WaitForSingleObject. Why you should never use it.

Updated 30/1/2013: Fixed a typo and formatting issue or two.  Added a sentence for clarity.  See also the short follow-up post.

I’ve really struggled with how to frame this post.  It could be about the dangers of WaitForSingleObject and WaitForMultipleObjects.  Or about how Delphi’s TThread.Synchronize seems so handy, and yet because it must use WaitForSingleObject, is so fraught with complications.  Or yet, about how pressing Alt+Left Shift to switch languages could hang an application.  In the end, it’s about all three of these things.

Some people, when confronted with a problem, think, “I know, I’ll use threads,” and then two they hav erpoblesms.

Let’s start with TThread.Synchronize.  It seems a handy little function: it will, in a thread-safe manner, call a procedure in the context of the main VCL thread (the thread that owns the Delphi VCL GUI windows), and wait, using the Windows API WaitForSingleObject(INFINITE), for the procedure to return.  Simpler than fiddling with synchronisation primitives, right?  Everyone knows threading is hard, so use the well-tested thread utilities where you can?

Except that WaitForSingleObject and its big brother WaitForMultipleObjects are dangerous.  The basic problem is that these calls can cause deadlocks, if you ever call them from a thread that has its own message loop and windows.  That’s okay, you say, I don’t have any UI except in my main thread.  But any thread that uses COM can have hidden COM helper windows when doing RPC (and see below for more on this).  And other libraries can create their own windows as well, such as the ADO libraries.

So what causes the deadlock? Well, I’ll illustrate with the scenario we ran into.  Some old code (that I wrote, okay, okay) created a thread (we’ll call it BackgroundThread).  BackgroundThread used TThread.Synchronize to periodically update the UI status about a background database process it was running.  It doesn’t really matter what it was doing, but the use of Microsoft’s ADO database library meant that this thread was creating a hidden window, with the class ADODB.AsyncEventMessenger.  Behind the scenes, a second window was automatically created by Windows once we had the first window in the thread, and this one had the class name IME.

Every now and then, our BackgroundThread would call Synchronize(RefreshStatus).  This would signal an event which the main thread would check periodically from its message loop.  Eventually it would call the RefreshStatus procedure.  BackgroundThread would in the meantime have called WaitForSingleObject(INFINITE) to wait for an event to be signaled by the main thread indicating that the RefreshStatus procedure had finished.

Where are we going?  Well, if the main thread receives a message that it then decides to send on to other windows in the process, while BackgroundThread is getting ready to synchronize, we can end up in a deadlock.  And, it turns out that in Windows XP, this can happen when WM_INPUTLANGCHANGEREQUEST (0x50) is received, e.g. when the user presses Alt+Left Shift.  (Note, for this scenario to play, the Input Method Manager must be enabled — install Far Eastern language support in Windows XP).  Remember, this is just one possible scenario which can trigger a deadlock.

Let’s pull this apart.  I’ve loaded the stalled process into WinDbg, and am now looking at two call stacks which have deadlocks, in user mode.  First the main VCL GUI thread:

