How to call API with PUCHAR?

Question

Hi, I'm trying to use an API under Delphi. Here's the API documentation

OKERR ENTRY SCardCLMifareStdAuthent 
    (IN SCARDHANDLE ulHandleCard,IN ULONG ulMifareBlockNr,
    IN UCHAR ucMifareAuthMode,IN UCHAR ucMifareAccessType,IN UCHAR ucMifareKeyNr,
    IN PUCHAR pucMifareKey,IN ULONG ulMifareKeyLen);

Whereas pucMifareKey: A pointer to the six byte Mifare key. The code i've been trying so far;

function Auth():Integer;
type
  TSCardCLMifareStdAuthent = function(SCARDHANDLE: cardinal; ulMifareBlockNr: ULONG;
   ucMifareAuthMode, ucMifareAccessType, ucMifareKeyNr: byte; pucMifareKey: puchar;
   ulMifareKeyLen: cardinal):LONG;
var
  SCardCLMifareStdAuthent: TSCardCLMifareStdAuthent;
  hDLL: Integer;
  CardHandle: cardinal;
  i: integer;
  Key: array of UCHAR;
begin
  Result:=1;
  //CardHandle is defined here
  SetLength(Key, 6);
  for i := low(key) to high(key) do
    Key[i] := $FF;
  hDLL := LoadLibrary('scardsyn.dll');
  @SCardCLMifareStdAuthent := GetProcAddress(hDLL, 'SCardCLMifareStdAuthent');
  if @SCardCLMifareStdAuthent <> nil then
    Result:=SCardCLMifareStdAuthent(CardHandle, $00, 96, 0, 0, ^Key, 6);
  FreeLibrary(hDLL);
end;

The error i'm getting is Incompatible types: 'Byte' and 'Char' at the line of Result:=SCardCL.... due ^Key pointer. Any ideas?

Answer 1

You wouldn't happen to be in Delphi 2009? If so, "char" is defined as 2 bytes. Try "AnsiChar" instead.

Answer 2

type
  TSCardCLMifareStdAuthent = function(SCARDHANDLE: cardinal; ulMifareBlockNr: ULONG;
    ucMifareAuthMode, ucMifareAccessType, ucMifareKeyNr: byte; pucMifareKey: puchar;
    ulMifareKeyLen: cardinal):LONG;

I think you should replace 'byte' with 'UCHAR':

ucMifareAuthMode, ucMifareAccessType, ucMifareKeyNr: byte;

It is a bad practice to treat 'byte' as a 'char'.

Answer 3

Try this:

1. Introduce new type for array of UCHAR.
2. Introduce new local variable of pointer type.
3. Set this variable to be pointer to your Key array

4. Use this pointer in function call

   type 
     TArrayOfUchar = array of UCHAR;
   
   
   var 
     ...
     Key  : TArrayOfUchar;
     PKey : ^TArrayOfUchar; 
   begin
     ...
     PKey = @Key;
     if @SCardCLMifareStdAuthent <> nil then
       Result:=SCardCLMifareStdAuthent(CardHandle, $00, 96, 0, 0, PKey, 6);
   

Answer 4

I don't know the DLL in question, but for a Delphi pre 2009 I would do the following:

function Auth: integer;
type
  TSCardCLMifareStdAuthent = function(SCARDHANDLE: cardinal;
    ulMifareBlockNr: ULONG; ucMifareAuthMode, ucMifareAccessType,
    ucMifareKeyNr: byte; pucMifareKey: PAnsiChar;
    ulMifareKeyLen: Cardinal): longint;
var
  SCardCLMifareStdAuthent: TSCardCLMifareStdAuthent;
  hDLL: Integer;
  CardHandle: Cardinal;
  Key: string;
begin
  Result := 1;
  //CardHandle is defined here...
  Key := StringOfChar(Chr($FF), 6);
  hDLL := LoadLibrary('scardsyn.dll');
  if hDLL <> 0 then begin
    @SCardCLMifareStdAuthent := GetProcAddress(hDLL, 'SCardCLMifareStdAuthent');
    if @SCardCLMifareStdAuthent <> nil then begin
      Result := SCardCLMifareStdAuthent(CardHandle, $00, 96, 0, 0,
        PChar(Key), Length(Key));
    end;
    FreeLibrary(hDLL);
  end;
end;

The access violation you get probably comes from the C string not being 0-terminated. Using a string and casting it to PChar will make things easier.

Answer 5

You stated that the key is a 6 byte (uchar) array. You are declaring a dynamic array to pass along, why not just declare an array?

var
  {...}
  MyKey: array[0..5] of UCHAR;
begin
  {...}
  Result:=SCardCLMifareStdAuthent(CardHandle, $00, 96, 0, 0, @MyKey, 6);