Exceptionally Serial

You may remember Kara, who recently found some "interesting" serialization code. Now, this code happens to be responsible for sending commands to pieces of machine equipment.

Low-level machine interfaces remain one of the domains where serial protocols rule. Serial communications use simple hardware and have minimal overhead, and something like RS232 has been in real-world use since the 60s. Sure, it's slow, sure it's not great with coping with noise, sure you have to jump through some hoops if you want a connection longer than 15m, but its failures are well understood.

Nothing is so well understood that some developer can't make a mess of it.

 Public Function SendCommand(ByVal cmd As String) As String Dim status As Integer ' Write cmd to the serial port using a protocol that is too painful to reproduce here. ' status receives an appropriate value along the way as the protocol checks for various error ' conditions including timeout If status <> 0 Then Throw MakeComPortException(status) End Function Private Function MakeComPortException(ByVal status As Integer) As ComPortException Dim code As Integer Dim message As String = Nothing GetErrorCode(status, code, message) Return New ComPortException(code, message) End Function Private Sub GetErrorCode(ByVal ErrorNum As Integer, ByRef code As Integer, ByRef message As String) code = ErrorNum Select Case ErrorNum Case 129 : message = "Hardware error occured during Send Data" ' Talk Error' Case 130 : message = "System asked to talk but did not recieve Previous Talk Command" ' Nothing to say Case 131 : SendCommand("ERRMS?") Dim EMsg As String = GetResponse() Dim EmsgStart As Integer = EMsg.IndexOf(" (") Try If EMsg.Contains("ERR=") Then code = CInt(EMsg.Substring(4, EmsgStart - 4)) Catch ex As Exception End Try message = EMsg.Substring(EmsgStart) Case 132 : message = "H/W Error while system trying to accept data" 'Listen Error Case 133 : message = "More than 80 characters received before term char" Case 134 : message = "Archive media is full" Case 135 : message = "Listen state interrupted by ESC key" ' Interrupted from keyboard Case 136 : message = "Listen state interrupted by controller sending '*'" ' Interrupted by Controller Case 137 : message = "Error Occured in UART" Case StatusCodes.PortDeviceNotFoundErrorCode : message = "No device Found" Case StatusCodes.PortTimeoutErrorCode : message = "COM port Timeout Error" ' This next occurs if cable is unplugged at controller Case StatusCodes.PortDisconnectedErrorCode : message = "Serial cable disconnected" Case Else : message = "Error #: " & ErrorNum End Select End Sub

So, the SendCommand method takes a string and passes it down the serial port. The protocol details were elided here, but we know that we receive a status number. MakeComPortException takes that number and helpfully looks up the message which goes with it, using GetErrorCode.

GetErrorCode is one gigantic switch statement. And let's pay close attention to the message lookup process for error 131. You'll note that we call SendCommand to ask the remote device to tell us what the error message was. But in some cases, it's going to reply to that request with an error code. Error 131, to be exact.

So if we trace this: we call SendCommand which gets a 131 error, which forces it to throw the results of calling MakeComPortException, which calls GetErrorCode, which calls SendCommand, which throws a new MakeComPortException, which"

There's an interesting side effect of this approach. Despite looking like a series of recursive calls, throw unwinds the stack, so this code will never actually trigger a stack overflow. It's actually more of an exception-assisted infinite loop.

For a bonus, note the PortTimeoutErrorCode entry. On the hardware side, they use a custom serial cable which wires a loopback on the RS232 "Ready to Send" and "Clear to Send" pins, which the software uses to detect that the cable is unplugged. It also has the side effect of ensuring that no off-the-shelf RS232 cables will work with the software. This is either a stupid mistake, or a fiendishly clever way to sell heavily marked-up replacement cables.

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