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DLL access violation

Posted on 1998-10-13
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Last Modified: 2010-04-06
I am trying to put my routines into a dll.
If I do a simple thing in the dll (e.g. addition, etc)
then it works fine.But if I do a bit complex thing (e.g. passing a tquery object and making a query and returning the resultset) then it gives me the access violation error
at the third or fourth call.
Can that be a memory allocation problem.(a growing DLL cause access violation ) or any oyher thing.
any suggestion will be appreciated!
thanks in advance
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Question by:guvencum
2 Comments
 
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Expert Comment

by:JimBob091197
ID: 1342735
Hi

Certain things cannot be done in a DLL.  E.g. you can't make a form, query, etc. in a DLL with the owner being your main form.

Perhaps you could give some code samples.

In general, passing VCL objects (forms, queries, etc.) from your main app to a DLL is OK, but they should be created and destroyed in your main app.

Cheers,
JB
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Accepted Solution

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Roadrunner100598 earned 100 total points
ID: 1342736
Hello,

I had the same problem. It's true that you cannot pass a query or database object from your main application to your DLL.

My solution was to have my own TDatabase object in my DLL as well as the TQuery. You don't need a form for this - just create the objects dynamically. The results of the query are passed by a TList to the main application.

This also solved another problem: I wanted to have BDE access from within a thread and I could not get this working (you need a second TSession...). The DLL routine can be called within a thread - it has it's own TSession it will work.

Hope you like this answere.

Beep, beep... Roadrunner
--------------------------------------
library CompanyData;

uses
  SysUtils,
  Classes,
  Windows,
  Dialogs,
  dbtables;

var
  DB     : TDatabase;

procedure GetData(ResultList:TList); stdcall; far;
var
  qry       : TQuery;
  NewItem   : TMyListItem;
begin
  if assigned(ResultList) then begin
    qry:=TQuery.Create(NIL);
    qry.DatabaseName:=DB.DatabaseName;
    qry.SQL.Text:=....
    qry.Open;

. move data into ResultList...

    qry.Free;
  end;
end;


procedure DLLHandler(Reason:integer);
begin
  case Reason of
    DLL_PROCESS_ATTACH : begin
                           DB:=TDatabase.Create(NIL);
                           DB.Name        :='dbCOMPANYS';
                           DB.LoginPrompt :=false;
                           DB.DatabaseName:='dbCOMPANYS';
                           DB.Params.Add('DATABASE NAME=dbCOMPANYS');
                           DB.AliasName   :='COMPANYS';
                           DB.Connected:=true;
                         end;
    DLL_PROCESS_DETACH : begin
                           DB.Connected:=false;
                           DB.Free;
                         end;
  end;
end;


exports
  GetData;

begin
  DLLProc:=@DLLHandler;
  DLLHandler(DLL_PROCESS_ATTACH); { Otherwise no initialization would be done }
end.

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