Question

I have an error that is confusing the hell out of me. The following code sends a line from a TidTCPClient to a TidTCPServer. The first time it executes, it works perfectly. The second time it executes, and every time after that, it adds a line break to the start of every string. What am I missing? (I know it does it in an odd way, but the list of clients is necessary in the full code)

   procedure TClientForm.ButtonSendStringClick(Sender: TObject);
    var
      I: integer;
      List: TList;
    begin
      List := ClientList.LockList;
      try
       for I := 0 to (List.Count- 1) do
        begin
          TidTCPClient(List[I]).IOHandler.WriteLn('Hello'+'|x|');
        end;
      finally
       ClientList.UnlockList;
      end;
      Edit1.Text := '';
    end;

    procedure TClientForm.IdTCPServer1Execute(AContext: TIdContext);
    var
    LLine: string;
    begin
    LLine := Acontext.Connection.IOHandler.ReadLn('|x|');
    OutputDebugString(PChar(LLine));
    end;

Answer 1

WriteLn() appends a CRLF to the end of the string you pass to it, but ReadLn() stops reading when it encounters the terminator string that you specify. So you are sending 'Hello|x|#13#10' but you are only reading 'Hello|x|', leaving #13#10 in the socket buffer for the next read to grab.

To solve your problem, you have two choices:

1) If you want to keep using a custom terminator in ReadLn(), change WriteLn() to Write() so the implicit CRLF is not sent anymore. No change is needed in your ReadLn() call.

Write('Hello|x|');
LLine := ReadLn('|x|');

2) Stop using a custom terminator altogether. Pass just your main string by itself to WriteLn() and let it append a CRLF, then do not pass any terminator to ReadLn() as its default terminator is LF (which includes handling for CRLF).

WriteLn('Hello');
LLine := ReadLn();