I know that I must call Synchronize to update the vcl from a thread that did not create the controls or send a message to the window.
I have often heard the word not thread safe but I can't find an actual explanation about what is happening.
I know the application might crash with an access violation, but again I don't know why?
Please shed a light on this topic.
One of the biggest causes of the thread-unsafety in the VCL UI controls is the TWinControl.Handle property getter. It is not just a simple read-only accessor of the control's HWND. It also creates the HWND if it does not exist yet. If a worker thread reads the Handle property when no HWND exists yet, it creates a new HWND within the worker thread context, which is bad because HWNDs are tied to the creating thread context, which would render the owning control pretty much inoperable at best since Windows messages for the control would not go through the main message loop anymore. But worse, if the main thread reads the same Handle property at the same time the worker thread does (for instance, if the main thread is dynamically recreating the Handle for any number of reasons), there is a race condition between which thread context creates the HWND that gets assigned as the new Handle, as well as a potential handle leak potential if both threads end up creating new HWNDs but only one can be kept and the other gets leaked.
Another offender to thread-unsafety is the VCL's MakeObjectInstance() function, which the VCL uses internally for assigning the TWinControl.WndProc() non-static class method as the message procedure of the TWinControl.Handle window, as well as assigning anyTWndMethod-typed object method as the message procedure of the HWND created by the AllocateHWnd() function (used by TTimer for example). MakeObjectInstance() does quite a bit of memory allocating/caching and twiddling of that memory content which are not protected from concurrent access by multiple threads.
If you can ensure a control's Handle is allocated ahead of time, and if you can ensure the main thread never recreates that Handle while the worker thread is running, then it is possible to safely send messages to that control from the worker thread without using Synchronize(). But it is not advisable, there are just too many factors that the worker thread would have to take into account. That is why it is best that all UI access be done in the main thread only. That is how the VCL UI system is meant to be used.
About GDI thread safety in Windows, see this reference article.
It clearly states that you can access safely handles from multiple threads, but that it should not be made at the same time. You need to protect access to GDI handles, e.g. using critical sections.
Remember that GDI handles, like most Windows handles, are pointers of internal structures mapped to an integer (NativeUInt under newer Windows, for 64 bit compatibility). Like always in multi-thread computing, accessing the same content concurrently can be source of problems, which are very difficult to identify and fix.
The UI part of the VCL itself was never meant to be thread-safe, from the beginning, since it was relying on the non-thread-safe Windows API. For instance, if you release a GDI object in a thread, which is still needed in another thread, you'll face potential GPF.
Embarcadero (at this time) could have made the VCL thread-safe, serializing all UI access via critical sections, but it may have added complexity, and decreased overall performance. Note that even Microsoft .Net platform (in both WinForms and WPF) also requires a dedicated thread for UI access, AFAIK.
So, to refresh UI from multiple threads, you have several patterns:
Use Synchronize calls from the thread;
Send a GDI custom message (see WM_USER) from the background threads to notify the UI thread that a refresh is needed;
Have a stateless approach: the UI will refresh its content from time to time, from the logic layer (using a timer or when you press some buttons which may change the data).
From my point of view, I prefer option 2 for most UIs, and an additional option 3 (which can be mixed with option 2) for remote client-server access. Therefore, you do not have to want from the server side to trigger some update event to the UI. In a HTTP/AJAX RESTful world, this does definitively make sense. Option 1 is somewhat slow, IMHO. In all cases, options 2 and 3 expect a clear n-Tier layered architecture, in which logic and UI are not mixed: but this is a good pattern to follow anyway, for any serious development.
Windows controls with handles are not thread-safe (i.e. they cannot be accessed safely by two different threads at the same time), and Delphi wraps the Windows controls to give you the VCL controls. Since the controls ARE accessed by the main GUI thread, you need to leave them alone if you are executing another thread.
Related
Everywhere is noticed that VCL is not thread-safe and we must synchronize access to it. So it's VCL faults that is not thread-safe.
How VCL itself can be thread-safe?
