My application continuously syncs data with another database and I am running this process in a thread of which I have very little knowledge.
I am pulling in data from an Actian Zen database. Formerly Pervasive.SQL and updating my database in NexusDB.
It is a continuous operation of refreshing data in NexusDB.
The thread is called from the main form.
The work is done here:
procedure tImportThread.Execute();
begin
while (not Terminated) do
Begin
TThread.Sleep(3000);
CheckPause;
//My work
end;
end;
At certain points in my application I want to pause and resume the thread.
I have read this post "Pausing" A Thread With A Property and the wiki it refers to.
I have implemented the solution offered in the above post for lack of other examples and lack of knowledge on my part. For the most part the thread is working fine except that I would like to pause and unpause the running at certain times and would like to see a log that it was actually paused or resumed.
What I want to know is:
Do I just use PAUSE and UNPAUSE in the parts of my application to suspend the thread and restart it?
I want to test with a button to Pause and Resume the thread but not sure how to implement:
If thread running then Pause else Unpause
Part of the solution offered includes this code:
procedure TMyThread.CheckPause;
var
SignaledEvent: THandleObject;
begin
while not Terminated do
begin
case TEvent.WaitForMultiple(FWaitEvents, INFINITE, False, SignaledEvent) of
wrSignaled: begin
if SignaledEvent = FRunEvent then Exit;
Break;
end;
wrIOCompletion: begin
// retry
end;
wrError: begin
RaiseLastOSError;
end;
end;
SysUtils.Abort;
end
Where to create an entry into a log file to show that the thread is now paused. I have a simple writelog function to create an entry but where in the above should I put it? The same when the thread is resumed.
Does it matter whether the thread is paused or running when the application terminate? Do I just use this code?
if Assigned(TMyThread) then
begin
TMyThread.Free;
end;
I need to develop a TCP server and client with persistent connections using Indy and Delphi XE2. Almost everything is going well.
This service is a critical service, so I need to put in some protection in the server to prevent unnecessary processing or freezes. Because of this, I create a thread to check a timeout for critical processes.
I made this TIdSync class:
type
TSync = class(TIdSync)
protected
procedure DoSynchronize; override;
end;
procedure TSync.DoSynchronize;
var
oTimeOut: TThreadTimeOut;
begin
...
oTimeOut := TThreadTimeOut.Create(AContext, WaitTimeOut*2, Self);
oTimeOut.Start;
...
// the code below is just a test, **this is the key to my question**
// if something goes wrong in any subroutine of DoSynchronize, I want
// to stop execution of this object and destroy it. In the thread above
// I want to test when the timeout elapses. If this IdSync object still
// exists and if this routine is still executing, I want to stop execution
// of any routine or subroutine of this object to avoid freezing the
// service and stop memory consumption and CPU usage
while true do begin
Sleep(100);
end;
//If everything is OK
oTimeOut.Stop;
end;
procedure TThreadTimeOut.execute;
var
IniTime: DWORD;
begin
IniTime := GetTickCount;
while GetTickCount < IniTime + TimeOut do begin
Sleep(SleepInterval);
if StopTimeOut then
Exit;
end;
if ((Terminated = False) or (StopTimeOut)) and (IoHandler <> nil) then begin
IOHandler.Connection.IOHandler.Close;
IdSync.Free; //here I try to make things stop execution but the loop to test is still running
end;
end;
This code above works fine to stop receiving and sending data when the timeout elapses, but not to stop execution of TIdSync. How can I do that?
There is no timeout logic in TIdSync (largely because there is no timeout logic in TThread.Synchronize(), which TIdSync uses internally).
You cannot destroy a TIdSync object while it is running. A synced procedure cannot be aborted prematurely once it has been queued for execution, or has started running. It must be allowed to run to completion.
TIdSync.DoSynchronize() (or any method synced with TThread.Queue() or TThread.Synchronize()) is executed in the context of the main UI thread. Long-running code should be executed in its own thread, not in the main UI thread. Make sure the main UI thread is not blocked from processing new messages and sync requests in a timely manner.
