Choose true statements in context of monitor based synchronization scheme that uses condition type -variable x:
I: Only wait() and signal() operations can be invoked on x.
II: If process P invokes x. wait() operation; then P is suspended until another process invokes x. signal().
III: The x. signal() operation resumes exactly one suspended process and if no process is suspended, then it has no effect.

Question

Choose true statements in context of monitor based synchronization scheme that uses condition type -variable x: 
I: Only wait() and signal() operations can be invoked on x. 
II: If process P invokes x. wait() operation; then P is suspended until another process invokes x. signal(). 
III: The x. signal() operation resumes exactly one suspended process and if no process is suspended, then it has no effect.

Accepted Answer

The correct option is (4):
The concept of the monitor, introduced by C.A.R. Hoare, is a high-level synchronization construct designed to simplify concurrent programming by encapsulating shared data and procedures that operate on that data. Central to the monitor scheme are condition variables, such as 'x', which processes use to wait for specific conditions to be met. Statement I is fundamentally true: a condition variable is a specialized synchronization aid that supports only the `wait()` and `signal()` operations. Unlike general variables, no other operations are permitted. Statement II accurately describes the waiting mechanism: when a process P calls `x.wait()`, it is suspended and is placed onto a waiting queue for the condition variable `x`. The process remains suspended until another process executes an `x.signal()` call. Finally, Statement III is also a defining characteristic of condition variables in the monitor model: the `x.signal()` operation wakes up exactly one process from the waiting queue of 'x'. If no process is currently waiting on 'x', the `signal()` operation simply has no effect, which is a crucial distinction from semaphore signaling where a signal would be remembered as an increment. Since all three statements correctly describe the behavior of a monitor's condition variable, the option combining them is the right choice.

The incorrect option is (1):
This option, "I and II," is incorrect because it excludes Statement III, which is a true and critical rule of the monitor model. The rule that `x.signal()` wakes up at most one waiting process and has no effect if the queue is empty is what distinguishes a condition variable from a counting semaphore. Therefore, an option that omits this defining behavioral statement is incomplete and fails to fully capture the correct concept.

The incorrect option is (2):
This option, "II and III," is incorrect because it excludes Statement I. Statement I is an absolute rule of the monitor construct, limiting condition variable operations strictly to `wait()` and `signal()` to maintain the integrity of the synchronization scheme. Monitors rely on this operational simplicity to ensure mutual exclusion within the critical section, and ignoring this rule renders the option fundamentally flawed.

The incorrect option is (3):
This option, "I and III," is incorrect because it excludes Statement II. Statement II describes the core action of a process using a condition variable: a process explicitly suspends itself (`wait()`) when it cannot proceed due to a false condition, which is the primary use case of condition variables. Without the ability to wait and be suspended until signaled, the condition variable would not be a usable synchronization tool.