What, precisely, does "thread-safe" mean to you? What about someone else? Every time I see this brought up, it ends up boiling down to this: "I want VCL to be thread-safe so I don't have to think about threading and synchronization issues. I want to write my code as if it is still single-threaded."
No matter how much work went into making VCL so-called "thread-safe", there will always be situations where you can get into trouble. How would you go about making it thread-safe? I don't say this to be combative, rather I merely want to demonstrate that it is not a simple problem with a simple, "works-in-all-cases" solution. To highlight this, let's look at some potential "solutions."
The simplest and most direct approach I see is each component has some kind of "lock", say a mutex or critical section. Every method on the component grabs the lock on entry and then releases the lock just prior to exit. Let's continue down this path with a thought experiment. Consider how Windows processes messages:
Main thread obtains a message from the message queue and then dispatches it to the appropriate WndProc. This message is then routed to the appropriate TWinControl component. Since the component has a "lock", as the message is routed to the appropriate message handler on the component, the lock is acquired. So far so good.
Now take the proverbial button-click message processing. The OnClick message handler is now called which will most likely be a method on the owning TForm. Since the TForm descendant is also a TWinControl component, the TForm's lock is now acquired while the OnClick handler is processed. Now the button component is locked and the TForm component is also locked.
Continuing on this line of thinking, suppose the OnClick handler now wants add an item to a listbox, listview, or some other visual list or grid component. Now suppose some other thread (not the main UI thread) is already in the midst of accessing this same component. Once a method is called on the list from the UI thread it will attempt to acquire the lock, which it cannot since the other thread is currently holding it. As long as the non-UI thread doesn't hold that lock for very long, the UI thread will only block for a brief period.
So far so good, right? Now suppose, that while the non-UI thread is holding the list control's lock, a notification event is called. Since, it will most likely be a method on the owning TForm, upon entry to the event handler, the code will attempt to acquire the lock for the TForm.
Do you see the problem? Remember the button OnClick handler? It already has the TForm lock in the UI thread! It is now blocked waiting for the lock on the list control, which the non-UI thread owns. This is a classic dead-lock. Thread A holds lock A and attempts to acquire lock B which is held by thread B. Thread B is at the same time attempting to acquire lock A.
Clearly, if every control/component has a lock that is automatically acquired and released for every method isn't a solution. What if we left the locking up to the user? Do you see how that also doesn't solve the problem either? How can you be certain that all the code you have (including any third-party components) properly locks/unlocks the controls/components? How does this keep the above scenario from happening?
What about a single shared lock for the whole of VCL? In this scenario, for each message that is processed, the lock is acquired while the message is processed regardless of what component the message is routed to. Again, how does this solve a similar scenario I described above? What if the user's code added other locks for synchronization with other non-UI threads? Even the simple act of blocking until a non-UI thread terminates can cause a dead lock if it is done while the UI thread holds the VCL lock.
What about non-UI components? Database, serial, network, containers, etc...? How should they be handled?
As excellently explained by the other answers, Windows already does a pretty decent job of properly segregating UI message processing to only the thread on which each HWND is created. In fact, learning precisely how Windows works in this regard will go a long way to understanding how you can write your code to work with Windows and VCL in a manner that avoids most of the pitfalls I highlighted above. The bottom line is writing multi-threaded code is difficult, requires a rather drastic mental shift, and lots of practice. Read as much as you can on multi-threading from as many sources as possible. Learn and understand as many coding examples of "thread-safe" code as you can, in any language.
Hopefully this was informative.
The VCL is not thread safe. It is a wrapper around Win32. Win32 is thread safe but has threading rules that give meaning to that statement. Most specifically a window has affinity to the thread that created it.
The design of the Windows message queue means that it is almost always preferable to have all your GUI windows created by the main thread. The VCL designers decided that it was reasonable only to support that mode of operation. And so all VCL code must be executed from the main thread.
There's nothing that can be done to change this. This is by design. If you wish to execute VCL code, it must be done on the main thread. Use TThread.Synchronize or TThread.Queue to arrange that.
