VB.Net SerialPort 读取返回碎片数据集

Question

问题

我有一个 USB 设备，可以在 Windows 上创建虚拟串行端口。我正在使用 VB.Net 从端口写入和读取。我的设备以特定大小的字节集进行响应，但我发现 SerialPort.Read(byte-array,offset,number-bytes) 不会返回完整的字节数，但它也不会超时或生成异常。重复调用会返回额外的片段（最多需要 3 次调用）。我不明白为什么这个读取方法的行为如此？它是否认为请求的字节数只是一个建议？ :-)。我希望它会等待完成整个请求，除非它首先超时。

使用 pySerial 的 Python 代码不有同样的问题。

那么，我在这里做错了什么？我是否期望太多？

一些场景是：

1. 我向端口写入命令并期望得到 4 个字节的响应。我首先得到 1 个字节，然后在后续调用中得到 3 个字节。
2. 我写了一条命令，期望得到 21120 字节的响应。我在 3 次调用中从端口读取数据，分别得到 1、12671 和 8448 字节。

以下是我的代码的一些摘录：

Private Sub SetupVirtualSerialPort()
  Dim portName As String = "COM" + (m_DeviceContext * -1).ToString
  Const baud As Int32 = 9600    '7680000
  Const parity As Parity = parity.None
  Const databits As Int32 = 8
  Const stopbits As StopBits = stopbits.One
  m_SerialPort = New SerialPort(portName, baud, parity, databits, stopbits)
  m_SerialPort.WriteTimeout = VSPtimeout
  m_SerialPort.ReadTimeout = VSPtimeout
  m_SerialPort.ReadBufferSize = 2 * RETURN_BUFFER_SIZE
  m_SerialPort.WriteBufferSize = 2 * COMMAND_BUFFER_SIZE
  m_SerialPort.Open()

  ' Register event handlers
  AddHandler m_SerialPort.ErrorReceived, AddressOf m_DriverInterface.Handle_VSP_Error
End Sub


Public Function WriteReadVSPort(ByVal commandLength As Int32, ByVal returnLength As Int32)             As Int32

  Const RetryLimit As Int32 = 5
  Dim NumberRetries As Int32 = 0
  Dim Offset As Int32 = 0
  Dim ExceptionOccurred As Boolean = False
  Dim NumberBytes As Int32 = 0

  Try '  Writing
    m_SerialPort.Write(m_CommandBuffer, 0, commandLength)
  Catch exc As InvalidOperationException
    MessageBox.Show("InvalidOperationException", Application.ProductName)
    ExceptionOccurred = True
  Catch exc As TimeoutException
    MessageBox.Show("TimeoutException", Application.ProductName)
    ExceptionOccurred = True
  End Try

  If Not ExceptionOccurred Then

    Try ' Reading

      ' Working around a problem here: reads are returning fewer 
      ' bytes than requested, though not signalling a timeout exception.
      ' Therefore, we retry if we got fewer bytes than expected, up to five times.
      While NumberRetries < RetryLimit And returnLength > Offset

        NumberBytes = m_SerialPort.Read(m_ReturnBytes, Offset, returnLength - Offset)
        Offset += NumberBytes
        NumberRetries += 1
        If returnLength <> NumberBytes Then
          System.Diagnostics.Debug.Print("Number of bytes read (" & NumberBytes &
            ") not what was requested (" & returnLength & "). Accumulated " & Offset)
        End If

      End While

    Catch exc As InvalidOperationException
      MessageBox.Show("InvalidOperationException", Application.ProductName)
      ExceptionOccurred = True
    Catch exc As TimeoutException
      MessageBox.Show("TimeoutException", Application.ProductName)
      ExceptionOccurred = True
    End Try

  If ExceptionOccurred Then
    Return 1
  Else
    Return 0
  End If

End Function

谢谢。

Answer 1

这对于处理 IO（包括流和端口）来说是完全正常的。基本上，您需要检查返回值和循环。例如：

int offset = 0, read, remaining = ...;
while(remaining > 0 &&
    (read = source.Read(buffer, offset, remaining) > 0)
{
    offset += read;
    remaining -= read;
}
if(remaining > 0) throw new EndOfStreamException();

如果您的消息不是固定长度的，您可能需要添加长度前缀（在每个消息之前）或消息分隔符（在每个消息之后）。

Answer 2

我采纳了使用 DataReceived 事件并使代码事件驱动而不是循环的建议。我发现虚拟串行端口仍然无法在一次读取操作中处理 21120 字节消息。较短的消息长度可以正确完成。然而，当我将串口数据接收阈值设置为21119字节并将串口读缓冲区设置为我的消息大小的10倍时，我发现
1. 只有 12672 字节可用（而不是 21119）时，将触发 DataReceived 事件，并且在对完整大小执行 Read() 时返回相同的数字。
2. 由于字节数不等于我的阈值，如果我当时不进行读取，则不会触发进一步的 DataReceived 事件
3. 但是，如果（且仅当）我读取 12672 字节时，另一个 DataReceived 事件会与剩余的 8448 字节一起出现。

我不明白为什么会这样。欢迎进一步评论。

但是，我想我应该分享我当前的代码以造福他人。

一些类变量是：

Private m_SerialPort As SerialPort = Nothing
Private Debug As Int16
Private m_CommandBuffer(COMMAND_BUFFER_SIZE) As Byte
Private m_ReturnBytes(RETURN_BUFFER_SIZE) As Byte
Private m_WaitingOnBytes As Int32
Private m_VSP_Offset As Int32 = 0
Private m_waitHandle As New System.Threading.ManualResetEvent(True) ' Initialize to signaled
Private m_waitHandle2 As New System.Threading.ManualResetEvent(False) ' Initialize to UN-signaled

