RE: RE: DLLs for Beginners

Richard

I also would love a copy of this. May I suggest it could be placed in the
files section?

Thanks
Garry

-----Original Message-----
From: Harry Russell [mailto:[email protected]]
Sent: Wednesday, 9 July 2003 5:43 AM
To: [email protected]
Subject: Re: [Delphi] RE: DLLs for Beginners


I wouldn't mind a copy too please, as I'm completely unaware of how/why 1
does .dlls in delphi...

----- Original Message -----
From: "Duncan Ewan" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 08, 2003 9:23 PM
Subject: RE: [Delphi] RE: DLLs for Beginners


> A copy of it to me if you don't mind. I'm writing a DLL and any extra info
I can get would always help.
>
> Many thx in adv.
>
> -----Original Message-----
> From: Andre du Plessis [mailto:[email protected]]
> Sent: 08 July 2003 10:51
> To: [email protected]
> Subject: [Delphi] RE: DLLs for Beginners
>
>
> Richard,
>
> First of all congratulations your DLL document seems to be quite popular
> however,
> I would like to add to your DLL document, I noticed that you did not add
> some of the stuff I mentioned before, which though I think is important,
> you also mentioned threads might be a problem. I think this is an
> important topic because whether you use threads in your DLL or not
> multithreaded apps might want to use your dll, so I would like to add a
> very basic simple techniques, Delphi has a IsMultiThread variable,
> whenever beginthread is called IsMultiThread is set to true, this tells
> the memory manager to wrap itself around a critical section making the
> memory manager thread safe, without it, it is not thread safe!.
>
> So every DLL that might be used in a multithreaded app, must set this
> variable to true! Because the BeginThread is called from the main
> process and not the DLL, the dll does not know to set it's IsMultiThread
> variable.
>
> Secondly when using global variables they must use the threadvar
> directive, this is done on compiler level but I assume utilizes critical
> sections as well. Secondly the best way to deal with this is to use
> Thread local storage, the windows API includes:
>
> TLSAlloc
> TLSFree
> TLSGetValue and
> TLSSetValue
>
> The api stores these in the thread's local storage and is truly
> thread-safe.
> You should initialize these values in your dll's Entry procedure
>
> Below I wrote a simple example of a thread-safe dll
> The function MyObject returns the instance of the global variable that
> uses Thread local storage.
>
> It is also important to note that TLSGetValue Must be called on the
> initialization for the main thread and then on Thread Attach for every
> new thread.
>
> Interface
>
>   Type
>     TMyObject = class(TObject);
>
> Implementation
>
> threadvar
>   FObjectIndex : DWORD = 0;
>
> Function MyObject : TMyObject;
> Begin
>   Result := TMyObject(TLSGetValue(FObjectInstance))
> End;
>
> Procedure InitMyObject;
> Begin
>   TLSSetValue(FObjectInstance, Pointer(TMyObject.Create));
> End;
>
> Procedure FreeMyObject;
> Begin
>   TMyObject(TLSGetValue(FObjectInstance)).Free;
> End;
>
> procedure DllEntry(Reason: Integer);
> begin
>   case Reason of
>     DLL_THREAD_ATTACH:
>       Begin
>          InitMyObject;
>               end;
>     DLL_THREAD_DETACH:
>       Begin
>         FreeMyObject;
>               end;
>   end;
> end;
>
>
> initialization
>   IsMultiThread := True;
>   FObjectIndex := TLSAlloc;
>   DLLProc := @DllEntry;
>   InitMyObject;
>
> Finalization
>   FreeMyObject;
>   TlsFree(FObjectInstance);
>
>
> Lastly we can look at the topic of the RTTI and DLL's another day.
>
>
>
> -----Original Message-----
> From: Richard Le Mesurier (Investec Employee Benefits)
> [mailto:[email protected]]
> Sent: Friday, July 04, 2003 3:59 PM
> To: Brent Boswell (E-mail); Mike Currin (E-mail); Ivor Chalton (E-mail)
> Cc: '[email protected]'; '[email protected]';
> '[email protected]'; '[email protected]'
> Subject: DLLs for Beginners
>
> Hi, guys,
>
> Well, here is what I have put together. As I said, its very very
> informal,
> and really its just my attempt at solidifying my own thoughts on the
> topic
> before I go away for close on a month's leave - without this doc I am
> sure
> returning to work would be a lot harder (to the SA guys... hehe I'm off
> to
> 30 degree weather in France! :)
>
> So in the spirit of sharing that's gotten me so far in my own learning,
> here's what I have learnt.
>
> There are two projects in the attachment. The one is the DLL and the
> other
> is a program that calls the DLL.
>
> The actual DLL itself is useless - it does pretty much nothing (it
> reverses
> a string...). The code was not as important to me as cracking how to
> write a
> DLL - I was scared of this strange concept and now I have found out that
> there is nothing too hard about it. A few extra rules to follow... I'm
> sure
> a real working DLL will have its complex parts, and also surely some
> extra
> tricks to remember, but that's the same with any REAL vs LEARNING
> programs
> (anyone tried to learn to write a REAL program after copying a "Hello
> world"
> program?)
>
> Andre mentioned that it would be handy to know why and when to write a
> DLL,
> and I guess also why and when to link statically or dynamically. I don't
> really know, haven't given it much thought, sorry. My aim was to learn
> HOW,
> so that when I got a valid reason at work to use one, I could.
>
> Please, if anyone has anything to add, please do. Make this a living
> document and maybe between us all we can help a new generation of guys
> along. If its handy, pass it along; its not really my work, all I did
> was
> rewrite other people's work and arrange it my own way. But most
> importantly,
> if I have made errors (in code, concept or just judgement) please
> correct
> them to me. I can read about them when I get back.
>
> One last favour to the guys on the mailing list... I am on leave from
> Monday, so if anyone else from the list is interested in this stuff, can
> one
> of you please send it along. I won't be around to read their requests.
>
> Cheers
> Richard
>
>  <<DoPWord.zip>>
> (this is a Winzip file)
>
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