Operating Systems - Semaphore vs Mutex - When to Use Which
Consider a binary semaphore initialized to 1 protecting a critical section. If a thread calls wait() twice consecutively without a post(), what happens?
Consider a binary semaphore initialized to 1 protecting a critical section. If a thread calls wait() twice consecutively without a post(), what happens?
medium🧠🧾 Concept Trace Q4 of Q15
Step-by-Step Solution
Solution:
Step 1: Understand binary semaphore behavior
Binary semaphore initialized to 1 allows one wait() to proceed, decrementing count to 0.Step 2: Analyze second wait() call
Second wait() finds count 0, so thread blocks until a post() increments count.Step 3: Evaluate other options
Proceeding both times (A) ignores blocking semantics. Negative count (C) is not standard; semaphores block instead. Automatic reset (B) does not happen.Final Answer:
Option C -> Option CQuick Check:
Second wait() blocks on zero count [OK]
Quick Trick: Second wait() on binary semaphore blocks if count is zero [OK]
Common Mistakes:
MISTAKES- Assuming binary semaphore allows multiple entries without post
- Believing semaphore count can go negative causing error
- Thinking semaphore resets automatically
Trap Explanation:
PITFALL- Candidates often confuse binary semaphore with mutex or think it resets automatically after wait.
Interviewer Note:
CONTEXT- Checks understanding of semaphore wait/post blocking behavior and count semantics.
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