There are a lot of reasons why the VCL (especially UI controls) is not thread safe.
Race conditions on message input, especially in code that directly calls TControl.Perform()/TObject.Dispatch() instead of using PostMessage()/SendMessage(). The former does not perform any synchronizing of the control's message handlers, but the latter does. So it is not safe to perform non-HWND based messages from outside of the main thread.
An HWND has thread affinity. It receives and processes messages only on, and can be destroyed only on, the thread context that creates it. A TWinControl can destroy and recreate its HWND at any time, even multiple times, during its lifetime. The TWinControl.Handle property getter creates a new HWND if none exists yet. So if the control is in the process of recreating its HWND when another thread reads from the Handle property, the control can end up with a new HWND that was created in the wrong thread context, making the control no longer responsive to the main message loop (and can potentially leak a second HWND as well). So it is not safe to read from the TWinControl.Handle property from outside of the main thread.
The VCL has a MakeObjectInstance() function that creates a dynamic proxy to allow a TWndMethod class method to be used as a Win32 WNDPROC window callback procedure. All TWinControl controls, and some utility classes like TTimer, use this function. Internally, it maintains a global linked list of proxies, and that list is not protected from concurrent access across threads. So it is not safe to create/destroy HWND-based VCL controls from outside of the main thread.
I'm sure there are other reasons, but these are the big ones.
Everywhere is noticed that VCL is not thread-safe and we must synchronize access to it. So it's VCL faults that is not thread-safe.
How VCL itself can be thread-safe?
What, precisely, does "thread-safe" mean to you? What about someone else? Every time I see this brought up, it ends up boiling down to this: "I want VCL to be thread-safe so I don't have to think about threading and synchronization issues. I want to write my code as if it is still single-threaded."
No matter how much work went into making VCL so-called "thread-safe", there will always be situations where you can get into trouble. How would you go about making it thread-safe? I don't say this to be combative, rather I merely want to demonstrate that it is not a simple problem with a simple, "works-in-all-cases" solution. To highlight this, let's look at some potential "solutions."
The simplest and most direct approach I see is each component has some kind of "lock", say a mutex or critical section. Every method on the component grabs the lock on entry and then releases the lock just prior to exit. Let's continue down this path with a thought experiment. Consider how Windows processes messages:
Main thread obtains a message from the message queue and then dispatches it to the appropriate WndProc. This message is then routed to the appropriate TWinControl component. Since the component has a "lock", as the message is routed to the appropriate message handler on the component, the lock is acquired. So far so good.
Now take the proverbial button-click message processing. The OnClick message handler is now called which will most likely be a method on the owning TForm. Since the TForm descendant is also a TWinControl component, the TForm's lock is now acquired while the OnClick handler is processed. Now the button component is locked and the TForm component is also locked.
Continuing on this line of thinking, suppose the OnClick handler now wants add an item to a listbox, listview, or some other visual list or grid component. Now suppose some other thread (not the main UI thread) is already in the midst of accessing this same component. Once a method is called on the list from the UI thread it will attempt to acquire the lock, which it cannot since the other thread is currently holding it. As long as the non-UI thread doesn't hold that lock for very long, the UI thread will only block for a brief period.
So far so good, right? Now suppose, that while the non-UI thread is holding the list control's lock, a notification event is called. Since, it will most likely be a method on the owning TForm, upon entry to the event handler, the code will attempt to acquire the lock for the TForm.
Do you see the problem? Remember the button OnClick handler? It already has the TForm lock in the UI thread! It is now blocked waiting for the lock on the list control, which the non-UI thread owns. This is a classic dead-lock. Thread A holds lock A and attempts to acquire lock B which is held by thread B. Thread B is at the same time attempting to acquire lock A.
Clearly, if every control/component has a lock that is automatically acquired and released for every method isn't a solution. What if we left the locking up to the user? Do you see how that also doesn't solve the problem either? How can you be certain that all the code you have (including any third-party components) properly locks/unlocks the controls/components? How does this keep the above scenario from happening?