If you want to stop a synced procedure, you need to have it handle a TEvent object or other flag which worker threads can signal when needed, and that the procedure checks periodically so it can exit as soon as possible (either gracefully or by raising an exception).
Synched operations of any nature should be short, to prevent blockages/deadlocks, resource starvation, etc. You need to re-think your design. You are doing things the wrong way.
Our program creates a background thread at the beginning of the program. The background thread does some database integrity checks and checks for stuff in the Internet using Indy. After 10 seconds, the background thread should be finished and since FreeOnTerminate is true, it will also clean itself up.
We have noticed that in some cases, if the user closes the program too quickly, the process will still be alive until the background thread is finished.
Since we couldn't exactly reproduce the issue, I have created a demo project to try a few things:
type
TBackgroundThread = class(TThread)
protected
procedure Execute; override;
end;
{ TForm1 }
var
bt: TBackgroundThread;
procedure TForm1.FormCreate(Sender: TObject);
var
i: integer;
begin
// Create a background thread which runs X seconds and then terminates itself.
bt := TBackgroundThread.Create(false);
bt.FreeOnTerminate := true;
end;
procedure TForm1.FormShow(Sender: TObject);
begin
// The user closes the app while the background thread is still active
Sleep(2000);
Close;
end;
{ TBackgroundThread }
procedure TBackgroundThread.Execute;
var
i: integer;
x: cardinal;
begin
inherited;
// Simulate some work that the background thread does
x := MaxInt;
for i := 0 to MaxInt do
begin
x := Random(x);
end;
end;
The result is a bit surprising to me: After I close the MainForm, the process will be immediately terminated and the background thread will get hard-killed.
Now I have a few questions about this:
After the closing of the MainForm (= exit of the main thread), should I manually terminate all created threads via .Terminate or will that be done automatically?
Shall my threads only check for Self.Terminated or should they also check for Application.Terminated ?
Why does my busy thread as shown above gets immediately killed when I close the application? I expected that the process Project1.exe will run until all threads have finished by themselfes. (And as described above, we had seen an application where the main form is closed, but a thread is preventing the process of being closed).
How is it possible then, that our real application's process does not terminate because of a running background thread? Might it have something to do with the Internet stuff, which might cause the app to wait until a connection timeout is reached?
Closing the main form is not synonymous with exiting the main thread. Code continues to run after the form is closed. In particular, units are finalized.
If you handle your test thread's OnTerminate event, or put a breakpoint in the Terminate method, you'll see that it's not called automatically when your program exits. You'll have to call it yourself. But note also that a thread doesn't stop running just because Terminate is called. It continues running until it stops itself or it's terminated forcefully. Call WaitFor to wait for it to terminate.
Don't bother checking Application.Terminated; the thread's property should be sufficient.
Your thread gets terminated forcefully as your program exits because eventually your program calls ExitProcess, and one of the things the OS does there is to terminate all other threads. It doesn't call Terminate on them because the OS doesn't know about Delphi classes and methods.
You'll have to do some more debugging to determine why your program doesn't terminate promptly for your customers. You say you can't reproduce the problem in house, and you've written a test program that doesn't exhibit the problem, either. You'll have to find a customer who will cooperate with your further debugging efforts. Do you really know it's the thread that's holding things up, or is that just a guess so far?
I have a message queue based on a thread that processes messages being thrown at it. As some activities in the thread may be VCL commands they are performed in Synchronize. As soon Synchronize is called the thread hangs. Sample code:
TMessageQ = class (TThread)
...
procedure TMessageQ.do_msg;
begin
case CurrentMessage.Command of
cQSize: if Assigned (OnSize) then OnSize (CurrentMessage);
cQReady: if Assigned (OnReady) then OnReady (CurrentMessage);
end; // case
end; // do_msg /
procedure TMessageQ.doTask (Sender: TObject);
begin
while FQ.Count > 0 do
begin
FSection.Enter;
try
CurrentMessage := FQ.Dequeue;
finally
FSection.Leave;
end; // try..finally
Synchronize (do_msg);
end; // while
end; // doTask //
No statement in do_msg is ever being processed. Does somebody know what I am doing wrong?
