Question

I previously asked a question about a delphi and a C/C++ DLL.

I have now another question about a record / struct.
The DLL should be able to dynamically CHANGE the VALUE of the pointer vars from the MainAPP.

My delphi MAINAPP has the following record:

type MyRec = record
 MyInteger    : Pointer;
 MyWideString : pwidechar;
 MyString     : pchar;
 MyBool       : Pointer
end;

type
 TMyFunc = function ( p  : pointer ): pointer; stdcall;

procedure test;
var
 MyFunction  : TMyFunc;
 TheRecord   : MyRec;
 AnInteger   : Integer;
 AWideString : WideString;
 AString     : String;
 ABool       : Bool;
begin
 AnInteger                := 1234;
 AWideString              := 'hello';
 AString                  := 'hello2';
 ABool                    := TRUE;
 TheRecord.MyInteger      := @AnInteger;
 TheRecord.MyWideString   := pwidechar(AWideString);
 TheRecord.AString        := pchar(AString);
 TheRecord.ABool          := @ABool;
 [...]
 @MyFunction := GetProcAddress...
 [...]
 MyFunction  (@TheRecord);  // now the DLL should be able to change the values dynamically.
 MessageBoxW (0, pwidechar(AWideString), '', 0); // Show the results how the DLL changed the String to...
end;

C/C++ Code (just example)

typedef struct _TestStruct{
void    *TheInteger;    // Pointer to Integer
wchar_t *TheWideString; // Pointer to WideString
char    *TheAnsiString; // Pointer to AnsiString  
bool    *TheBool        // Pointer to Bool
}TestStruct;

__declspec(dllexport) PVOID __stdcall MyExportedFunc (TestStruct *PTestStruct)
{
MessageBoxW(0 ,PTestStruct->TheWideString, L"Debug" , 0); // We read the value.
    PTestStruct->TheWideString = L"Let me change the value here.";
return 0;
}

For some reasons it crashes etc.
What do I do wrong?

Thanks for help.

Answer 1

You are mismanaging the string fields. PWideChar and PChar are not the same thing as “pointer to WideString” and “pointer to AnsiString”. Delphi has PWideString (WideString*) and PAnsiString (AnsiString*) types for that purpose instead. You should also use Delphi’s PInteger (int*) and PBoolean (bool*) types instead of Pointer (void*). Delphi and C++ are both type-safe languages. Stay away from untyped pointers when possible, your code will be better for it.

type
  PMyRec = ^MyRec;
  MyRec = record
    MyInteger    : PInteger;
    MyWideString : PWideString;
    MyAnsiString : PAnsiString;
    MyBool       : PBoolean;
  end;

  TMyFunc = function ( p  : PMyRec ): Integer; stdcall;

procedure test;
var
  MyFunction  : TMyFunc;
  TheRecord   : MyRec;
  AnInteger   : Integer;
  AWideString : WideString;
  AAnsiString : AnsiString;
  ABool       : Bool;
begin
 AnInteger                := 1234;
 AWideString              := 'hello';
 AAnsiString              := 'hello2';
 ABool                    := TRUE;
 TheRecord.MyInteger      := @AnInteger;
 TheRecord.MyWideString   := @AWideString;
 TheRecord.MyAnsiString   := @AAnsiString;
 TheRecord.MyBool         := @ABool;
 [...]
 @MyFunction := GetProcAddress...
 [...]
 MyFunction  (@TheRecord); 
 MessageBoxW (0, PWideChar(AWideString), '', 0);
end;

.

typedef struct _MyRec
{
    int        *MyInteger;    // Pointer to Integer
    WideString *MyWideString; // Pointer to WideString
    AnsiString *MyAnsiString; // Pointer to AnsiString  
    bool       *MyBool;       // Pointer to Bool
} MyRec, *PMyRec;

__declspec(dllexport) int __stdcall MyExportedFunc (PMyRec PRec)
{
    MessageBoxW(NULL, PRec->MyWideString->c_bstr(), L"Debug" , 0);
    *(PRec->MyWideString) = L"Let me change the value here.";
    return 0;
}

With that said, it can be very dangerous to manipulate AnsiString (and UnicodeString) values across a DLL boundary like this, especially if the EXE and DLL are written in different versions of Delphi/C++Builder due to RTL differences, memory manager differences, payload layout differences, etc. WideString is OK to pass around, though, because it’s memory and layout are controlled by the OS, not the RTL.