Sending Delphi string as a parameter to DLL - delphi

Question

I want to call a DLL function in Delphi 2010. This function takes a string and writes it to a printer with an USB interface. I do not know in which language is the DLL developed. According to the documentation, the syntax of the function is:
int WriteUSB(PBYTE pBuffer, DWORD nNumberOfBytesToWrite);
How can I declare and use my function in Delphi?
I declare the function like this:
var
function WriteUSB(myP:pByte;n:DWORD): integer ; external 'my.dll';
Should I use stdcall or cdecl in the declaration?
I call the DLL function like this:
procedure myProc;
var
str : string:
begin
str := 'AAAAAAAAAAAAAAAAAAAAA';
WriteUSB(str,DWORD(length(tmp)));
end;
But this code give me exception all the time. I know that the problem is that String is Unicode and each character > 1 byte. I tried to convert to different string types ( AnsiChar and ShortString) but I failed.
What is the correct way to do this?

Answer 1

A couple things. First off, if this is a C interface, which it looks like it is, then you need to declare the import like this:
function WriteUSB(myP:pAnsiChar; n:DWORD): integer; cdecl; external 'my.dll';
Then to call the function, you need to use an Ansi string, and convert it to a PAnsiChar, like so:
procedure myProc;
var
str : AnsiString;
begin
str := 'AAAAAAAAAAAAAAAAAAAAA';
WriteUSB(PAnsiChar(str), length(str));
end;
(The cast to DWORD is unnecessary.) If you do it like this, it should work without giving you any trouble.

Answer 2

You could convert the string to AnsiString (as already mentioned) if you're only going to use Ansi characters but if you want to use unicode strings AND the DLL/printer will accept them you could try something along the lines of (untested but I think it's generally corrext):
procedure myProc;
var
str: string;
buff: TBytes;
begin
str := 'blahblahblah'; // plus additional unicode stuff
buff := TEncoding.Default.GetBytes(str); // of TEncoding.UTF8 or... etc
WriteUSB(#buff[0], Length(buff));
end;
Don't know whether this will work with this particular DLL but it is a more general way of coping with the shift to unicode strings rather than having to assume (and cast to) AnsiString everywhere.

Answer 3

Thanks a lot for all the feedbacks. I make it work by combining your feedbacks. The solution is:
Declaration (I add cdecl):
function WriteUSB( pc:pByte;n:DWORD): integer ; cdecl; external 'my.dll';
And the call:
Procedure myProc;
Var
str : string;
buff : TBytes;
begin
str := 'My string";
buff := TEncoding.Default.GetBytes(str); // of TEncoding.UTF8 or... etc
WriteUSB(pByte(#buff[0]), Length(buff))
...
End;
I do have some problems with Swedish characters but I will solve it. Now I know that the DLL call is correct.
Thanks again for all feedback. This is a great forum.
BR
Delphi User

Answer 4

Try casting it with pchar in your call:
WriteUSB(pchar(str),DWORD(length(tmp)));