PostMessage in service applications - delphi

Question

There is a problem I am unable to solve. I created two service applications in Delphi and tried to post messages within them. Of course, there are no windows in such applications and PostMessage needs a window handle parameter to send a message.
Therefore, I created a window handle using the AllocateHWnd(MyMethod: TWndMethod) function and passed, as the 'MyMethod' parameter, a procedure I want to be called when a message is received. If it was a windowed application, PostMessage() called using the handle returned by the AllocateHWnd method would certainly send a message that would then be received by the 'MyMethod' procedure.
The situation, however, is different in my service applications. I do not understand why, but in one of them posting messages this way works fine, whereas in the second one it does not (the messages are not received at all). Only when the service is being stopped do I notice that two messages are received by 'MyMethod': WM_DESTROY and WM_NCDESTROY. The messages I send using PostMessage are never received by this procedure. On the other hand, the first service always receives all messages I send.
Could you please give me a clue that would help me find the reason of the second service not receiving my messages? I do not know in what way they can differ. I checked the settings of the services and they seem to be identical. Why then one of them works fine and the second one does not (as far as sending messages is concerned)?
Thanks for any advice.
Mariusz.

Answer 1

Without more information it will be difficult to help you debug this, especially why it works in one service but not in the other. However:
Instead of trying to fix the problem in your code you might want to remove the windows altogether, and use PostThreadMessage() instead of PostMessage(). For the posting of messages to work correctly you need a message loop, but not necessarily receiving windows.
Edit: I'm trying to reply to all your answers in one go.
First - if you want to make your life easy you should really check out OmniThreadLibrary by gabr. I don't know whether it does work in a Windows service application, I don't even know whether that has been tried yet. You could ask in the forum. It has however a lot of great features and is worth looking into, if only for the learning effect.
But of course you can also program this for yourself, and you will have to for Delphi versions prior to Delphi 2007. I will simply add some snippets from our internal library, which has evolved over the years and works in several dozen programs. I don't claim it to be bug-free though. You can compare it with your code, and if anything sticks out, feel free to ask and I'll try to clarify.
This is the simplified Execute() method of the worker thread base class:
procedure TCustomTestThread.Execute;
var
Msg: TMsg;
begin
try
while not Terminated do begin
if (integer(GetMessage(Msg, HWND(0), 0, 0)) = -1) or Terminated then
break;
TranslateMessage(Msg);
DispatchMessage(Msg);
if Msg.Message = WM_USER then begin
// handle differently according to wParam and lParam
// ...
end;
end;
except
on E: Exception do begin
...
end;
end;
end;
It is important to not let exceptions get unhandled, so there is a top-level exception handler around everything. What you do with the exception is your choice and depends on the application, but all exceptions have to be caught, otherwise the application will get terminated. In a service your only option is probably to log them.
There is a special method to initiate thread shutdown, because the thread needs to be woken up when it is inside of GetMessage():
procedure TCustomTestThread.Shutdown;
begin
Terminate;
Cancel; // internal method dealing with worker objects used in thread
DoSendMessage(WM_QUIT);
end;
procedure TCustomTestThread.DoSendMessage(AMessage: Cardinal;
AWParam: integer = 0; ALParam: integer = 0);
begin
PostThreadMessage(ThreadID, AMessage, AWParam, ALParam);
end;
Posting WM_QUIT will cause the message loop to exit. There is however the problem that code in descendant classes could rely on Windows messages being properly handled during shutdown of the thread, especially when COM interfaces are used. That's why instead of a simple WaitFor() the following code is used to free all running threads:
procedure TCustomTestController.BeforeDestruction;
var
i: integer;
ThreadHandle: THandle;
WaitRes: LongWord;
Msg: TMsg;
begin
inherited;
for i := Low(fPositionThreads) to High(fPositionThreads) do begin
if fPositionThreads[i] <> nil then try
ThreadHandle := fPositionThreads[i].Handle;
fPositionThreads[i].Shutdown;
while TRUE do begin
WaitRes := MsgWaitForMultipleObjects(1, ThreadHandle, FALSE, 30000,
QS_POSTMESSAGE or QS_SENDMESSAGE);
if WaitRes = WAIT_OBJECT_0 then begin
FreeAndNil(fPositionThreads[i]);
break;
end;
if WaitRes = WAIT_TIMEOUT then
break;
while PeekMessage(Msg, 0, 0, 0, PM_REMOVE) do begin
TranslateMessage(Msg);
DispatchMessage(Msg);
end;
end;
except
on E: Exception do
// ...
end;
fPositionThreads[i] := nil;
end;
end;
This is in the overridden BeforeDestruction() method because all threads need to be freed before the destructor of the descendant controller class begins to free any objects the threads might use.

