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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for December 30th, 2020

A garbage collector for C and C++ (and a wrapper for Delphi): The Boehm-Demers-Weiser conservative C/C++ Garbage Collector

Posted by jpluimers on 2020/12/30

I bumped into [WayBackA garbage collector for C and C++ a while ago, for which the source is at [WayBack] GitHub – ivmai/bdwgc: The Boehm-Demers-Weiser conservative C/C++ Garbage Collector (libgc, bdwgc, boehm-gc).

There is a (very old!) wrapper for Delphi too: [WayBack] 21646 API for Boehm Garbage Collector DLL

Barry Kelly <>,
19 April 2004
This archive contains a simple API unit for the Boehm Garbage Collector DLL, along with another unit which makes it easier to use with classes, and a demonstration application. Also included is the Boehm GC DLL binary, along with source code in the gc_dll_src directory.

The files:

This unit exports a dozen or so routines from the Boehm GC dll. Since the GC integrates with and replaces the Delphi default memory manager, you probably don’t need to use this unit unless you want to fine-tune the behaviour of the DLL. The DLL exports more routines than are in this unit; the C prototypes are in the gc_dll_src/gc.h header file, and can be imported as needed. If you allocate large chunks of memory (>100K) which don’t contain references to other chunks (and thus don’t need to be scanned for pointers), there are routines in this unit which you can use to increase performance.

General advice: don’t tweak until you need to tweak.

This is the main unit. Put this unit first in the uses clause of you project and the project will automatically use garbage collection. If you want to use objects which require finalization and you don’t want to have to call TObject.Free / TObject.Destroy on them manually, you can use the MarkForFinalization(TObject) function. The basic pattern is to register the object for finalization in its constructor and unregister it with UnmarkForFinalization in its destructor. This handles the two most common use cases for finalization: GC-invoked finalization and manual finalization. Note that it’s always safe to behave as if GC doesn’t exist, and use GetMem/FreeMem, New/Dispose, Create/Free etc. The use of these units simply allows you to also program with garbage collection.

GcTest.dpr & GcTest.exe
This program contains simple sample code demonstrating the garbage collector in action.

This contains the implementation of the garbage collector itself. The DLL can be recompiled from the source in gc_dll_src with various options, including multithreaded support, different pointer alignment granularities, etc.

The original Boehm GC source comes from:

I’m Barry Kelly:

You can do anything you like with my source code (*.pas, *.dpr).

See the file gc_dll_src/LICENSEa for permissions for the GC itself.


Although when trying to download, I got this for both and

Access to the path ‘\\\f\webcache\cc\2004\4\19\’ is denied.

An error has occurred while processing the page.

Please try to refresh the page, or return to the home page.


and [WayBackJeroen Pluimers auf Twitter: “It looks like the @EmbarcaderoTech code central file is broken: “Access to the path ‘\\f\webcache\cc\2004\4\19\’ is denied.” when exploring or downloading.…”

 Explore the files in this upload

File Exploration is Disabled

We’re sorry, but errors in the uploaded zip file prevent it from being explored.

The error generated by the Zip attachment is:

Access to the path ‘\\\f\webcache\cc\2004\4\19\’ is denied.You may still be able to repair the zip file contents if you download the entire zip locally. You may also want to ask the author to repost the attachment.

Via [WayBack] delphi – Reference-counting for objects – Stack Overflow which also points to:

Downloads of stable versions: [WayBack] Download · ivmai/bdwgc Wiki · GitHub


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Posted in C, C++, Delphi, Development, Software Development | Leave a Comment »

Windows command prompt: decrementing loop

Posted by jpluimers on 2020/12/30

I needed a decrementing loop on the Windows command prompt, but that seems very hard from batch files without programming your own kind of while loop:

PowerShell to the rescue to loop back from and including 463 down to and including 290:

PowerShell -Command "for ($i=463; $i -ge 290; $i--) { Write-Host "Value " $i}"

This outputs:

Value 463
Value 462
Value 291
Value 290

In a similar way, you can execute a cmd command, but then you need to be careful on how to pass parameters: the \" is important to you can have quotes within quoted strings..

PowerShell -Command "for ($i=463; $i -ge 290; $i--) { & echo \"Value $i\"}"

gives this:

Value 463
Value 462
Value 291
Value 290

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Posted in Batch-Files, CommandLine, Console (command prompt window), Development, PowerShell, PowerShell, Scripting, Software Development, Windows | 1 Comment »

Delphi processing all Windows messages

Posted by jpluimers on 2020/12/30

Since I had captures messages inside the main message loop, I forgot it is straightforward: create a [WayBackTApplicationEvents instance, then use the [WayBackOnMessage event and hook it to a method like

procedure TMainForm.ApplicationEventsMessage(var Msg: TMsg; var Handled: Boolean);
  // figure out if the TMsg should be handled; set Handled to True if you do not want it to be processed by your application any further
  // if you do not handle it: bail out as quickly as possible
  // for performance reasons, it might be wise to have only the decision in this method, and all actual handling (including any managed variables) inside another method.

Capturing these messages is limited to the ones processed through the main message loop (or message pump): these are the asynchronous ones either put there by PostMessage or related functions (like PostQuitMessage), or generated by Windows.

This means you will not see any synchronous messages sent to specific Windows using SendMessage.

Unlike the documentation for the OnMessage event, the type inside the method is TMsg from the unit  [WayBack]WinApi.Windows, which is an alias for the (documentation mentioned) tagMSG in the same unit; neither type is documented at or Luckily, it is documented in the Windows SDK as the structure [WayBacktagMSG.

That structure is not the same as the [WayBackTMessage type inside the unit WinApi.Messages [WayBack], as TMessage omits these fields from TMsg:

  • HWND hwnd;
  • DWORD time;
  • POINT pt;
  • DWORD lPrivate;

Luckily the [WayBack] TMessageEvent (that describes the type of the signature of the event method) is better in this regard:

TMessageEvent = procedure (var Msg: TMsg; var Handled: Boolean) of object;

TMessageEvent includes the following parameters:

  • Msg identifies the Windows message that triggered the event.
  • Handled indicates whether the event handler responded to the message. If the event handler sets Handled to True, the application assumes that the message has been completely handled and halts any subsequent processing of the message.

TApplicationEvents is actually a multi-cast component (one of the very few multi-casting things parts of the Delphi VCL): an undocumented TMultiCaster class inside the unit Vcl.AppEvnts [WayBack].

A bit reminder though: ALL queued (not sent!) Windows messages flow through this method, so make it as short and efficient as possible, so from the OnMessage documentation:

OnMessage only receives messages that are posted to the message queue, not those sent directly with the Windows API SendMessage function.

Warning: Caution:Thousands of messages per second flow though this event. Be careful when coding the handler, because it can affect the performance of the entire application.
Tip: Call the CancelDispatch method from an OnMessage event handler to prevent the application from forwarding the event to any other application events objects.
It took until Delphi 2010 for the method [Archive.isTCustomApplicationEvents.CancelDispatch to be documented:
Prevents other TCustomApplicationEvents objects from receiving the current event.

A few more relevant links if you want to also hook SendMessage based messages:

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Posted in Conference Topics, Conferences, Delphi, Development, Event, Software Development | Leave a Comment »

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