Difference between event object and condition vari

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...().

Answer 2

They are very similar, but event objects work across process boundaries, whereas condition variables do not. From the MSDN documentation on condition variables:

Condition variables are user-mode objects that cannot be shared across processes.

From the MSDN documentation on event objects:

Threads in other processes can open a handle to an existing event object by specifying its name in a call to the OpenEvent function.

Answer 3

The most significant difference is the Event object is a kernel object and can be shared across processes as long as it is alive when processes/threads are trying to acquire, on the contrary, Condition variable is a user mode object which is light(only has same size as a pointer and has nothing additional to be released after using it) and has better performance.

Typically, condition variable is often used along with locks, since we need to keep data synchronized properly. When considering Condition Variable, we treat it like keyed events which was improved since Vista.

Joe duffy has a blog post http://joeduffyblog.com/2006/11/28/windows-keyed-events-critical-sections-and-new-vista-synchronization-features/ that explained more detailed information.