Question

I am programming a multi-threaded chat server application. Thus critical section are very necessary. I just made a wrapper class for threaded tree view. It works perfectly in the client but the server raises an AV.

There are two forms, FormServer (which practically does almost no processing except for gui painting) and the DataModuleServer which does all the heavy lifting.

The FormServer gets created first.

Actually the threaded tree view is inside another wrapper class TRoomTree.

TDataModuleServer.Create

procedure TDataModuleServer.cr(Sender: TObject);
begin
  Rooms := TRoomTree.Create (FormServer.tvRooms);

tvRooms : the visual treeview

TRoomTree.Create

constructor TRoomTree.Create (TV : TTreeView);
begin

  if Assigned (TV) then
    fTreeView.Create (TV)
  else
    exit;

fTreeView : the threaded tree view;

TThreadTreeView.Create

constructor TThreadTreeView.Create (TreeView : TTreeView = nil);
begin

  fLock := TCriticalSection.Create;

  if Assigned (TreeView) then
    fTreeView := TreeView
  else
    fTreeView := TTreeView.Create (nil);

end;

fTreeView : the normal tree view;

Now the TCriticalSection.Create raises an AV exception in ntdll.dll.

Call Stack

:76cac41f KERNELBASE.RaiseException + 0x58
:0040469c NotifyNonDelphiException + $1C
:77ecb42b ; ntdll.dll
uThreadTreeView.TThreadTreeView.Create($26B4300) 
uRoomTree.TRoomTree.Create($26B4300)
uServer.TDataModuleServer.cr($26A48B0)
Classes.TDataModule.DoCreate
Classes.TDataModule.AfterConstruction
System.@AfterConstruction($26A48B0)
Classes.TDataModule.Create(???)
Forms.TApplication.CreateForm(???,(no value))
PlayburnServer.PlayburnServer
:773833aa kernel32.BaseThreadInitThunk + 0x12
:77ea9ef2 ntdll.RtlInitializeExceptionChain + 0x63
:77ea9ec5 ntdll.RtlInitializeExceptionChain + 0x36

Any help will be greatly appreciated.

Answer 1

In TRoomTree.Create, this line of code

fTreeView.Create(TV);

is responsible for your access violation. That’s because fTreeView has not been initialised before you attempt to call a method on it.

Delphi instances are created like this:

fTreeView := TThreadTreeView.Create(TV);

I’m not sure about your threading model, but I trust you are aware that VCL controls must only be accessed from the GUI thread.