Condition Variableの由来
yohhoyC.A.R. Hoare (1974). "Monitors: An Operating System Structuring Concept," Communications of the ACM.
In many cases, there may be more than one reason for waiting, and these need to be distinguished by both the waiting and the signalling operation. We therefore introduce a new type of "variable" known as a "condition"; and the writer of a monitor should declare a variable of type condition for each reason why a program might have to wait. Then the wait and signal operations should be preceded by the name of the relevant condition variable, separated from it by a dot:
condvariable.wait; condvariable.signal;Note that a condition "variable" is neither true nor false; indeed, it does not have any stored value accessible to the program. In practice, a condition variable will be represented by an (initially empty) queue of processes which are currently waiting on the condition; but this queue is invisible both to waiters and signallers. This design of the condition variable has been deliberately kept as primitive and rudimentary as possible, so that it may be implemented efficiently and used flexibly to achieve a wide variety of effects. There is a great temptation to introduce a more complex synchronization primitive, which may be easier to use for many purposes. We shall resist this temptation for a while.