In context of semaphores choose the option with valid codes for wait() and signal() functions:

Question

Accepted Answer

The correct option is (2):
The classical definition of a semaphore’s operations, particularly for a counting semaphore, involves two atomic operations: wait (also called P) and signal (also called V). The function of the **wait()** operation is to attempt to acquire a resource. It must first check if the resource is available, and if not, it must block or busy-wait. Option 2’s `Wait (s) { while (s) <= 0; s--}` correctly implements this: the `while (s) <= 0;` line enforces the necessary busy-wait until the count is positive, and the subsequent `s--` atomically decrements the count to acquire the resource. The **signal()** function is executed when a process is exiting its critical section, releasing the resource. This is simply implemented by atomically increasing the semaphore count so another waiting process can proceed, which Option 2’s `signal (s) {s++; }` correctly executes. This pair represents the canonical, busy-waiting implementation of semaphores.

The incorrect option is (1):
This option's wait function, `Wait (s) { while (s) <= 0; s++ }`, is fundamentally flawed because it increments the semaphore variable 's' while the process is *waiting* (inside the busy-wait loop). The primary action of the wait operation is to *acquire* a resource, which must be represented by a decrement (`s--`). Incrementing 's' within the waiting loop would incorrectly signal resource availability and lead to a massive race condition, entirely defeating the purpose of mutual exclusion and synchronization that semaphores are designed to enforce.

The incorrect option is (3):
Option 3 entirely swaps the core logic of the two operations. It uses a non-blocking decrement for `Wait (s) {s--;}`, which allows a process to enter the critical section even when the resource is unavailable (s would become negative, leading to deadlocks or violations). Conversely, it places the blocking/busy-wait logic into the `signal` function, `signal (s) { while (s) <= 0; s++ }`. The signal operation is meant to be a simple, non-blocking release of a resource (`s++`), not a synchronizing operation that causes the releasing process to potentially block itself.

The incorrect option is (4):
Option 4 is also incorrect because it completely reverses the required arithmetic for resource management. The wait operation, `Wait (s) { s++; }`, unconditionally *increments* the semaphore, which is the logic for *releasing* a resource, not acquiring one. This would allow all processes to enter the critical section simultaneously, violating mutual exclusion. The signal function, `signal (s) { while (s) <= 0; s--; }`, incorrectly attempts to busy-wait and then decrement the resource count, which is the behavior expected from the wait function, not the signal function.