In process synchronisation, when does the condition known as rendezvous arise between cooperating processes or threads?

Introduction / Context:
Rendezvous is a classical term used in operating systems and concurrent programming to describe a particular synchronisation pattern between cooperating processes or threads. Understanding this concept helps in designing correct communication protocols and avoiding race conditions. This question asks you to recognise the situation in which the rendezvous condition arises.

Given Data / Assumptions:

• Two or more processes or threads may need to coordinate at certain points.
• Communication and synchronisation primitives such as semaphores or message passing are available.
• Rendezvous refers to a specific requirement on when processes can continue execution.
• No numerical operations are needed.

Concept / Approach:
Rendezvous describes a meeting point in the control flow of multiple processes or threads. Each participant executes until it reaches the designated rendezvous point, then waits. Only when all participating processes or threads have reached that point is each one allowed to proceed. This pattern ensures that certain actions are coordinated, such as exchanging data at a specific time. It can be implemented using synchronous send and receive operations or carefully designed semaphore based protocols.

Step-by-Step Solution:
Step 1: Recall that rendezvous implies a meeting in time and space between participants. Step 2: In concurrent programming, interpret this as multiple processes reaching the same logical program point. Step 3: Understand that each process must wait at the rendezvous until all partners arrive. Step 4: After all have arrived, the synchronisation mechanism releases them so they can continue executing. Step 5: Choose the option that directly states that all processes must reach a specific point and wait until everyone is there.

Verification / Alternative check:
Many textbooks on operating systems present examples where a producer and a consumer perform a rendezvous when exchanging data, ensuring that the consumer waits until the producer has produced and the producer waits until the consumer is ready. This matches the description of both sides reaching a point and then continuing together, confirming the definition used in the correct option.

Why Other Options Are Wrong:
Option B is incorrect because a process running alone without interaction has no need for rendezvous. Option C describes system boot, which is unrelated to process synchronisation. Option D describes a deadlock like situation where processes are blocked indefinitely, which is the opposite of the coordinated progression that rendezvous is designed to achieve.

Common Pitfalls:
Learners sometimes confuse rendezvous with simple mutual exclusion, where only one process can enter a critical section at a time. In a rendezvous, the key property is that all participating processes meet and then continue together, not that they exclude each other. It is also easy to mix up rendezvous with barrier synchronisation; conceptually they are very close, and in many contexts the terms are used in a similar way.

Final Answer:
The rendezvous condition arises when two or more processes must each reach a specific point and wait there until all have arrived before any of them can proceed.