What about a single shared lock for the whole of VCL? In this scenario, for each message that is processed, the lock is acquired while the message is processed regardless of what component the message is routed to. Again, how does this solve a similar scenario I described above? What if the user's code added other locks for synchronization with other non-UI threads? Even the simple act of blocking until a non-UI thread terminates can cause a dead lock if it is done while the UI thread holds the VCL lock.
What about non-UI components? Database, serial, network, containers, etc...? How should they be handled?
As excellently explained by the other answers, Windows already does a pretty decent job of properly segregating UI message processing to only the thread on which each HWND is created. In fact, learning precisely how Windows works in this regard will go a long way to understanding how you can write your code to work with Windows and VCL in a manner that avoids most of the pitfalls I highlighted above. The bottom line is writing multi-threaded code is difficult, requires a rather drastic mental shift, and lots of practice. Read as much as you can on multi-threading from as many sources as possible. Learn and understand as many coding examples of "thread-safe" code as you can, in any language.
Hopefully this was informative.
The VCL is not thread safe. It is a wrapper around Win32. Win32 is thread safe but has threading rules that give meaning to that statement. Most specifically a window has affinity to the thread that created it.
The design of the Windows message queue means that it is almost always preferable to have all your GUI windows created by the main thread. The VCL designers decided that it was reasonable only to support that mode of operation. And so all VCL code must be executed from the main thread.
There's nothing that can be done to change this. This is by design. If you wish to execute VCL code, it must be done on the main thread. Use TThread.Synchronize or TThread.Queue to arrange that.
There are a lot of reasons why the VCL (especially UI controls) is not thread safe.
Race conditions on message input, especially in code that directly calls TControl.Perform()/TObject.Dispatch() instead of using PostMessage()/SendMessage(). The former does not perform any synchronizing of the control's message handlers, but the latter does. So it is not safe to perform non-HWND based messages from outside of the main thread.
An HWND has thread affinity. It receives and processes messages only on, and can be destroyed only on, the thread context that creates it. A TWinControl can destroy and recreate its HWND at any time, even multiple times, during its lifetime. The TWinControl.Handle property getter creates a new HWND if none exists yet. So if the control is in the process of recreating its HWND when another thread reads from the Handle property, the control can end up with a new HWND that was created in the wrong thread context, making the control no longer responsive to the main message loop (and can potentially leak a second HWND as well). So it is not safe to read from the TWinControl.Handle property from outside of the main thread.
The VCL has a MakeObjectInstance() function that creates a dynamic proxy to allow a TWndMethod class method to be used as a Win32 WNDPROC window callback procedure. All TWinControl controls, and some utility classes like TTimer, use this function. Internally, it maintains a global linked list of proxies, and that list is not protected from concurrent access across threads. So it is not safe to create/destroy HWND-based VCL controls from outside of the main thread.
I'm sure there are other reasons, but these are the big ones.
I know that you need synchronize (yourprocedure) to set e.g. a label's text.
But what about:
Reading a label's text.
Toggle/Set the label's enabled property.
Call other labels procedures/functions (e.g. onclick event).
Is there an easy rule to know/remember when I need to use synchronize?
PS.: Is synchronize similar to PostMessage/SendMessage?
Easy rule of thumb: ANY access to VCL UI components needs to be synchronized. That includes both reading and writing of UI control properties. Win32 UIs, most notably dialogs like MessageBox() and TaskDialog(), can be used directly in worker threads without synchronizing.
TThread.Synchronize() is similar to SendMessage() (in fact, it used to be implemented using SendMessage() internally in Delphi 5 and earlier). TThread.Queue() is similar to PostMessage().
Any time you access a VCL UI component, you need to implement some type of thread safety measure. This is also, typically, the case when you're accessing a variable or procedure that exists or will be accessed by another thread. However, you don't need to use the Synchronize method in all of these situations. There are other tools at your disposal, and Synchronize is not always your best solution.
