Difference between event object and condition variable

Question

What is the difference between event objects and condition variables? I am asking in context of WIN32 API.

Answer 1

Event objects are kernel-level objects. They can be shared across process boundaries, and are supported on all Windows OS versions. They can be used as their own standalone locks to shared resources, if desired. Since they are kernel objects, the OS has limitations on the number of available events that can be allocated at a time.

Condition Variables are user-level objects. They cannot be shared across process boundaries, and are only supported on Vista/2008 and later. They do not act as their own locks, but require a separate lock to be associated with them, such as a critical section. Since they are user- objects, the number of available variables is limited by available memory. When a Conditional Variable is put to sleep, it automatically releases the specified lock object so another thread can acquire it. When the Conditional Variable wakes up, it automatically re-acquires the specified lock object again.

In terms of functionality, think of a Conditional Variable as a logical combination of two objects working together - a keyed event and a lock object. When the Condition Variable is put to sleep, it resets the event, releases the lock, waits for the event to be signaled, and then re-acquires the lock. For instance, if you use a critical section as the lock object, SleepConditionalVariableCS() is similar to a sequence of calls to ResetEvent(), LeaveCriticalSection(), WaitForSingleObject(), and EnterCriticalSection(). Whereas if you use a SRWL as the lock, SleepConditionVariableSRW() is similar to a sequence of calls to ResetEvent(), ReleaseSRWLock...(), WaitForSingleObject(), and AcquireSRWLock...().