Delphi & DLLs II

• Creating DLL's using Delphi
• Getting the path of a DLL
• see also:
  • Delphi Programming Main Index
  • dynamic link libraries - DLLs
  • Delphi Reports in DLLs
  • Creating and Using DLLs with Exported Classes

Creating DLL's using Delphi:

• creating a DLL within Delphi is very similar to creating a normal application, except:
  • when starting a new project, from the menu, select File:New then select DLL
    • this creates a project file as usual except:
      • instead of it being headed "program", it is headed "library"
      • the application.run line is not included
      • a form is not automatically created for you to use
  • add a new unit to your project via File:New and select Unit
    • write the function code as usual in the implementation section and the corresponding function name and parameters in the interface section but suffixing it with "export; stdcall;"
      • eg.
        • interface
          • function MyFunction(S:String):String; export; stdcall;
        • implementation
          • function MyFunction(S:String):String;
            • begin
            • result := UpperCase(S);
            • end;
      • nb. the suffix "export" was only needed in Delphi 1 to deal with Win 3.x and its segmented memory, so it is no longer needed
      • nb. the suffix "stdcall" should be used in Delphi 2 and higher to ensure Delphi compiles the DLL so any other Window's application can access it, not just Delphi. It tells Delphi to use Window API's stdcall convention of pushing all arguments onto the stack in a standard order rather than Delphi's method of loading them into registers which results in faster code execution but only Delphi programs can access the DLL.
  • the new unit will be added to the project unit automatically
    • ie. in its uses clause the line will be added:
      • UnitName in 'Unitname.pas';
  • now you just need to itemise the exported functions in your project file:
    • open the project file (eg. from menu, Project:View Source)
    • after the uses clause and before the begin end section, create a section as follows:
      • exports
        • MyFunction1;
        • MyFunction2;
        • etc;
  • now its ready to compile, just choose from menu, Project:Compile;
    • NB. if you try to run it, it will compile it but not run it as DLL's cannot be ran alone
• see how easy it is to create a DLL in Delphi!!

Getting the path of a DLL:

• if you call ExtractFilePath(ParamStr(0)) from a DLL you will get the path of the web server, so if you want the path of the DLL itself use:
  • function ScriptPath: String;
  • var path: array[0..MaxPathLength-1] of char;
  • begin
  • if IsLibrary then
    • SetString(Result, path, GetModuleFileName(HInstance, path, SizeOf(path)))
    • else Result := ParamStr(0);
  • end;
•  

Some notes relating to creating DLLs using Delphi:

• Delphi 4 gives a warning that if Pascal strings are passed within any exported routines either as strings or within records, then you must place "ShareMem" as the 1st item in the project's uses clause and you must deploy Borland's share memory DLL with your DLL ie. BorlNDMM.dll
  • to avoid this, pass PChars or ShortStrings instead of Pascal strings as PChars are the native string type in Windows