Semaphores (counting and binary) in Operating Systems - Step-by-Step Execution

Concept Flow - Semaphores (counting and binary)

Start

↓

Check semaphore value

↓

Decrement semaphore

→Allow access

↓

Block process

→Wait for semaphore increment

↓

End

The semaphore controls access by checking its value. If positive, it decrements and allows access; if zero, the process waits until the semaphore is incremented.

Execution Sample

Operating Systems

semaphore = 2
process1: wait(semaphore)
process2: wait(semaphore)
process3: wait(semaphore)
process1: signal(semaphore)

Three processes try to enter a critical section controlled by a semaphore initialized to 2.

Analysis Table

Step	Process	Semaphore Value Before	Action	Semaphore Value After	Process State
1	process1	2	wait: value>0, decrement	1	enters critical section
2	process2	1	wait: value>0, decrement	0	enters critical section
3	process3	0	wait: value=0, blocks	0	blocked, waiting
4	process1	0	signal: increment	1	exits critical section
5	process3	1	unblocked, wait: value>0, decrement	0	enters critical section

💡 All processes have entered or are waiting; semaphore controls access by blocking when value is zero.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
semaphore	2	1	0	0	1	0
process1 state	waiting	in critical section	in critical section	in critical section	exited	exited
process2 state	waiting	in critical section	in critical section	in critical section	in critical section	in critical section
process3 state	waiting	waiting	waiting (blocked)	waiting (blocked)	waiting (blocked)	in critical section

Key Insights - 3 Insights

Why does process3 block at step 3 even though it called wait?

At step 3 in the execution_table, the semaphore value is 0, so process3 cannot decrement it and must wait until another process signals (increments) the semaphore.

What happens to the semaphore value when a process signals?

As shown at step 4, signaling increments the semaphore value by 1, allowing blocked processes to proceed.

How does a binary semaphore differ from a counting semaphore in this flow?

A binary semaphore only allows values 0 or 1, so it controls access for one process at a time, while a counting semaphore can have values greater than 1 to allow multiple simultaneous accesses.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the semaphore value and what happens to process3?

A Semaphore is 2; process3 enters critical section

B Semaphore is 0; process3 blocks waiting

C Semaphore is 1; process3 enters critical section

D Semaphore is 0; process3 exits

💡 Hint

Check the 'Semaphore Value Before' and 'Process State' columns at step 3 in execution_table.

At which step does the semaphore value increase?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look for the 'signal' action in the 'Action' column in execution_table.

If the semaphore was initialized to 1 instead of 2, what would happen at step 2?

A process2 would block waiting

B process2 would enter critical section

C process3 would enter critical section

D semaphore value would be 2

💡 Hint

Refer to the semaphore value changes in variable_tracker and how wait works when value is 0.

Concept Snapshot

Semaphore controls access to resources.
Counting semaphore: value ≥ 0, allows multiple accesses.
Binary semaphore: value 0 or 1, allows single access.
wait() decrements if >0, else blocks.
signal() increments and may unblock waiting processes.

Full Transcript

Semaphores are tools to control access to shared resources. They hold a value representing available resources. When a process wants access, it calls wait(). If the semaphore value is greater than zero, wait() decrements it and allows the process to enter. If zero, the process blocks until another process calls signal(), which increments the semaphore and may unblock waiting processes. Counting semaphores allow multiple simultaneous accesses, while binary semaphores allow only one. This flow ensures safe access and prevents conflicts.