   0  Id: bf8.cb8 Suspend: 1 Teb: 7ffde000 Unfrozen
ChildEBP RetAddr
0012fadc 7e4194be ntdll!KiFastSystemCallRet
0012fb30 7e43652f USER32!NtUserMessageCall+0xc
0012fb50 7e418734 USER32!EditWndProcW+0x5d
0012fb7c 7e418816 USER32!InternalCallWinProc+0x28
0012fbe4 7e42a013 USER32!UserCallWinProcCheckWow+0x150
0012fc14 7e42a039 USER32!CallWindowProcAorW+0x98
0012fc34 004c0e7d USER32!CallWindowProcW+0x1b
0012fda4 004c0d80 audit4_home!Vcl.Controls.TWinControl.DefaultHandler+0xdd
0012fdf0 004c03d3 audit4_home!Vcl.Controls.TWinControl.WndProc+0x5b8
0012fe20 00467b3e audit4_home!Vcl.Controls.TWinControl.MainWndProc+0x2f
0012fe38 7e418734 audit4_home!System.ClassesStdWndProc+0x16
0012fe64 7e418816 USER32!InternalCallWinProc+0x28
0012fecc 7e4189cd USER32!UserCallWinProcCheckWow+0x150
0012ff2c 7e418a10 USER32!DispatchMessageWorker+0x306
0012ff3c 005a6980 USER32!DispatchMessageW+0xf
0012ff58 005a69c3 audit4_home!Vcl.Forms.TApplication.ProcessMessage+0xf8
0012ff7c 00d26d60 audit4_home!Vcl.Forms.TApplication.HandleMessage+0xf
0012ff9c 016e6903 audit4_home!S4s.Ui.Session.Appsession_main.TAppSession_Main.Run+0xcc
0012ffc0 7c817077 audit4_home!Audit4_home.initialization+0xc3
0012fff0 00000000 kernel32!BaseProcessStart+0x23

And then our BackgroundThread:

   9  Id: bf8.f30 Suspend: 1 Teb: 7ffd5000 Unfrozen
ChildEBP RetAddr
0494fdb8 7c90df5a ntdll!KiFastSystemCallRet
0494fdbc 7c8025db ntdll!NtWaitForSingleObject+0xc
0494fe20 7c802542 kernel32!WaitForSingleObjectEx+0xa8
0494fe34 0042d4b3 kernel32!WaitForSingleObject+0x12
0494fe80 00408a19 audit4_home!System.Sysutils.WaitForSyncWaitObj+0x7
0494fed0 004654f2 audit4_home!System.TMonitor.Wait+0x25
0494fedc 00c9f3a2 audit4_home!System.Classes.TThread.Synchronize+0x2e
0494fef8 00c9ef29 audit4_home!S4s.Br.Backgroundclasses.Backgroundclass.TBackgroundClass.SetStatus+0x8a
0494ff70 00464b11 audit4_home!S4s.Br.Backgroundclasses.Backgroundclass.TBackgroundClass.Execute+0x18d
0494ffa0 00409752 audit4_home!System.Classes.ThreadProc+0x45
0494ffb4 7c80b729 audit4_home!SystemThreadWrapper+0x2a
0494ffec 00000000 kernel32!BaseThreadStart+0x37

We can see that the main thread has sent a message somewhere.  It turns out it has sent a message to a window in the same thread (The window handle 0006040c is just the Edit window):

0012fb30 7e43652f 0006040c 00000050 00000001 USER32!NtUserMessageCall+0xc
0012fb50 7e418734 0006040c 00000050 00000001 USER32!EditWndProcW+0x5d

So why is it stalling?  It’s hard to determine here, because everything bad is happening in kernel mode, behind that KiFastSystemCallRet call. That must mean it’s time to step into kernel mode!  In WinDbg, press Ctrl+K, select Local.  I’m learning here, so this is as much for my own documentation as to explain to anyone else (in other words, if it doesn’t make sense to you, it won’t make sense to me, either, in 6 weeks time).  First, we find the process details:

lkd> !process 0 0 audit4_home.exe
PROCESS 894c3710  SessionId: 0  Cid: 0bf8    Peb: 7ffdf000  ParentCid: 0e0c
DirBase: 0a5c0700  ObjectTable: 00000000  HandleCount:   0.
Image: audit4_home.exe

PROCESS 893277e0  SessionId: 0  Cid: 0d94    Peb: 7ffd8000  ParentCid: 0e0c
DirBase: 0a5c0720  ObjectTable: 00000000  HandleCount:   0.
Image: audit4_home.exe

PROCESS 8950d9a8  SessionId: 0  Cid: 0f10    Peb: 7ffdb000  ParentCid: 0e0c
DirBase: 0a5c0740  ObjectTable: 00000000  HandleCount:   0.
Image: audit4_home.exe