TThread.Synchronize() blocks until the main thread processes the request. The main thread requires an active message loop in order to process requests (unless you manually call Application.ProcessMessages() or CheckSynchronize() periodically). If your thread's main work is always being invoked by TThread.Synchronize(), there is no point in using a thread at all. You could just use a timer or custom window message in the main thread instead and take out all of the unnecessary complexity.
I have a main thread and a separate thread in my program. If the separate thread finishes before the main thread, it should free itself automatically. If the main thread finishes first, it should free the separate thread.
I know about FreeOnTerminate, and I've read that you have to be careful using it.
My question is, is the following code correct?
procedure TMyThread.Execute;
begin
... Do some processing
Synchronize(ThreadFinished);
if Terminated then exit;
FreeOnTerminate := true;
end;
procedure TMyThread.ThreadFinished;
begin
MainForm.MyThreadReady := true;
end;
procedure TMainForm.Create;
begin
MyThreadReady := false;
MyThread := TMyThread.Create(false);
end;
procedure TMainForm.Close;
begin
if not MyThreadReady then
begin
MyThread.Terminate;
MyThread.WaitFor;
MyThread.Free;
end;
end;
You can simplify this to:
procedure TMyThread.Execute;
begin
// ... Do some processing
end;
procedure TMainForm.Create;
begin
MyThread := TMyThread.Create(false);
end;
procedure TMainForm.Close;
begin
if Assigned(MyThread) then
MyThread.Terminate;
MyThread.Free;
end;
Explanation:
Either use FreeOnTerminate or free the thread manually, but never do both. The asynchronous nature of the thread execution means that you run a risk of not freeing the thread or (much worse) doing it twice. There is no risk in keeping the thread object around after it has finished the execution, and there is no risk in calling Terminate() on a thread that has already finished either.
There is no need to synchronize access to a boolean that is only written from one thread and read from another. In the worst case you get the wrong value, but due to the asynchronous execution that is a spurious effect anyway. Synchronization is only necessary for data that can not be read or written to atomically. And if you need to synchronize, don't use Synchronize() for it.
There is no need to have a variable similar to MyThreadReady, as you can use WaitForSingleObject() to interrogate the state of a thread. Pass MyThread.Handle as the first and 0 as the second parameter to it, and check whether the result is WAIT_OBJECT_0 - if so your thread has finished execution.
BTW: Don't use the OnClose event, use OnDestroy instead. The former isn't necessarily called, in which case your thread would maybe continue to run and keep your process alive.
Have the main thread assign a handler to the worker thread's OnTerminate event. If the worker thread finishes first, then the handler can signal the main thread to free the thread. If the main thread finishes first, it can terminate the worker thread. For example:
procedure TMyThread.Execute;
begin
... Do some processing ...
end;
procedure TMainForm.Create;
begin
MyThread := TMyThread.Create(True);
MyThread.OnTerminate := ThreadFinished;
MyThread.Resume; // or MyThread.Start; in D2010+
end;
const
APPWM_FREE_THREAD = WM_APP+1;
procedure TMainForm.ThreadFinished(Sender: TObject);
begin
PostMessage(Handle, APPWM_FREE_THREAD, 0, 0);
end;
procedure TMainForm.WndProc(var Message: TMessage);
begin
if Message.Msg = APPWM_FREE_THREAD then
StopWorkerThread
else
inherited;
end;
procedure TMainForm.StopWorkerThread;
begin
if MyThread <> nil then
begin
MyThread.Terminate;
MyThread.WaitFor;
FreeAndNil(MyThread);
end;
end;
procedure TMainForm.Close;
begin
StopWorkerThread;
end;
No, your code is not good (though it probably will work in 99.99% or even 100% cases). If you are planning to terminate work thread from main thread, don't set FreeOnTerminate to True (I don't see what are you trying to gain in the above code by setting FreeOnTerminate to True, it at least makes your code less understandable).
