I'm trying to spawn and then join two threads using MS VS 6.0 (2003), MS .NET Framework 1.1.
The following seems to be a reasonable solution:
CWinThread* thread1 = AfxBeginThread(worker, ¶llel_params);
CWinThread* thread2 = AfxBeginThread(worker, ¶llel_params);
WaitForSingleObject(thread1->m_hThread, INFINITE);
WaitForSingleObject(thread2->m_hThread, INFINITE);
but my main concern has to do with this statement in the documentation: "If this handle is closed while the wait is still pending, the function's behavior is undefined." When do handles get closed? Does ending the worker proc close the handle? If so, am I in trouble? Is this really a reasonable solution??
In order to be able to safely wait on the thread's handle, you should:
Start the thread suspended
Set the m_bAutoDelete member to false, so that the returned CWinThread* is not deleted automatically once the thread exits
Resume the thread
And finally wait on the handle the way you do.
Alternatively, you can start the thread suspended, then duplicate the handle, leave m_bAutoDelete as is, and finally wait on the new handle. This way the CWinThread* will indeed be deleted, but you'll still have a handle to wait on. Also, don't forget to close the handle once your done waiting on it.
If you leave your code as is, you might not even get to wait on a closed handle. If the thread exits you get to the wait function, the CWinThread* pointer might point to a deleted object, and you'll get an exception.
Related
what is the rigth way to close Thread in Winapi, threads don't use common resources.
I am creating threads with CreateThread , but I don't know how to close it correctly in ,because someone suggest to use TerminateThread , others ExitThread , but what is the correct way to close it .
Also where should I call closing function in WM_CLOSE or WM_DESTROY ?
Thx in advance .
The "nicest" way to close a thread in Windows is by "telling" the thread to shutdown via some thread-safe signaling mechanism, then simply letting it reach its demise its own, potentially waiting for it to do so via one of the WaitForXXXX functions if completion detection is needed (which is frequently the case). Something like:
Main thread:
// some global event all threads can reach
ghStopEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
// create the child thread
hThread = CreateThread(NULL, 0, ThreadProc, NULL, 0, NULL);
//
// ... continue other work.
//
// tell thread to stop
SetEvent(ghStopEvent);
// now wait for thread to signal termination
WaitForSingleObject(hThread, INFINITE);
// important. close handles when no longer needed
CloseHandle(hThread);
CloseHandle(ghStopEvent);
Child thread:
DWORD WINAPI ThreadProc(LPVOID pv)
{
// do threaded work
while (WaitForSingleObject(ghStopEvent, 1) == WAIT_TIMEOUT)
{
// do thread busy work
}
return 0;
}
Obviously things can get a lot more complicated once you start putting it in practice. If by "common" resources you mean something like the ghStopEvent in the prior example, it becomes considerably more difficult. Terminating a child thread via TerminateThread is strongly discouraged because there is no logical cleanup performed at all. The warnings specified in the `TerminateThread documentation are self-explanatory, and should be heeded. With great power comes....
Finally, even the called thread invoking ExitThread is not required explicitly by you, and though you can do so, I strongly advise against it in C++ programs. It is called for you once the thread procedure logically returns from the ThreadProc. I prefer the model above simply because it is dead-easy to implement and supports full RAII of C++ object cleanup, which neither ExitThread nor TerminateThread provide. For example, the ExitThread documentation :
...in C++ code, the thread is exited before any destructors can be called
or any other automatic cleanup can be performed. Therefore, in C++
code, you should return from your thread function.
Anyway, start simple. Get a handle on things with super-simple examples, then work your way up from there. There are a ton of multi-threaded examples on the web, Learn from the good ones and challenge yourself to identify the bad ones.
Best of luck.
So you need to figure out what sort of behaviour you need to have.
Following is a simple description of the methods taken from documentation:
"TerminateThread is a dangerous function that should only be used in the most extreme cases. You should call TerminateThread only if you know exactly what the target thread is doing, and you control all of the code that the target thread could possibly be running at the time of the termination. For example, TerminateThread can result in the following problems:
If the target thread owns a critical section, the critical section will not be released.
If the target thread is allocating memory from the heap, the heap lock will not be released.
If the target thread is executing certain kernel32 calls when it is terminated, the kernel32 state for the thread's process could be inconsistent.
