C++ function to Delphi XE7 - c++

Question

I have a C++ function :
long CSP2_API csp2GetPacket(char szData[], long nBcrNr, long nMaxDataSz)
parameters are :
szData: buffer where the barcode data is stored in
nBcrNr: the desired barcode’s index (which starts from 0L)
nMaxDataSz: the maximum size of the szData buffer
My way of dealing with this is :
type
TBarcode = Array[0..99] of AnsiChar;
function csp2GetPacket(szBarData : TBarCode;
nBarcodeNumber: LongInt;
nMaxLength: LongInt): LongInt cdecl {$IFDEF WIN32}stdcall {$ENDIF}; external 'CSP2.DLL';
and called using...
procedure TForm1.Button1Click(Sender: TObject);
var
BarCode : TBarcode;
begin
BarCode := #0;
csp2GetPacket(BarCode,0,SizeOf(BarCode));
end;
The C++ documentation confirms it is using standard ASCII text for the barcodes.
Is there a better (more correct way) of calling the C++ function? or is this acceptable?

Answer 1

If you know the barcode data will never exceed 100 bytes, then your approach is fine, though your function declaration should be more like this instead:
type
TBarcode = Array[0..99] of AnsiChar;
function csp2GetPacket(szBarData: TBarCode; nBarcodeNumber: LongInt; nMaxLength: LongInt): LongInt; cdecl; external 'CSP2.DLL';
Or, if the function really does use stdcall on Windows instead of cdecl:
function csp2GetPacket(szBarData: TBarCode; nBarcodeNumber: LongInt; nMaxLength: LongInt): LongInt; stdcall; external 'CSP2.DLL';
Either way, a more correct translation of the function would be to declare the szBarData parameter as PAnsiChar instead:
function csp2GetPacket(szBarData: PAnsiChar; ...) ...
You can still pass a fixed array variable to a PAnsiChar parameter.

Answer 2

This:
long CSP2_API csp2GetPacket(char szData[], long nBcrNr, long nMaxDataSz)
is translated like this:
function csp2GetPacket(szData: PAnsiChar; nBcrNr, nMaxDataSz: Longint): Longint; <calling-convention>;
Your translation is not correct. In C, one can degrade an array to a pointer, but in Delphi, you would really pass the entire array per value, so the translation is not binary compatible with the original.
I assume you have the calling convention right. If not, simply remove the stdcall.
I would simply use an AnsiString as parameter, cast to a PAnsiChar, more or less like:
var
BarCode: AnsiString;
L: Longint;
begin
SetLength(L, 100);
L := csp2GetPacket(PAnsiChar(BarCode), 0, Length(BarCode));
// If csp2GetPacket returns the length, then:
SetLength(BarCode, L);
// Otherwise, this will work too:
BarCode := PAnsiChar(BarCode); // Copies up to and including terminating #0
I don't know the limits for the length of the buffer, so I assumed the same length as your code.
More info in my article on the subject of conversions: Pitfalls of converting.