WaitForInputIdle waits for any thread, which might not be the thread you care about

We saw last time that the WaitForInputIdle function waits only once for a process to go input idle. Even if the process later stops processing messages, WaitForInputIdle will return immediately and say, "Yeah, he's idle."

The way a process is determined to be input idle is that it is waiting for user input when there is none. This translates into the process sitting in a function like GetMessage or WaitMessage when there are no messages. But what if a process has more than one thread? And what if one of the threads is waiting for input, while the other is busy and unresponsive? The WaitForInputIdle function will treat the process as having gone input idle, even if the ready thread is just displaying your splash screen and the busy thread is the one preparing the main window. The WaitForInputIdle function doesn't know that the main window is more important than the splash screen; as far as the window manager is concerned, you've got two threads, each with a window.

What this means for your application is that you need to know that, if you create multiple threads, then the moment any of them goes input idle, the entire process is treated as input idle, and you need to be ready for people who were waiting for you via WaitForInputIdle to start trying to talk to your application.

As I noted last time, the WaitForInputIdle function is really just a stopgap to help with the transition from 16-bit Windows to 32-bit Windows. Whereas 16-bit programs could just charge ahead knowing that the program it launched is ready (because if it weren't ready, then it would still have control of the CPU), 32-bit programs need to wait for this faked-up version of Yield. And since it was created merely to aid in porting 16-bit programs, the WaitForInputIdle function didn't really worry about multiple threads. After all, 16-bit Windows didn't support multiple threads per process, so all 16-bit programs were necessarily single-threaded. If you're porting one of these programs, your initial 32-bit version is also going to be single-threaded. (At least I hope you're not going to try to add multiple threads right off the bat. The first step in porting is just to get the program to run without adding any new features!)

In fact, back in the old days, the WaitForInputIdle function tried a bit too hard to emulate the Yield behavior from 16-bit Windows. When the target application received a message, it was taken out of the input idle state, and went back into the state when it once again reached a state where it was waiting for input. In other words, the one-line summary of the WaitForInputIdle function was actually correct at the time it was written.

The old mechanism for WaitForInputIdle, taking the process in and out of the input idle state, mirrored the 16-bit behavior of Windows, but only if you had a single-threaded application. If you had multiple threads, then the input-idle state starts getting all wonky as each thread updates the global idle state:

Thread 1 Thread 2
1 Busy Busy
2 Idle
(application marked as idle)
3 Idle
(application marked as idle)
4 Busy
(application marked as busy)
5 Busy
(application marked as busy)

There is only one state that gets updated by each thread as they go idle or busy. All you really know is that if all threads are busy, then the input idle state will be busy and that if all threads are idle, then the input idle state will be idle. But if one thread is idle and the other is busy, then the process idle state is unpredictable; it depends on what the most recent transition was. For example, notice that at step 2, we have one idle thread (thread 1) and one busy thread (thread 2), and the process is marked idle. On the other hand, at step 4, we also have one idle thread (thread 2) and one busy thread (thread 1), but this time, the process is marked busy.

Of course, since this behavior was intended to mimic the 16-bit programming model, the fact that it got all confused when applications created multiple threads was "out of scope". Creating multiple threads meant that you have gone past the simple "Just trying to get it to work" stage and have moved on to adding Win32-specific enhancements. You were creating a situation that the WaitForInputIdle function was not designed to handle.

My guess is that the unpredictable nature of the original design led the window manager folks to take a deep breath and go back to the spirit of the WaitForInputIdle function: To tell when a program has finished its initialization.

So now it decides that when you've finished initialization, you've finished initialization. It's a one-way door.

Comments (6)
  1. Pierre B. says:

    I don’t have the knowledge pool of hundreds of application (MS might have a better clue on this) but I would have thought that, in the case of multiple threads having windows, the initial thread could be chosen as the real one to wait to go idle.

    I’m sure there is some wily software out there where the main window thread ends up being some other thread and the initial thread handles the splash screen, but for most software, my guess is that the initial thread ends up being the main window thread.

    I know that’s how I’ve coded it in the past (the startup code launching a thread to display the spalsh screen).

    It’s a small improvement to an aguably unfixable function, but an improvement nonetheless.

  2. Matthew says:

    @ Pierre:

    There is a class of programs that invalidates your assumption that the initial thread is most likely to be the "main" thread: those which operate based primarily on modular plugins, with minimal built-in functionality. Here, the more usual case is that the initial thread performs fairly minimal activities, as the "real" thread is launched via a desired plug-in DLL.

    Trying to apply extremely broad assumptions where there is no actual, logical constraint is a very, very bad practice.

  3. Is there a “not” too many in the first paragraph?

    [Indeed there is. Fixed, thanks. -Raymond]
  4. Back when the global busy/idle state was updated by each thread, would the state get updated (and how) when a thread exited? I’m envisioning a program where both threads are idle, then one becomes busy and the global state indicates busy; then that thread exits while still busy, leaving only one thread, which is in fact idle, but the global status indicates busy.

    [No such cleanup took place when a thread exited. So you can’t even say that “busy” means that there is at least one busy thread! The flag was basically garbage for multi-threaded programs. -Raymond]
  5. Gabe says:

    I don’t understand the "WaitForSingleObject" reference. The link points to a post about GetDateFormat and this post is about WaitForInputIdle.

  6. Messiant R says:

    @ Gabe

    Surely WaitForInputIdle is meant instead of WaitForSingleObject.

    And the link simply explains another case of description changing as a result of changes in functionality, the actual functions don’t matter.

    [Fixed the function name, thanks. And the link target is correct, as you figured out. -Raymond]

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