Question

I’m writing a dll library in Delphi with multiple threads created by it. Let me describe the problem step by step. I’m sorry for a lengthy description in advance :-(.

Let’s forget about the library for a while. I created a windows application that is going to present views from several cameras. I created a window which is meant to show the view from a single camera and it contains a TImage control. There is a thread (a TThread descendant) that downloads the current image from the camera every couple of milliseconds and assigns it to the TImage control of that window (using the Synchronize() method). The application creates several instances of that window on startup (with a separate thread for each of them), so you can see the live view from several cameras at once. What’s more, all those viewing windows are parented by the main application window, so they appear within it.

Everything worked fine until I decided to put those two windows into a dll library. I just found it necessary for some reasons, but they are not important now. So I created a new dll library, added the existing main window and the camera-view window to the project and exported a function that creates and returns an instance of the main window. When the main window is created, it creates several camera-view windows, making itself their parent.

Then, for testing purposes, I created an app that imports the above mentioned dll function from the library and calls it at startup to get an instance of the main window; then just shows it on the screen (in a non-modal state).

When I started the app it came out that I couldn’t get a single image from any camera then. When I debugged it, I noticed that when the thread calls the Synchronize() method, it hangs forever. It didn’t happen before putting both those windows into a dll.

And this is my problem. To be honest, this is my first approach to libraries I have had to get through many other problems so far. You might wonder why I use windows instead of frames… So whenever I created an instance of a TFrame in a dll, I would get an exception saying “the control xxx does not have a parent window”. I did not know what to do about that so I used windows instead :-(.

Could you please tell me what to do with the synchronization problem? The main thread does not seem to be blocked in any way when the application is started for it accepts clicking buttons etc. What is the problem then?

Please, help!

Thank you in advance!!

Answer 1

When you call TThread.Synchronize the thread and method pointer are added to a global SyncList: TList in Classes.pas. In the main exe’s TApplication.Idle routine calls CheckSynchronize, which checks the SyncList, but it’s going to check the version in the exe instead of the one in the DLL. End result, your synchronized methods are never called.

The easiest fix would be to switch from DLLs to packages, which would eliminate the duplicate SyncList.

Another approach would be to override the exe’s Application.OnIdle callback, and call your DLL’s CheckSynchronize manually. You would need some help from the application for that though, since your DLL will have an Application object too, and that one won’t work.