PROCESS 89566758  SessionId: 0  Cid: 0520    Peb: 7ffde000  ParentCid: 0e0c
DirBase: 0a5c0840  ObjectTable: e127a330  HandleCount: 467.
Image: audit4_home.exe

We’ll look at the last process listed, being the one with the problem (the other three are defunct):

lkd> !process 89566758 2
PROCESS 89566758  SessionId: 0  Cid: 0520    Peb: 7ffde000  ParentCid: 0e0c
DirBase: 0a5c0840  ObjectTable: e127a330  HandleCount: 467.
Image: audit4_home.exe

THREAD 8953a920  Cid 0520.009c  Teb: 7ffdd000 Win32Thread: e1210eb0 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
8953aabc  Semaphore Limit 0x2

THREAD 8963bda0  Cid 0520.0a98  Teb: 7ffd9000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
8963bf3c  Semaphore Limit 0x2

THREAD 8953b9a0  Cid 0520.03c8  Teb: 7ffd8000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
8953bb3c  Semaphore Limit 0x2

THREAD 895a2c08  Cid 0520.021c  Teb: 7ffd7000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
895a2da4  Semaphore Limit 0x2

THREAD 898f8cc8  Cid 0520.0bd8  Teb: 7ffd4000 Win32Thread: e2d75870 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
898f8e64  Semaphore Limit 0x2

THREAD 893287f8  Cid 0520.0eec  Teb: 7ff4f000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89328994  Semaphore Limit 0x2

THREAD 89308c30  Cid 0520.00f0  Teb: 7ff4e000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89308dcc  Semaphore Limit 0x2

THREAD 89ab4020  Cid 0520.01b0  Teb: 7ff4d000 Win32Thread: e2ed4260 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89ab41bc  Semaphore Limit 0x2

THREAD 893269b0  Cid 0520.0830  Teb: 7ff4c000 Win32Thread: e23bf2c0 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89326b4c  Semaphore Limit 0x2

THREAD 89895778  Cid 0520.0ce0  Teb: 7ff4b000 Win32Thread: e2fa06e8 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89895914  Semaphore Limit 0x2

THREAD 8933ba80  Cid 0520.0e08  Teb: 7ff4a000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
8933bc1c  Semaphore Limit 0x2

THREAD 89322728  Cid 0520.0f28  Teb: 7ff49000 Win32Thread: 00000000 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
893228c4  Semaphore Limit 0x2

THREAD 89449960  Cid 0520.0ab0  Teb: 7ff48000 Win32Thread: e12d5590 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
89449afc  Semaphore Limit 0x2

THREAD 8952d020  Cid 0520.09dc  Teb: 7ffdc000 Win32Thread: 00000000 WAIT: (Executive) KernelMode Non-Alertable
SuspendCount 1
b8c197d4  SynchronizationEvent

And we can see the first thread has the address 8953a920.  So let’s look at that (flag 16 means show the full stack in the process context, with parameters).