Synchronize blocks both the main thread and the calling thread while it's performing the procedure that you pass to it, so overusing it can detract from the benefits of multi-threading. Synchronize is probably most commonly used for updating your UI, but if you find that you're having to use it really frequently, then it might not be a bad idea to check and see if you can restructure your code. I.E. do you really need to read labels from within your thread? Can you read the label before starting the thread and pass it into the thread's constructor? Can you handle any of these tasks in the thread's OnTerminate event handler?
I would like to have three threads in a sample application.
Thread #1 (Main Thread) - User Interface/GUI
Thread #2 - Tied to a serial port device receiving data via events passing to a data queue.
Thread #3 - Activated when a queue entry is made, process data node, frees data object.
The goal is to
a) Prevent the loss of data when a button or the form is held by the mouse on the main form.
b) Quickly get the data from the event, stuff it in the queue, go back to sleep
c) Process data when we have it, otherwise sleep.
Can packages like AsyncoPro tie event handling to a non-main thread?
I've never done much with serial port event driven apps, most of what I've work with are polled and I want to do some testing.
You can definitely tie event handling to a non-main thread. What you can't do is tie screen updating to a non-main thread. The Windows API is not threadsafe, and so the Delphi VCL, which is built on top of the Windows API, isn't either. But your design is basically a good, workable idea; just remember to use the Synchronize or Queue methods of TThread to send any UI updates back to be executed on the main thread.
The easiest should be to define some user messages, then sent it from sub-threads to the main thread.
It's perfectly thread-safe, and even process-safe.
Use PostMessage() with the Handle of the main form. But don't broadcast this WM_USER+n message to the whole UI, because you could confuse some part of the VCL which defines its own custom messages.
If you want to copy some textual data accross threads or processes, you can see WM_COPY_DATA. In practice, this is very fast, faster than named pipes for small messages.
For User Interface, I discovered than a stateless implementation is sometimes a good idea. That is, you don't call-back the main thread via a Synchronize() call or a GDI message, but your main GUI thread has a timer which check a shared memory buffer for pending updates. This is how the web works, and in practice, it's pretty easy to work with: you don't have to write any callback, each thread is independent, do its own stuff, and refresh when necessary.
But of course, the solution depends on your exact project architecture.
For a simple but proven library, see AsyncCalls, working from Delphi 5 up to XE. For latest versions of the IDE (Delphi 2007 and later), take a look at OmniThreadLibrary. By using such libraries, you'll ensure that your software implementation won't break anywhere: it's very common for a multi-threaded application to work as expected most of the time, then, for unknown reasons, going into an endless loop. And, of course, it happens only on the customer side, not yours... If you don't want to spend hours debugging your program, just trust those proven libraries, which are known to be well designed and debugged.
Sure you can do this, one way or another. Not used Apro since D5 - the Apro I have does not work on my D2009, (unicode/string/ANSIstring issues), & I have my own serial classes. Most of the available serial components have the option of firing dataRx events on either the rx thread or the main GUI thread - obviously in your case you should select the rx thread, (Thread #2). Shove the rx data into some buffer class and push it onto a producer-consumer thread to (Thread #3). Process it there. If you need to do a GUI update from there, PostMessage the reference to the GUI thread and handle it in a user-defined message-handler procedure.
Done this sort of stuff loadsa times - it will work OK.
Rgds,
Martin
I don't want multiple windows, each with its own UI thread, nor events raised on a single UI thread, not background workers and notifications, none of that Invoke, BeginInvoke stuff either.
I'm interested in a platform that allows multiple threads to update the same window in a safe manner. Something like first thread creates three buttons, the second thread another five, and they both can access them,change their properties and delete them without any unwanted consequences.
I want safe multi-threaded access to the UI without Invoking, a platform where the UI objects can be accessed directly from any thread without raising errors like "The object can only be accessed from the thread that created it". To let me do the synchronizing if I have to, not prevent me from cross-tread accessing the UI in a direct manner.
I'm gonna get down voted but ... Go Go Gadget Soapbox.
