Question

I’m trying to receive a buffer through a TCP socket using the TServerSocket component (I’m maintaining a legacy application, so migrating to Indy or anything else is out of question for now).

I have implemented a method of the OnClientRead event that reads this buffer in a non-blocking socket (again, I cannot make drastic changes to this legacy application).

The function looks like this:

procedure TFrmMain.ServerSocket1ClientRead(Sender: TObject;
  Socket: TCustomWinSocket);
var
  Size: Integer;
  Bytes: TBytes;
begin
  Size := Socket.ReceiveLength;
  SetLength(Bytes, Size);
  Socket.ReceiveBuf(Bytes, Size);
end;

However, this gives me the following exception:

Asynchronous socket error 10053

If I change it to this:

procedure TFrmMain.ServerSocket1ClientRead(Sender: TObject;
  Socket: TCustomWinSocket);
var
  Size: Integer;
  Bytes: array[0..1024*256] of Byte;
begin
  Size := Socket.ReceiveLength;
  Socket.ReceiveBuf(Bytes, Size);
end;

It works. However, the dynamic approach is more adequate to my problem domain.

What could be causing this? My objective is to read a binary buffer through a TCP socket with this component.

Thanks in advance.

Answer 1

The first parameter of ReceiveBuf is an untyped var parameter. It needs direct access to the place it’s going to start writing.

When you pass it a dynamic array, it will overwrite the dynamic-array variable itself instead of the contents of the array. Pass a reference to the first element of the array instead of a reference to the variable:

Socket.ReceiveBuf(Bytes[0], Size);

That syntax will work with non-dynamic arrays, too. In that case, a reference to the first element is the same as a reference to the variable itself.