A more important situation with terminating work threads is that you are trying to close an application while work thread is in wait state. The thread will not be awaken if you just call Terminate, generally you should use additional syncronization object (usually event) to wake up the work thread.
And one more remark - there is no need for
begin
MyThread.Terminate;
MyThread.WaitFor;
MyThread.Free;
end;
if you look at TThread.Destroy code, it calls Terminate and WaitFor, so
MyThread.Free;
is enough (at least in Delphi 2009, have no Delphi 7 sources at hand to check).
Updated
Read mghie answer. Consider the following situation (better on 1 CPU system):
main thread is executing
procedure TMainForm.Close;
begin
if not MyThreadReady then
begin
MyThread.Terminate;
MyThread.WaitFor;
MyThread.Free;
end;
end;
it checked MyThreadReady value (it is False) and was switched off by scheduler.
Now scheduler switches to work thread; it executes
Synchronize(ThreadFinished);
and forces scheduler to switch back to main thread. Main thread continues execution:
MyThread.Terminate; // no problem
MyThread.WaitFor; // ???
MyThread.Free;
can you say what will happen at WaitFor? I can't (requires a deeper look into TThread sources to answer, but at first glance looks like a deadlock).
Your real error is something different - you have written an unreliable code and trying to find out is it correct or not. That is bad practice with threads - you should learn to write a reliable code instead.
As for resources - when the TThread (with FreeOnTerminate = False) is terminated the only resources that remains allocated is Windows thread handle (it does not use substantial Windows resources after thread is terminated) and Delphi TThread object in memory. Not a big cost to be on the safe side.
Honestly, your
... Do some processing
Is the real problem here. Is that a loop for doing something recursively? If not and, instead, thats a huge task, you should consider split this task in small procedures / functions, and put all together in the execute body, calling one after another with conditional if's to know the thread state, like:
While not Terminated do
begin
if MyThreadReady then
DoStepOneToTaskCompletion
else
clean_and_or_rollback(Something Initialized?);
if MyThreadReady then
DoStepTwoToTaskCompletion
else
clean_and_or_rollback(Something Initialized?, StepOne);
if MyThreadReady then
DoStepThreeToTaskCompletion
else
clean_and_or_rollback(Something Initialized?, StepOne, StepTwo);
Self.DoTerminate; // Not sure what to expect from that one
end;
It is dirty, almost a hack, but will work as expected.
About FreeOnTerminate, well... just remove the declaration and always
FreeAndNil(ThreadObject);
I'm not a fan of syncronise. I like more critical sections, for the flexibility to extend the code to handle more shared data.
On the form public section, declare:
ControlSection : TRTLCriticalSection;
On form create or somewhere else before thread.create ,
InitializeCriticalSection(ControlSection);
Then, every time you write to a shared resource (including your MyThreadReady variable), do
EnterCriticalSection ( ControlSection );
MyThreadReady := True; //or false, or whatever else
LeaveCriticalSection ( ControlSection );
Before you go (exit), call
DeleteCriticalSection ( ControlSection );
and free your thread as you always do.
Regards
Rafael
I would state that mixing models is simply not recommended. You either use FreeOnTerminate and never touch the thread again, or you don't. Otherwise, you need a protected way for the two to communicate.
Since you want fine control over the thread variable, then don't use FreeOnTerminate. If your thread finishes early, clear the local resources that the thread has consumed as you normally would, and then simply let the main thread free the child thread when the application is finished. You'll get the best of both worlds - resources freed by the child thread as soon as it can be, and no worries about thread synchronization. (And it's got the added bonus of being much simpler in design/code/understanding/support...)