Call and Reuse the DLL functions dynamically

Question

I have created a DLL file with some functions and wish to reuse in a program multiple times in its different functions. But the Access-violation error comes after 2nd function of the program when calls the same DLL functions.

I'm currently using GetProcAddress. For example:

function xyz:boolean
var
   dllHandle : cardinal;
   EnBFStr : TEnBFStr;
   StrToHex : TStrToHex;
   Encodeddata , HexString : UnicodeString;

   begin
     dllHandle := LoadLibrary('Utilities.dll') ;
     if dllHandle <> 0 then
     begin
        Encodeddata:='Sample';
        @EnBFStr := GetProcAddress(dllHandle, 'EncodeBlowFishString') ;
        @StrToHex := GetProcAddress(dllHandle, 'UniStrToUniHexStr') ;
        if Assigned (EnBFStr) then
           Encodeddata:=EnBFStr('Key','Text') ; //Sample would be replaced

        if Assigned (StrToHex ) then
           HexString :=StrToHex(Encodeddata) ; //call the function

        FreeLibrary(dllHandle) ;
 end;

There are other functions which is loading the library and calling these DLL functions multiple times. Also, within the same procedure/function, we are calling these DLL functions multiple times in (IF Else) conditions.

In earlier part of the program, I have tried to check for the DLL file is present. Also, I tried to directly load the functions as another alternative:

function EncodeBlowFishString (Const Key:UnicodeString; Const DecodedString:UnicodeString; ): UnicodeString; stdcall;
external 'Utilities.dll' name 'EncodeBlowFishString';

function UniStrToUniHexStr(Const aString:UnicodeString): UnicodeString; stdcall;
external 'Utilities.dll';

Remy Lebeau · Accepted Answer · 2016-08-01 19:17:40Z

3

You are breaking the rules of memory allocation for DLLs. The return value is allocated by the callee but deallocated by the caller. Two solutions:

Use ShareMem as described in the comment at the top of a new library project.
Use standard interoperability techniques to ensure that allocation and deallocation always happens in the same module.

As an aside it is greatly wasteful to load and unload a DLL each time you want to use it. Load the DLL once only.

Furthermore I would like to point out that encryption operates on binary data and in my view you are storing up a world of pain by working instead with text.

edited Aug 1, 2016 at 19:17

Remy Lebeau

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answered Aug 1, 2016 at 17:10

David Heffernan

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5 Comments

Satyendra Shukla Over a year ago

You are correct. I'm using the Binary Data to encrypt it. It was just an example.

David Heffernan Over a year ago

You are passing text and not binary.

Satyendra Shukla Over a year ago

Sorry, Actually, that EncodeBlowFishString custom function itself is doing everything for me. That is why, I passed Unicode String and then it casted everything inside it. It was inline then I decided to have a DLL for all these custom-functions

David Heffernan Over a year ago

That's the problem in my view. You have lost control of text encoding. If it were me I'd explicitly specify an encoding. Anyway that is an aside. I dealt with your actual question.

Satyendra Shukla Over a year ago

@david-heffeman I have added a wrapper inside DLL so that it does most of the work instead of reusing the same functions outside the scope of any caller's procedure or function. I referred your example in stackoverflow. Since I was coming from Pascal Lazarus to Delphi, some things are trickier. Thank you for your help

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