Multi threaded GUI are not possible in the general case. It has been attempted time and time again and it never comes out well. It is not a coincidence that all of the major windowing frameworks follow the single threaded ui model. They weren't copying each other, it's just that the constraints of the problem lead them to the same answer. Many people smarter than you or i have tried to solve this.
It might be possible to implement a multi-thread ui for a particular project. I'm only saying that it can't be done in the general case. That means it's unlikely you'll find a framework to do what you want.
The gist of the problem is this. Envision the gui components as a chain (in reality it's more like a tree, but a chain is simple to describe). The button connects to the frame, connects to the box, connects to the window. There are two source of events for a gui the system/OS and the user. The system/OS event originate at the bottom of the chain (the windowing system), the user event originate at the top of the chain (the button). Both of these events must move through the gui chain. If two threads are pushing these events simultaneously they must be mutex protected. However, there is no known algorithm for concurrently traversing a double linked list in both directions. It is prone to dead lock. GUI experts tried and tried to figure out ways to get around the deadlocking problem, and eventually arrived at the solution we use today called Model/View/Controller, aka one thread runs the UI.
You could make a thread-safe Producer/Consumer queue of delegates.
Any thread that wants to update a UI component would create a delegate encapsulating the operations to be performed, and add it to the queue.
The UI thread (assuming all components were created on the same thread) would then periodically pull an item from the queue, and execute the delegate.
I don't believe a platform like that exists per se
There is nothing stopping you from saying taking .Net and creating all new controls which are thread safe and can work like that(or maybe just the subset of what you need) which shouldn't be an extremely large job(though definitely no small job) because you can just derive from the base controls and override any thread-unsafe methods or properties.
The real question though is why? It would definitely be slower because of all the locking. Say your in one thread that is doing something with the UI, well it has to lock the window it's working on else it could be changed without it knowing by the other thread. So with all the locking, you will spend most of your drawing time and such waiting on locks and (expensive) context switches from threads. You could maybe make it async, but that just doesn't seem safe(and probably isn't) because controls that you supposedly just created may or may not exist and would be about like
Panel p=new Panel();
Button b=new Button();
WaitForControlsCreated(); //waits until the current control queue is cleared
p.Controls.Add(b);
which is probably just as slow..
So the real question here is why? The only "good" way of doing it is just having an invoke abstracted away so that it appears you can add controls from a non-UI thread.
I think you are misunderstanding how threads really work and what it takes to actually make an object thread safe
Accept that any code updating the GUI has to be on the GUI thread.
Learn to use BeginInvoke().
On Windows, Window handles have thread affinity. This is a limitation of the Window manager. It's a bad idea to have multiple threads accessing the same window on Windows.
I'm surprised to see these answers.
Only the higher level language frameworks like C# have thread restrictions on GUI elements.
Windows, at the SDK layer, is 100% application controlled and there are no restrictions on threads except at insignificant nitty gritty level. For example if multiple threads want to write to a window, you need to lock on a mutex, get the device context, draw, then release the context, then unlock the mutex. Getting and releasing a device context for a moment of drawing needs to be on the same thread... but those are typically within 10 lines of code from each other.
There isn't even a dedicated thread that windows messages come down on, whatever thread calls "DispatchMessage()" is the thread the WINPROC will be called on.
Another minor thread restriction is that you can only "PeekMessage" or "GetMessage" a window that was created on the current thread. But really this is very minor, and how many message pumps do you need anyway.
Drawing is completely disconnected from threads in Windows, just mutex your DC's for drawing. You can draw anytime, from anywhere, not just on a WM_PAINT message.
BeOS / Haiku OS
Based on my guessing of your requirement, you want a single Windows Form and having ways to execute certain routines asynchronously (like multi-threading), yes?
Typically (for the case of .NET WinForms) Control.Invoke / Control.BeginInvoke is used to a certain effect what I think you want.
Here's an interesting article which might help: http://www.yoda.arachsys.com/csharp/threads/winforms.shtml