Answer 2

I'd suggest you consider using named pipes for IPC. That is what they are designed to do:
Looking for an alternative to windows messages used in inter-process communication

Answer 3

As Mghie mentioned, you need a message processing loop. That's why PeekMessage returns the messages correctly. It's not that the messages aren't there, it's that you aren't processing them. In a standard application, Delphi creates a TApplication class and calls Application.Run. This IS the message processing loop for a normal app. It basically consists of:
repeat
try
HandleMessage;
except
HandleException(Self);
end;
until Terminated;
If you want your service application to handle messages, you'll need to perform the same kind of work.
There's an example of using a service and handling PostThreadMessage dispatches here. Keep in mind, as Mick mentioned, you cannot use message handling between applications of differing security contexts (particularly in Vista). You should use named pipes or similar. Microsoft discusses this here.
Edit:
Based on the code snippet that you posted, you may just be fighting a threading issue. AllocHWnd is not thread safe. See here for a really detailed explanation of the issue and a version that works correctly in threads.
Of course, this still leads us back to why you aren't using PostThreadMessage instead. The way your code sample is structured, it would be trivial to make the message handling a function of the thread and then pass it down into the class for disposition.

Answer 4

Thanks for all your answers. I think we can forget about the problem. I created a new service application and performed quick post message tests. The messages were delivered correctly, so I hope I can now state that normally everything works fine and something is wrong only with this one service I described. I know it is stupid, but I will just try to copy one fragment of code after another from the 'bad' service to a new one. Maybe this will help me find the reason of the problem.
I hope I can now consider the message-waiting loop unnecessary as long as everything works fine without it, can't I?
If it comes to the privileges, Microsoft says: "UAC uses WIM to block Windows messages from being sent between processes of different privilege levels.". My Vista's UAC is off and I did not set any privileges for those services I described. Apart from that I do not send messages between different processes. Messages are sent within one process.
To give you the idea of what I am doing, I'll show you a code snippet from a test service application.
uses ...;
type
TMyThread = class;
TMyClass = class
private
FThread: TMyThread;
procedure ReadMessage(var Msg: TMessage);
public
FHandle: HWND;
constructor Create;
destructor Destroy; override;
end;
TMyThread = class(TThread)
private
FMyClass: TMyClass;
protected
procedure Execute; override;
constructor Create(MyClass: TMyClass); reintroduce;
end;
implementation
{ TMyClass }
constructor TMyClass.Create;
begin
inherited Create;
FHandle := AllocateHWnd(ReadMessage);
FThread := TMyThread.Create(Self);
end;
destructor TMyClass.Destroy;
begin
FThread.Terminate;
FThread.WaitFor;
FThread.Free;
DeallocateHWnd(FHandle);
inherited Destroy;
end;
procedure TMyClass.ReadMessage(var Msg: TMessage);
begin
Log.Log('message read: ' + IntToStr(Msg.Msg));
end;
{ TMyThread }
constructor TMyThread.Create(MyClass: TMyClass);
begin
inherited Create(True);
FMyClass := MyClass;
Resume;
end;
procedure TMyThread.Execute;
begin
while not Terminated do
begin
//do some work and
//send a message when finished
if PostMessage(FMyClass.FHandle, WM_USER, 0, 0) then
Log.Log('message sent')
else
Log.Log('message not sent: ' + SysErrorMessage(GetLastError));
//do something else...
Sleep(1000);
end;
end;
This is only an example, but functioning of my real code bases on the same idea. When you create an object of this class, it will create a thread that will start sending messages to that class. Log.Log() saves data into a text file. When I use this code in a new service application, everything works fine. When i put it into the 'broken' service, it does not. Please note that I do not use any message-waiting loop to receive messages. I created a new service and just put the code above into it, then created an object of the class. That's all.
If I get to know why this does not work in the 'broken' service, I'll write about it.
Thanks for the time you devoted me.
Mariusz.