If the target thread is manipulating the global state of a shared DLL, the state of the DLL could be destroyed, affecting other users of the DLL."
So if you need your thread to terminate at any cost, call this method.
About ExitThread, this is more graceful. By calling ExitThread, you're telling to windows you're done with that calling thread, so the rest of the code isn't going to get called. It's a bit like calling exit(0).
"ExitThread is the preferred method of exiting a thread. When this function is called (either explicitly or by returning from a thread procedure), the current thread's stack is deallocated, all pending I/O initiated by the thread is canceled, and the thread terminates. If the thread is the last thread in the process when this function is called, the thread's process is also terminated."
I create a process using CreateProcess() with the CREATE_SUSPENDED and then go ahead to create a little patch of code inside the remote process to load a DLL and call a function (exported by that DLL), using VirtualAllocEx() (with ..., MEM_RESERVE | MEM_COMMIT, PAGE_EXECUTE_READWRITE), WriteProcessMemory(), then call FlushInstructionCache() on that patch of memory with the code.
After that I call CreateRemoteThread() to invoke that code, creating me a hRemoteThread. I have verified that the remote code works as intended. Note: this code simply returns, it does not call any APIs other than LoadLibrary() and GetProcAddress(), followed by calling the exported stub function that currently simply returns a value that will then get passed on as the exit status of the thread.
Now comes the peculiar observation: remember that the PROCESS_INFORMATION::hThread is still suspended. When I simply ignore hRemoteThread's exit code and also don't wait for it to exit, all goes "fine". The routine that calls CreateRemoteThread() returns and PROCESS_INFORMATION::hThread gets resumed and the (remote) program actually gets to run.
However, if I call WaitForSingleObject(hRemoteThread, INFINITE) or do the following (which has the same effect):
DWORD exitCode = STILL_ACTIVE;
while(STILL_ACTIVE == exitCode)
{
Sleep(500);
if(!GetExitCodeThread(hRemoteThread, &exitCode))
break;
}
followed by CloseHandle() this leads to hRemoteThread finishing before PROCESS_INFORMATION::hThread gets resumed and the process simply "disappears". It is enough to allow hRemoteThread to finish somehow without PROCESS_INFORMATION::hThread to cause the process to die.
This looks suspiciously like a race condition, since under certain circumstances hRemoteThread may still be faster and the process would likely still "disappear", even if I leave the code as is.
Does that imply that the first thread that gets to run within a process becomes automatically the primary thread and that there are special rules for that primary thread?
I was always under the impression that a process finishes when its last thread dies, not when a particular thread dies.
Also note: there is no call to ExitProcess() involved here in any way, because hRemoteThread simply returns and PROCESS_INFORMATION::hThread is still suspended when I wait for hRemoteThread to return.
This happens on Windows XP SP3, 32bit.
Edit: I have just tried Sysinternals Process Monitor to see what's happening and I could verify my observations from before. The injected code does not crash or anything, instead I get to see that if I don't wait for the thread it doesn't exit before I close the program where the code got injected. I'm thinking whether the call to CloseHandle(hRemoteThread) should be postponed or something ...
Edit+1: it's not CloseHandle(). If I leave that out just for a test, the behavior doesn't change when waiting for the thread to finish.
The first thread to run isn't special.
For example, create a console app which creates a suspended thread and terminates the original thread (by calling ExitThread). This process never terminates (on Windows 7 anyway).
Or make the new thread wait for five seconds then exit. As expected, the process will live for five seconds and exit when the secondary thread terminates.
I don't know what's happening with your example. The easiest way to avoid the race is to make the new thread resume the original thread.
Speculating now, I do wonder if what you're doing isn't likely to cause problems anyway. For example, what happens to all the DllMain calls for the implicitly loaded DLLs? Are they unexpectedly happening on the wrong thread, are they being skipped, or are they postponed until after your code has run and the main thread starts?
Odds are good that the thread with the main (or equivalent) function calls ExitProcess (either explicitly or in its runtime library). ExitProcess, well, exits the entire process, including killing all threads. Since the main thread doesn't know about your injected code, it doesn't wait for it to finish.
I don't know that there's a good way to make the main thread wait for yours to complete...
