

A Multiple processes may enter the critical section simultaneously, breaking mutual exclusion.

B The semaphore will cause a deadlock immediately after the first wait operation.

C The semaphore will automatically reset to 1 after the first signal operation.

D The semaphore will behave like a counting semaphore with no effect on mutual exclusion.

💡 Hint

Think about what happens if the lock allows more than one holder.

If a binary semaphore is initialized to 2, it allows two processes to enter the critical section simultaneously, violating mutual exclusion and potentially causing race conditions.