Answer 5

Here's what I would try:
Check the return value and GetLastError of PostMessage
Is this a Vista/2008 machine? If yes, check if the sending application have sufficient priviliges to do send the message.
I have to have more information to help you further.

Answer 6

I spent long hours trying to find the reason of the messages not being received. As I showed in my code snippet, the constructor of the class creates a window handle which I used to send messages to. As long as the class was constructed by the main thread, everything worked fine for the window handle (if I understand it correctly) existed in the context of the main thread which, by default, awaits messages. In the 'broken' service, as I called it by mistake, my class was created by another thread, so the handle must have existed in the context of that thread. Therefore, when I sent messages using this window handle, they were received by that thread, not by the main one. Because of the fact that this thread did not have any message-waiting loop, my messages were not received at all.
I just did not know it worked this way. To solve the problem in an easy way, I create and destroy the class in the main thread even though I use it in the second one.
Thanks for your time and all the information you gave me.

Answer 7

Mghie, I think you are absolutely right. I implemented a message waiting loop this way:
procedure TAsyncSerialPort.Execute;
var
Msg: tagMSG;
begin
while GetMessage(Msg, 0, 0, 0) do
begin
{thread message}
if Msg.hwnd = 0 then
begin
case Msg.message of
WM_DATA_READ: Log.Log('data read');
WM_READ_TIMEOUT: Log.Log('read timeout');
WM_DATA_WRITTEN: Log.Log('data written');
WM_COMM_ERROR: Log.Log('comm error');
else
DispatchMessage(Msg);
end;
end
else
DispatchMessage(Msg);
end;
end;
I'm doing it for the first time, so please, could you check the code whether it is correct? In fact, this is my real class code snippet (the logs will be substituted with a real code). It handles overlapped comm port. There are two threads that send thread messages to the thread above, informing it that they wrote or received some data from comm port, etc. When the thread gets such a message, it takes an action - it gets the received data from a queue, where the threads first put it and then calls an external method that, lets say, analyses the received data. I don't want to go into details for it is unimportant :). I send thread messages like this: PostThreadMessage(MyThreadId, WM_DATA_READ, 0, 0).
This code works properly as I checked, but I would like to be sure everything is correct, so I'm asking you about that. I would be grateful if you answered.
To free the thread I do the following:
destructor TAsyncSerialPort.Destroy;
begin
{send a quit message to the thread so that GetMessage returns false and the loop ends}
PostThreadMessage(ThreadID, WM_QUIT, 0, 0);
{terminate the thread but wait until it finishes before the following objects
(critical sections) are destroyed for the thread might use them before it quits}
Terminate;
if Suspended then
Resume;
WaitFor;
FreeAndNil(FLock);
FreeAndNil(FCallMethodsLock);
inherited Destroy;
end;
I hope this is the proper way to end the message loop.
Thank you very much for your help.
BTW, I hope my English language is understandable, isn't it? :) Sorry if you have difficulties understanding me.

Answer 8

There's one trick in message loops in threads. Windows won't create a message queue for a thread immediately so there will be some time when posting messages to a thread will fail. Details are here. In my msg loop thread I use the technique MS proposes:
constructor TMsgLoopThread.Create;
begin
inherited Create(True);
FEvMsgQueueReady := CreateEvent(nil, True, False, nil);
if FEvMsgQueueReady = 0 then
Error('CreateEvent: '+LastErrMsg);
end;
procedure TMsgLoopThread.Execute;
var
MsgRec: TMsg;
begin
// Call fake PeekMessage for OS to create message queue for the thread.
// When it finishes, signal the event. In the main app execution will wait
// for this event.
PeekMessage(MsgRec, 0, WM_USER, WM_USER, PM_NOREMOVE);
SetEvent(FEvMsgQueueReady);
...
end;
// Start the thread with waitinig for it to get ready
function TMsgLoopThread.Start(WaitInterval: DWORD): DWORD;
begin
inherited Start;
Result := WaitForSingleObject(FEvMsgQueueReady, WaitInterval);
end;
But in your case I'd strongly recommend using other means of IPC.