I'm creating a worker thread in MFC with AfxBeginThread, but the thread is not getting scheduled. Here's the code:
CWinThread* worker = AfxBeginThread(initialUpdateWorkerThread, this);
DWORD dwExitCode = 0;
while(GetExitCodeThread(worker->m_hThread, &dwExitCode))
{
if(dwExitCode != STILL_ACTIVE)
break;
::Sleep(100);
}
When I run this, this loop just livelocks because initialUpdateWorkerThread is never called (I've put break points and message boxes at the top of it) so dwExitCode is always STILL_ACITVE. But if I put in a call to AfxMessageBox before the loop but after AfxBeginThread then the function is called. This makes me think that somehow I'm not calling the right function to get the thread scheduled, but a call to AfxMessageBox causes it to get scheduled.
How can I force the thread to be scheduled? I would think sleep would do that, but in this case it doesn't seem to.
Your worker thread is probably trying to send your main thread a message, but since you aren't processing messages on on the main thread, the worker thread simply waits. You can confirm this by simply breaking into the debugger to see what the worker thread is doing.
if a threadA is sleeping, how will another thread threadB call threadA to start ?
Please provide an example if possible.
Instead of sleeping you will want to create an EventWaitHandle and use WaitOne with a timeout.
When you want the thread to wake-up early, you will simply set the event to signaled.
First create the EventWaitHandle:
wakeUpEvent = new EventWaitHandle(false, EventResetMode.ManualReset);
Then in your thread:
wakeUpEvent.WaitOne(new TimeSpan(1, 0, 0));
When the main program wants to wake up the thread early:
wakeUpEvent.Set();
Note: You can either set the event to auto reset or manual reset. Auto reset means once WaitOne returns from the event, it will set it back to non signaled. This is useful if you are in a loop and you signal multiple times.
A thread can be started by waiting on a WaitObject and having the other thread calling the Set method on it. Look at the WaitHandle.WaitOne method.
Here's article that may be of help as well.
I'm using Delphi 2006 and have a bit of a problem with an application I'm developing.
I have a form that creates a thread which calls a function that performs a lengthy operation, lets call it LengthyProcess. Inside the LengthyProcess function we also call several Dll functions which also create threads of their own.
The problem that I am having is that if I don't use the Synchronize function of my thread to call LengthyProcess the thread stops responding (the main thread is still responding fine). I don't want to use Synchronize because that means the main thread is waiting for LengthyProcess to finish and therefore defeats the purpose of creating a separate thread.
I have tracked the problem down to a function inside the dll that creates a thread and then calls WaitFor, this is all done using TThread by the way. WaitFor checks to see if the CurrentThreadID is equal to the MainThreadID and if it is then it will call CheckSychronization, and all is fine. So if we use Synchronize then the CurrentThreadID will equal the MainThreadID however if we do not use Synchronize then of course CurrentThreadID <> MainThreadID, and when this happens WaitFor tells the current thread (the thread I created) to wait for the thread created by the DLL and so CheckSynchronization never gets called and my thread ends up waiting forever for the thread created in the dll.
I hope this makes sense, sorry I don't know any better way to explain it. Has anyone else had this issue and knows how to solve it please?
If your secondary thread "stops responding," then I assume it has a message pump. (Otherwise, you need to explain what it stops responding to.) You appear to also wish for the thread to be able to detect when the tertiary thread finishes running. (The "primary" thread here is the VCL thread, which isn't involved at all.)
You tried using WaitFor, but were disappointed when you discovered that it blocks. That's what it has always been designed to do, though. Its behavior in the main thread is where it gets weird, so it's safe to call from the VCL thread even though it was never really meant to be used that way originally.
To process messages and wait for threads to finish running, you need to use one or more of the wait functions from the Windows API. Start with MsgWaitForMultipleObjects. It can wait for various types of kernel handles, including thread handles, but also notify you when messages are available. The idea is that you'll call that function in a loop. When it says messages are available, handle them, and then loop again to continue waiting.
The following is just an outline. You'll want to check the documentation for all the API functions used, and combine that with the rest of the knowledge you have about your own threads.
procedure TSecondaryThread.Execute;
var
ret: DWord;
ThreadHandle: THandle;
Msg: TMsg;
begin
ThreadHandle := TertiaryThread.Handle;
repeat
ret := MsgWaitForMultipleObjects(1, ThreadHandle, False, Infinite, qs_AllEvents);
case ret of
Wait_Object_0: begin
// The thread terminated. Do something about it.
