Condition variable - Definition and Overview

A condition variable is a type representing a queue of suspended (delayed) processes or threads waiting on some condition. It can be considered as part of the monitor synchronization mechanism, and it used by a process or thread to delay until the monitor's state satisfies some condition; it is also used to awaken a delayed process when the condition becomes true.

Two basic operations are defined:

• wait: enqueue the calling process on the condition variable.
• signal: wake up one process waiting on the condition variable (if there is one). (Different implementations of signal are possible; it may be guaranteed to awaken the process at the head of the queue, or it may be simply guaranteed to awaken some process from the queue.)

Example: A monitor for the producer-consumer problem.

monitor ProducerConsumer
{
    ItemType buffer[slots];
    Condition bufferHasSpace, bufferHasData;

    procedure putItem(ItemType & item)
    {
        if (itemCount == slots)
            bufferHasSpace.wait();
        buffer.add(item);
        bufferHasData.signal();
    }

    procedure getItem(ItemType & item)
    {
        if (itemCount == 0)
            bufferHasData.wait();
        item = buffer.removeFirst();
        bufferHasSpace.signal();
    }
}