CODEWITHSUNDEEP
c++windowswinapisynchronization
David Haim
David Haim

Reputation: 26506

Can you really wait on Condition Variable with WaitFor...Object(s)?


I'm trying to implement some sort of waiting on many CONDITION_VARIABLE.
The answers here imply that WaitForMultipleObjects and such are valid options when dealing with Windows API (and many more places over the internet), but it appears that it is not the case.

first of all, nowhere in the MSDN documentation it is written that a Windows Condition variable is a valid argument for WaitFor... functions.
Second of all, it appears that WaitFor... only accepts HANDLE type as argument, which is basically a kernel object. but PCONDITION_VARIABLE is not really a HANDLE.
finally, trying to use a condition variable (both as a PCONDITION_VARIABLE and the undocumented CONDITION_VARIABLE::Ptr) makes the functions return error code 6 (invalid handle)

for example:

    CONDITION_VARIABLE cv;
    InitializeConditionVariable(&cv);
    auto res = WaitForSingleObject(cv.Ptr, INFINITE); //returns immediately
    if (res != WAIT_OBJECT_0) { 
        auto ec = GetLastError();
        std::cout << ec << "\n";
    }

so, can you really wait on a condition variable or it's just an urban legend?

Upvotes: 4

Views: 1972

Answers (2)

Cody Gray
Cody Gray

Reputation: 244843

The simple answer to your question is no. You cannot use the WaitForXxx functions with the condition variables provided by the Windows synchronization APIs. From the linked documentation:

Condition variables are synchronization primitives that enable threads to wait until a particular condition occurs. Condition variables are user-mode objects that cannot be shared across processes.

The WaitForXxx functions accept parameters of the generic HANDLE type, which represents a handle to a kernel object. Condition variables are user-mode objects, not kernel objects, so you cannot use them with these functions, since they work only with kernel objects.

Moreover, the documentation for these functions is pretty explicit about which types of objects they can wait on, and condition variables are not on that list. For instance, WaitForMultipleObjects says:

The WaitForMultipleObjects function can specify handles of any of the following object types in the lpHandles array:

  • Change notification
  • Console input
  • Event
  • Memory resource notification
  • Mutex
  • Process
  • Semaphore
  • Thread
  • Waitable timer

They all have the same list, so no confusion there.

Technically speaking (and we're diving into undocumented implementation details here, so you shouldn't rely on this as gospel), the Win32 WaitForSingleObject and WaitForMultipleObjects functions are built upon the KeWaitForSingleObject and KeWaitForMultipleObjects functions provided by the kernel subsystem. You can divide the objects supported by the kernel into three basic categories: dispatcher objects, I/O objects/data structures, and everything else. The first category, dispatcher objects, are the lowest level objects and they are all represented using the same DISPATCHER_HEADER data structure in their bodies. Dispatcher objects are the only types of objects that are "waitable". It is this DISPATCHER_HEADER structure that makes an object waitable, by definition. If the object is represented using this data structure, then it can be passed to the kernel synchronization functions. Thus, the same rules would apply to the Win32 functions.

This entire question seems to be based around a single statement that Managu makes in his answer: "Windows has WaitForMultipleObjects as aJ posted, which could be a solution if you're willing to restrict your code to Windows synchronization primitives." Perhaps he doesn't consider condition variables (as they are implemented by Windows) to be synchronization primitives, or perhaps he is just wrong. aJ's answer, to which he refers, is pretty clear about stating that WaitForMultipleObjects is used "to wait for multiple kernel objects," and we have already established that condition variables are not kernel objects. Either way, I don't see any evidence for an "urban legend" that you can do this.

Obviously you cannot use the WaitForXxx family of functions with boost::condition_variable, or std::condition_variable, or anything else. I'm sure you already knew that, but your question has confused some people because it links to a question that refers to the Boost implementation.

It is not especially clear to me why you would need to wait on multiple condition variables simultaneously. I guess you could write your own implementation of condition variables, based on the classic Win32 synchronization primitives, such as mutexes, which you can then wait on with WaitForMultipleObjects. You can probably find examples of such code online, since condition variables did not become part of the operating system until Vista. For example, this article discusses strategies for implementing condition variables in Windows as they are defined by the POSIX Pthreads specification. You could also look into using Event Objects.

Upvotes: 6

conio
conio

Reputation: 3718

I don't think so and it doesn't make any sense.

First of all, the WaitForXxx functions operate (mostly) on dispatcher objects - a subset of kernel objects including timers, events, mutexes, sempahores, threads and process (and a few internal object types like KAGTEs and KQUEUEs, but not access tokens or file mapping objects) that have a DISPATCHER_HEADER. It certainly won't work on user mode constructs that the kernel is unaware of.

Second, note that when you sleep ("wait") on a condition variable you have to specify whether this is critical section-based condition variable or a SRWL-based condition variable by using the correct function - either SleepConditionVariableCS or SleepConditionVariableSRW. So again, Windows (not only the kernel) has no idea what kind of condition variable you're passing it, but it needs this information to operate correctly. Since you don't provide this information to WaitForXxx it follows that they cannot be used with condition variables.

Upvotes: 7

Related Questions