consuming C dlls with Delphi

Question

I have a Dll function with this signature:

UInt32 Authenticate(uint8 *Key);

I'm doing this on Delphi:

function Authenticate(Key:string) : UInt32; external 'mylib.dll' name 'Authenticate';

But always, the function return 10 (error code) and the application brakes :\

There is a way to do this right?

UPDATE: thanks guys! you're the best!

Answer 1

There are some problems with your code.

1) uint8 is the equivilent of Byte in Delphi, not String.

2) the C code is using the compiler's default calling convention, which is usually __cdecl. Delphi's default calling convention, on the other hand, is register instead. They are not compatible with each other. If you mismatch the calling convention, the stack and CPU registers will not be managed correctly during the function call at runtime.

A literal translation of the C code would be this instead:

function Authenticate(Key: PByte) : UInt32; cdecl; external 'mylib.dll';

However, assuming the function is actually expecting a null-terminated string then do this instead:

// the function is expecting a pointer to 8-bit data,
// so DO NOT use `PChar`, which is 16-bit in Delphi 2009+...
function Authenticate(Key: PAnsiChar) : UInt32; cdecl; external 'mylib.dll';

I would stick with the first declaration, as it matches the original C code. Even if the function is expecting a null-terminated string as input, you can still pass it in using PByte via a type-cast:

var
  S: AnsiString;
begin
  Authenticate(PByte(PAnsiChar(S)));
end;

Or, if the function allows NULL input for empty strings:

var
  S: AnsiString;
begin
  Authenticate(PByte(Pointer(S)));
end;