如何将字符串从 DLL 返回到 Inno Setup？

Question

I need to return a string value to the calling Inno Setup script. Problem is I can't find a way to manage the allocated memory. If I allocate on the DLL side, I don't have anything to deallocate with on the script side. I can't use an output parameter, because there is no allocation function in the Pascal Script either. What should I do?

Answer 1

Here is a sample code of how to allocate a string that returns from a DLL:

[Code]
Function GetClassNameA(hWnd: Integer; lpClassName: PChar; nMaxCount: Integer): Integer; 
External 'GetClassNameA@User32.dll StdCall';

function GetClassName(hWnd: Integer): string;
var
  ClassName: String;
  Ret: Integer;
begin
  { allocate enough memory (pascal script will deallocate the string) }
  SetLength(ClassName, 256); 
  { the DLL returns the number of characters copied to the buffer }
  Ret := GetClassNameA(hWnd, PChar(ClassName), 256); 
  { adjust new size }
  Result := Copy(ClassName, 1 , Ret);
end;

Answer 2

A very simple solution for the case where the DLL function is called only once in the installation - use a global buffer in your dll for the string.

DLL side:

char g_myFuncResult[256];

extern "C" __declspec(dllexport) const char* MyFunc()
{
    doSomeStuff(g_myFuncResult); // This part varies depending on myFunc's purpose
    return g_myFuncResult;
}

Inno-Setup side:

function MyFunc: PChar;
external 'MyFunc@files:mydll.dll cdecl';

Answer 3

The only practical way to do this is to allocate a string in Inno Setup, and pass a pointer to that along with the length to your DLL that then writes to it up to the length value before returning.

Here's some example code taken from the newsgroup .

function GetWindowsDirectoryA(Buffer: AnsiString; Size: Cardinal): Cardinal;
external 'GetWindowsDirectoryA@kernel32.dll stdcall';
function GetWindowsDirectoryW(Buffer: String; Size: Cardinal): Cardinal;
external 'GetWindowsDirectoryW@kernel32.dll stdcall';

function NextButtonClick(CurPage: Integer): Boolean;
var
  BufferA: AnsiString;
  BufferW: String;
begin
  SetLength(BufferA, 256);
  SetLength(BufferA, GetWindowsDirectoryA(BufferA, 256));
  MsgBox(BufferA, mbInformation, mb_Ok);
  SetLength(BufferW, 256);
  SetLength(BufferW, GetWindowsDirectoryW(BufferW, 256));
  MsgBox(BufferW, mbInformation, mb_Ok);
end;

Also see this thread for more up to date discussion.