CloseHandle(ThreadHandle);
PostQuitMessage(0);
// Put *something* in the parameter so further calls to MWFMO
// will have a valid handle. May as well use a handle to something
// that will never become signaled so all we'll get are more
// messages. I'm pretty sure you can't pass an empty array of
// handles; there must be at least one, and it must be valid.
ThreadHandle := Self.Handle;
end;
Wait_Object_0 + 1: begin
// At least one message is available. Handle *all* of
// them before calling MsgWaitForMultipleObjects again
while PeekMessage(Msg, 0, 0, 0, pm_Remove) do
case Msg.Message of
wm_Quit: begin
// Do something about terminating the tertiary thread.
// Then stop the message loop and the waiting loop.
Exit;
end;
else begin
TranslateMessage(Msg);
DispatchMessage(Msg);
end;
end;
end;
Wait_Timeout: Assert(False, 'Infinity has passed');
Wait_Failed: RaiseLastOSError;
else Assert(False, 'Unexpected return value');
end;
until False;
end;
The part about handling all the messages is important. As soon as you call GetMessage, PeekMessage, or WaitMessage, the OS marks all messages in the queue as "old," but MsgWaitForMultipleObjects will only return when there is a "new" message on the queue — one that arrived after the last call to PeekMessage.
HI, Thanks for your reply, yes i realize that my question isn't very clear and somewhat confusing; so i'll try to clarify things a bit, here goes..
All of the threads described below are derived from TThread.
I have a form which starts a thread but does not wait for it. The thread started by the form calls a function that performs a long task.
The function calls another function in a DLL, the function in the DLL starts a thread and waits for it. The thread started by the DLL function calls another function via synchronize.
Form->Starts a thread but does not wait->The thread calls a functions->The function calls a DLL function->The Dll function starts a thread and waits->The thread started by the DLL function calls another function via synchronize i.e Synchronize(UpdateRecords).
The problem is that the call to synchronize never returns because, from what i can see, it has entered some sort of dead lock.
How synchronize works: Synchronize puts the method call into a queue and sets an event, Synchronize then waits for the event to become signaled. When the main thread is idle it will process the method calls that are waiting in the queue, after it has processed a method call it will signal the associated event so that the thread that initiated the synchronization can continue on.
The thread that was started by the form does not use synchronize to call the function that performs the long task, if it does use synchronize then the application does not dead lock, but this defeats the purpose of use a thread for the long process.
I've tracked down the problem, it seems to be that the TApplication object created by the dll is not processing messages and has a handle of 0, how this happened I don't know (I didn't write the DLL, it was written by someone else), but it is a cause of the problem because it will never process the method called queued by synchronize.
I mentioned earlier that if i call the function that performs the long process from my thread using synchronize then the application does not dead lock. This is because the main thread will be responsible for calling the function that performs the long process. So the long process function calls a DLL function which starts another thread and then calls WaitFor. WaitFor checks to see if the current thread is the main thread, and if it is, it processes method calls which have been queued by synchronize, continuously in a loop until the thread the thread that it is waiting for is released (i.e. the method it queued via synchronize gets called and the wait event is signaled).
In WaitFor, if the current thread is not the main thread then WaitFor simply blocks until the thread it is waiting for is released.
Anyway i can't do anything about the application object in the dll because that dll is quite complex and used by a larger system. I guess i can expose a method in the dll which can process the methods in the synchronization queue, i can then call this method from my application while its idle.
Anyway again thanks for your help but i've solved this problem now.
Using the TThread class or even the Application object in a Delphi DLL is extremely unsafe. The RTL and VCL core classes, globals and singleton objects are designed to exist once per process and can't handle the namespace duplication and incomplete initialization in a stadard DLL library.
You may get around it by building with runtime packages (RTL & VCL are enough; you may also build your own with just the system units you reallt need), in the EXE and all DLLs referencing the runtime units (Forms and Classes, especially) - they get single shared namespace and the full EXE initialization sequence this way.
If you can't modify the DLL at all, you may try to set it's Application.Handle, MainThreadID, SyncEvent and WakeMainThread to the corresponding values in the main EXE module - this may work, but it's just as ugly as it looks like and it doesn't cover all edge-cases (the classes and important globals will still be duplicated).