Sending a Delphi string as a parameter to a DLL

Question

I want to call a DLL function in Delphi 2010. This function takes a string and writes it to a USB printer. I do not know what language the DLL is developed. According to the documentation, the syntax of the function:

int WriteUSB(PBYTE pBuffer, DWORD nNumberOfBytesToWrite); 

How can I declare and use my function in Delphi?

I declare the function as follows:

 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 as follows:

 procedure myProc; var str : string: begin str := 'AAAAAAAAAAAAAAAAAAAAA'; WriteUSB(str,DWORD(length(tmp))); end; 

But this code gives me an exception all the time. I know that the problem is that String is Unicode and each character> 1 byte. I tried converting to different types of strings ( AnsiChar and ShortString ), but I failed.

What is the right way to do this?

Answer 1

Few things. First of all, if it is a C-interface that looks like this, then you need to declare the import as follows:

 function WriteUSB(myP:pAnsiChar; n:DWORD): integer; cdecl; external 'my.dll'; 

Then, to call the function, you need to use the Ansi string and convert it to PAnsiChar, for example:

 procedure myProc; var str : AnsiString; begin str := 'AAAAAAAAAAAAAAAAAAAAA'; WriteUSB(PAnsiChar(str), length(str)); end; 

(A cast to DWORD is optional.) If you do it this way, it should work without any problems.

Answer 2

You can convert the string to AnsiString (as already mentioned) if you intend to use only Ansi characters, but if you want to use Unicode strings. And that the DLL / printer will accept them, you can try something line by line (untested, but I think it usually matches):

 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; 

I don’t know if this will work with this particular DLL, but this is a more general way to deal with the shift in Unicode strings, instead of accepting (and discarding) AnsiString everywhere.

Answer 3

Try it with pchar in your call:

  WriteUSB(pchar(str),DWORD(length(tmp))); 

Answer 4

Thanks so much for all the feedback. I make it work by combining your reviews. Decision:

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 have some problems with the Swedish characters, but I will solve it. Now I know that the DLL call is correct.

Thanks again for the feedback. This is a great forum.

BR User Delphi