lkd> !thread 8953a920 16
THREAD 8953a920  Cid 0520.009c  Teb: 7ffdd000 Win32Thread: e1210eb0 WAIT: (Suspended) KernelMode Non-Alertable
SuspendCount 1
FreezeCount 1
8953aabc  Semaphore Limit 0x2
Not impersonating
DeviceMap                 e1050c08
Owning Process            0       Image:
Attached Process          89566758       Image:         audit4_home.exe
Wait Start TickCount      213083         Ticks: 67800 (0:00:17:39.375)
Context Switch Count      20200  NoStackSwap    LargeStack
UserTime                  00:00:00.781
KernelTime                00:00:01.484
Win32 Start Address audit4_home!Audit4_home.initialization (0x016e6840)
Start Address kernel32!BaseProcessStartThunk (0x7c810705)
Stack Init b7c1f000 Current b7c1e840 Base b7c1f000 Limit b7c19000 Call 0
Priority 10 BasePriority 8 PriorityDecrement 0 DecrementCount 16
ChildEBP RetAddr  Args to Child
b7c1e858 80503864 8953a990 8953a920 804fb094 nt!KiSwapContext+0x2f (FPO: [Uses EBP] [0,0,4])
b7c1e864 804fb094 8953aa8c 8953a920 8953a954 nt!KiSwapThread+0x8a (FPO: [0,0,0])
b7c1e88c 80502fa0 00000000 00000005 00000000 nt!KeWaitForSingleObject+0x1c2 (FPO: [5,5,4])
b7c1e8a4 804ff8e0 00000000 00000000 00000000 nt!KiSuspendThread+0x18 (FPO: [3,0,0])
b7c1e8ec 80503882 00000000 00000000 00000000 nt!KiDeliverApc+0x124 (FPO: [3,10,0])
b7c1e904 804fb094 00000240 e1210eb0 00000000 nt!KiSwapThread+0xa8 (FPO: [0,0,0])
b7c1e92c bf802f15 00000000 0000000d 00000001 nt!KeWaitForSingleObject+0x1c2 (FPO: [5,5,4])
b7c1e968 bf835eb7 00000200 00000000 00000000 win32k!xxxSleepThread+0x192 (FPO: [3,5,4])
b7c1ea04 bf8141d2 bbe83720 00000287 00000019 win32k!xxxInterSendMsgEx+0x7f6 (FPO: [Non-Fpo])
b7c1ea50 bf80ecd9 bbe83720 00000287 00000019 win32k!xxxSendMessageTimeout+0x11f (FPO: [7,7,0])
b7c1ea74 bf92b42e bbe83720 00000287 00000019 win32k!xxxSendMessage+0x1b (FPO: [4,0,0])
b7c1eaa4 bf92c675 e2d75870 e13f10e0 e13f10e0 win32k!xxxImmActivateLayout+0x5b (FPO: [2,3,4])
b7c1ec08 bf8696d9 00000004 00000000 e13f10e0 win32k!xxxImmActivateThreadsLayout+0x10c (FPO: [3,82,4])
b7c1ec48 bf86862f e13f10e0 00000100 bbe88228 win32k!xxxInternalActivateKeyboardLayout+0xb7 (FPO: [3,8,4])
b7c1ec70 bf80b5a7 8989a2d0 04090c09 00000100 win32k!xxxActivateKeyboardLayout+0x4c (FPO: [4,3,0])
b7c1ecd4 bf80ec9f bbe88228 00000050 00000001 win32k!xxxRealDefWindowProc+0x56d (FPO: [4,16,0])
b7c1ecec bf81c176 bbe88228 00000050 00000001 win32k!xxxWrapRealDefWindowProc+0x16 (FPO: [5,0,0])
b7c1ed08 bf80eee6 bbe88228 00000050 00000001 win32k!NtUserfnNCDESTROY+0x27 (FPO: [7,0,0])
b7c1ed40 8054168c 0006040c 00000050 00000001 win32k!NtUserMessageCall+0xae (FPO: [7,3,0])
b7c1ed40 7c90e514 0006040c 00000050 00000001 nt!KiFastCallEntry+0xfc (FPO: [0,0] TrapFrame @ b7c1ed64)
0012fadc 7e4194be 7e428e0d 0006040c 00000050 ntdll!KiFastSystemCallRet (FPO: [0,0,0])
0012fb30 7e43652f 0006040c 00000050 00000001 USER32!NtUserMessageCall+0xc
0012fb50 7e418734 0006040c 00000050 00000001 USER32!EditWndProcW+0x5d (FPO: [4,0,4])
0012fb7c 7e418816 7e4364cf 0006040c 00000050 USER32!InternalCallWinProc+0x28
0012fbe4 7e42a013 00000000 7e4364cf 0006040c USER32!UserCallWinProcCheckWow+0x150 (FPO: [Non-Fpo])
0012fc14 7e42a039 7e4364cf 0006040c 00000050 USER32!CallWindowProcAorW+0x98 (FPO: [6,0,0])
0012fc34 004c0e7d 7e4364cf 0006040c 00000050 USER32!CallWindowProcW+0x1b (FPO: [5,0,0])
0012fda4 004c0d80 02940d99 02bb9430 fffffffe audit4_home!Vcl.Controls.TWinControl.DefaultHandler+0xdd
0012fdf0 004c03d3 0012fe04 004c03eb 0012fe20 audit4_home!Vcl.Controls.TWinControl.WndProc+0x5b8
0012fe20 00467b3e 00000050 00000001 04090c09 audit4_home!Vcl.Controls.TWinControl.MainWndProc+0x2f
0012fe38 7e418734 0006040c 00000050 00000001 audit4_home!System.ClassesStdWndProc+0x16
0012fe64 7e418816 02940d99 0006040c 00000050 USER32!InternalCallWinProc+0x28
0012fecc 7e4189cd 00000000 02940d99 0006040c USER32!UserCallWinProcCheckWow+0x150 (FPO: [Non-Fpo])
0012ff2c 7e418a10 0012ff60 00000000 0006040c USER32!DispatchMessageWorker+0x306 (FPO: [Non-Fpo])
0012ff3c 005a6980 0012ff60 00120100 0012ff9c USER32!DispatchMessageW+0xf (FPO: [1,0,0])
0012ff4c 7c910222 0000000f 028458e0 005a69c3 audit4_home!Vcl.Forms.TApplication.ProcessMessage+0xf8
0012ff9c 016e6903 0012ffb0 016e6916 0012ffc0 ntdll!RtlpAllocateFromHeapLookaside+0x42 (FPO: [Non-Fpo])
0012ffc0 7c817077 7c910222 0000000f 7ffde000 audit4_home!Audit4_home.initialization+0xc3
0012fff0 00000000 016e6840 00000000 00000018 kernel32!BaseProcessStart+0x23 (FPO: [Non-Fpo])

Highlighted above there is a call to win32k!xxxSendMessage.  This is passed the address of a window at bbe83720.  And when we look at that, we see the following:

lkd> dd bbe83720
bbe83720  000d01fe 00000006 e2d75870 8998ae50

And window 000d01fe is the IME window for the other, deadlocked thread!  Why is the kernel sending a message to another thread here and now?  And without a timeout?  That’s our deadlock.  We know how it happens, but I at least still don’t know why!

Still, that’s a step forward.  Next was to figure out why we were getting that.  After a fair bit of exploration, I looked at little harder at the call stack, and decided to investigate the parameters for xxxActivateKeyboardLayout:

8989a2d0 04090c09 00000100 win32k!xxxActivateKeyboardLayout+0x4c 

That third parameter in xxxActivateKeyboardLayout corresponds to the Flags parameter for ActivateKeyboardLayout.  Perusing the documentation for the Windows ActivateKeyboardLayout function, we can see that 0x100 is KLF_SETFORPROCESS.  Bingo!  That sounds pretty suspicious!  I worked through the assembly code for xxxActivateKeyboardLayout and xxxRealDefWindowProc, and sure enough, that was it: when xxxRealDefWindowProc processes WM_INPUTLANGCHANGEREQUEST, it sets the Flags parameter to 0x0100:

8211f1dd 56              push    esi
8211f1de 6800010000      push    100h
8211f1e3 ff7514          push    dword ptr [ebp+14h]
8211f1e6 6a00            push    0
8211f1e8 e8dfe40100      call    win32k!_GetProcessWindowStation (8213d6cc)
8211f1ed 50              push    eax
8211f1ee e838bdf9ff      call    win32k!xxxActivateKeyboardLayout (820baf2b)

In Windows Vista and later versions, this problem does not arise as frequently, because the Text Services Framework takes over the Alt+Left Shift command (before DefWindowProc gets a look-in) and calls ActivateKeyboardLayout without the KLF_SETFORPROCESS flag set.  However, the issue can still arise if you ever use that flag in your own code when calling ActivateKeyboardLayout.

To summarize, it’s helpful to remember that technically, we were using the functions as designed.  The primary issue is combination of design flaws, first, with how that KLF_SETFORPROCESS flag is handled: either the kernel code should be using a timeout when it sends the message to the other thread, or it should be queuing an event for the other thread to handle when it gets around to it; and second, with TThread.Synchronize, which unfortunately by design cannot be robust against deadlocks.

In any case, to make your own code more robust:

  1. Determine, when using synchronisation calls, if there is any chance that windows could be created by your thread, and if so, use MsgWaitForMultipleObjects and a message loop instead of WaitForSingleObject or WaitForMultipleObjects.  Check the libraries you are using, and if you are using COM, it’s safest to assume that windows will be created.
  2. Don’t use the TThread.Synchronize procedure.  Recent versions of Delphi include TThread.Queue, which is asynchronous, and so avoids this deadlock.
  3. Think carefully about whether a thread is the right solution to the problem.
  4. Don’t use the KLF_SETFORPROCESS flag!

21 thoughts on “WaitForSingleObject. Why you should never use it.

  1. The following breakpoint was useful to break on the IME window procedure except on the main GUI thread (check !teb to find the appropriate StackBase to test):

    bp ImeWndProcW “.if(poi(@$teb+4) == 190000) {gc}”

  2. no wonder you had a deadlock

    remove the informing of finishing the mainform refresh to the thread

    i assume the thread doesn’t do anything with the message it gets from a finished redraw of the main form anyway

  3. Anonymous, I’m not sure why you think notification of mainform refresh is going to the thread. That isn’t happening. The blog post describes exactly what is happening.

  4. He probably thinks that because you said “BackgroundThread would in the meantime have called WaitForSingleObject(INFINITE) to wait for an event to be signaled by the main thread indicating that the RefreshStatus procedure had finished.”

    1. Thanks Eric. I guess I wasn’t very clear on that being the behaviour of TThread.Synchronize — which is kinda the root issue anyway — as my final point 2 says 🙂

  5. I’ve never been all that happy with this blog post. I don’t do a very good job of explaining the principles of the issue, although I go into gory detail in the case study.

    I feel some frustration with Anonymous’s comment — the “no wonder” followed by “assume” (in this case his assumption was incorrect anyway).

    The thing is, there shouldn’t have been a deadlock. Just changing a factor of the problem without understanding the root cause is not a good programming practice. And in this situation, the root cause of the deadlock was certainly unexpected.

    Furthermore, this model of thread synchronisation — using TThread.Synchronize — is still taught in nearly all Delphi books and online resources I have seen. And it’s fundamentally unworkable for so many use cases. This really is just one example of its brokenness.

  6. Is TThread.Synchronize() implemented using WFSO? I found this post when trying to make my own Thread::join() implementation. Are you saying that to join() I can always use ‘MsgWaitForMultipleObjects’ ? Would you care to post a simple Thread class implementation?

    1. Thanks for the comment, John. Delphi’s TThread.Synchronize uses WaitForSingleObject deep down, yes. But it’s not safe to just swap MsgWaitForMultipleObjects into its place without some careful analysis, because that makes the thread re-entrant, which can of course cause all sorts of grief… IsGUIThread() may be able to tell you if the thread has a message queue but it’s not well documented and I’m unclear what it actually is reporting on. WaitForSingleObject would be much safer if you knew the thread had no message queue of its own.

      I don’t think there is a simple, robust Thread class implementation with synchronisation support, and even if there was one, I am probably not qualified to create it! Anything I whipped up would be certain to include bugs and corner cases.

  7. Wow! Great exploration! I suffered from this problem for many years (I have a background thread which creates main window and do everything). The first workaround I found is to handle WM_INPUTLANGCHANGEREQUEST and call ActivateKeyboardLayout(lparam,0) there. I didn’t know why it worked, but now it seems to be because of flags=0. This worked, but causes problems with PuntoSwitcher 🙁 Another workaround is to call ImmDisableIME(0) – this works too, but Asian customers may be disappointed… Finally, thanks to your publication, the best solution for me is to process windows messages in the initial thread in addition to the main thread.

  8. Very interesting article. Thank you.

    We did have problems years ago (around 2005 with .net 2.0 and XP). We did have an application built with vs2005 (c#), which was using threads. For some reason the .net threads were suspended (all of them ) occasionally. After studying the problem we found out that a separate process was using “WaitForSingleObject” and during that wait, all .net managed threads did not have any time slice.
    Fortunately the process in question was our own, so we changed the “WaitForSingleObject” calls to “MsgWaitForMultipleObjects” and then dispatched all windows messages during the wait.
    That solved the problem. What I wonder is that I have not found any articles about the issued, execpt this one.
    My point is that if this is not a well known “feature” in windows anybody can implement a harmful process unintentionally. I have not tested Windows 7 but in XP this was a real problem.

    1. That’s very interesting, and thanks for writing about it. I wonder what the WFSO process was doing that caused all .NET threads to block? Sounds like a SendMessage somewhere in the system, curious indeed!

  9. This article should be called TThread.Synchronize and why you should never use it.

    You shouldn’t communicate data from a subthread to a main thread by a synchronous RPC mechanism like TThread.Synchronize. The clue is in the name “Synchronize” rather than something like “Queue RPC” which is what you actually want. Synchronize … duh … synchronizes the threads as well as running the RPC. Your subthread doesn’t need any results back from the main thread, it’s just notifying it, so a design that causes it to wait is highly flawed – and as you’ve identified, synchronizing them using the hokey implementation in TThread::Synchronize causes a deadlock in some cases.

    There’s no bug in WaitForSingleObject. There is a bug in TThread::Synchronize. Frankly, TThread::Synchronize shouldn’t exist at all, and it certainly shouldn’t be aimed at beginners. Blaming an OS function for your deadlock is really lame, especially when it’s caused by your misuse of libraries and/or bugs in those libraries. Pro-tip: if you find an issue when calling a library, 9 times out of 10 its the library or your usage of it, not a fundamental flaw in the operating system that nobody even found before.

    Did you know this shit comes up as the 3rd hit on Google for “WaitForSingleObject”? #facepalm

    1. FWIW, the ActivateKeyboardLayout bug is an acknowledged bug in Windows. I never claimed there was a bug in WaitForSingleObject. But you probably shouldn’t use it in a thread with a message queue (which isn’t always readily obvious). Thanks for reading!

  10. Hi Marc:

    I am java developer and am not aware of windows programming so decided I will ask you a question to clear a doubt about WaitForSingleObject. In java we have a method for parking threads in LockSupport class which in turn calls our JNI (Java Native Interface) method – void Parker::park(bool isAbsolute, jlong time) and currently I am looking at the windows implementation of our JVM for version 7. And inside this method we have call to WaitForSingleObject as- WaitForSingleObject(_ParkEvent, time); ResetEvent(_ParkEvent);//The time in WaitForSingleObject is set to INFINITE
    My question: Will the thread that is executing this method return from WaitForSingleObject to execute ResetEvent and proceed further or will this thread be blocked on execution of WaitForSingleObject ?
    The docs for WaitForSingleObject (https://msdn.microsoft.com/en-us/library/windows/desktop/ms687032(v=vs.85).aspx) states the following:
    If dwMilliseconds is INFINITE, the function will return only when the object is signaled. My understanding, its in blocked state. The CPU will stop processing this thread.
    Thanks a lot in advance.

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