事件处理程序子例程

Public Sub Handle_VSP_DataReceived_Dev(ByVal sender As System.Object, ByVal e As System.EventArgs)
  Dim NumberBytes As Int32

  If m_SerialPort.BytesToRead > 0 And m_SerialPort.BytesToRead >= m_WaitingOnBytes Then
    ' This handles the case where the event was triggered, there was data and its length matched
    ' or exceeded the requested amount.
    System.Diagnostics.Debug.Print("DR-Dev: Bytes to read: " & m_SerialPort.BytesToRead & ", waiting for: " & m_WaitingOnBytes)
    NumberBytes = m_SerialPort.Read(m_ReturnBytes, m_VSP_Offset, m_WaitingOnBytes)
    System.Diagnostics.Debug.Print("DR-Dev: got " & NumberBytes & " bytes, released wait handle")
    m_WaitingOnBytes = 0
    m_waitHandle.Set() ' Release the wait handle so the thread running WriteReadVSPort can proceed
  ElseIf m_SerialPort.BytesToRead > 0 And m_WaitingOnBytes > 0 Then
    ' Handle the case where the full request is not delivered. Note: 
    ' This should not need to be done, but it seems that the 
    ' Serial Port is sending the event before all the data is 
    ' received and the threshold is crossed and then not 
    ' sending another event until the buffer is read.
    ' So, here we do a partial read, if we are waiting on a
    ' read operation and adjust the offset and remaining bytes
    ' we are waiting for.
    System.Diagnostics.Debug.Print("DR-Dev: Bytes to read: " & m_SerialPort.BytesToRead & ", waiting for: " & m_WaitingOnBytes)
    NumberBytes = m_SerialPort.Read(m_ReturnBytes, m_VSP_Offset, m_WaitingOnBytes)
    If NumberBytes = m_WaitingOnBytes Then
      ' We actually got all the data, though the serial port did not report it had it ready. Fine, 
      ' proceed as above
      System.Diagnostics.Debug.Print("DR-Dev: got " & m_WaitingOnBytes & " bytes, released wait handle")
      m_WaitingOnBytes = 0
      m_waitHandle.Set() ' Release the wait handle so the thread running WriteReadVSPort can proceed
    Else ' Mark this as a partial read
      System.Diagnostics.Debug.Print("DR-Dev: got partial " & NumberBytes & " while waiting for: " &
        m_WaitingOnBytes & " bytes, continue to hold WriteReadVSPort")
      m_WaitingOnBytes -= NumberBytes
      m_VSP_Offset += NumberBytes
    End If
  End If

End Sub

执行写入命令/读取响应活动的函数

Public Function WriteReadVSPort(ByVal commandLength As Int32, ByVal returnLength As Int32) As Int32

  Dim ExceptionOccurred As Boolean = False
  Dim NumberBytes As Int32 = 0
  Dim RetriesRemaining As Int32 = 4
  Dim Finished As Boolean = False

  ' Important to set up for reading response before the command is written
  ' because another thread will handle the DataReceived event and process
  ' the received data without intervention from the thread executing
  ' this(subroutine.
  m_VSP_Offset = 0
  m_WaitingOnBytes = returnLength
  ' Set the DataReceived event threshold
  m_SerialPort.ReceivedBytesThreshold = m_WaitingOnBytes - 1
  ' Set waitHandle so it will block the thread executing this routine until the data is received
  m_waitHandle.Reset()

  Try '  Writing
    m_SerialPort.Write(m_CommandBuffer, 0, commandLength)
  Catch exc As InvalidOperationException
    MessageBox.Show("InvalidOperationException when writing to Serial Port COM" & -1 * DeviceContext, Application.ProductName)
    ExceptionOccurred = True
  Catch exc As TimeoutException
    MessageBox.Show("TimeoutException when writing to Serial Port COM" & -1 * DeviceContext, Application.ProductName)
    ExceptionOccurred = True
  End Try

  If Not ExceptionOccurred Then

    Try ' Reading all done by Event Handler

      ' wait for event handler to complete its job, running in another thread
      System.Diagnostics.Debug.Print("WR_VSP: waiting on waitHandle, bytes avail: " &
              m_SerialPort.BytesToRead & ", want bytes: " & m_WaitingOnBytes)

      If m_waitHandle.WaitOne(VSPtimeout) Then
        ' The WaitOne call returned True, meaning that Handle_VSP_DataReceived_Dev was able to receive all the requested data
        System.Diagnostics.Debug.Print("WR_VSP: proceeding")
      Else
        ' The WaitOne call timed out. Give it some retries before throwing an exception
        While Not Finished And RetriesRemaining > 0
          System.Threading.Thread.Sleep(VSPtimeout)
          If m_SerialPort.BytesToRead > 0 And m_SerialPort.BytesToRead >= m_WaitingOnBytes Then
            NumberBytes = m_SerialPort.Read(m_ReturnBytes, m_VSP_Offset, m_WaitingOnBytes)
            System.Diagnostics.Debug.Print("WR_VSP: timeout mode, got " & m_WaitingOnBytes & " bytes")
            Finished = True
          Else
            RetriesRemaining -= 1
          End If
        End While
        If Not Finished Then
          Throw New TimeoutException("Device failed to send the requested number of bytes.")
        End If
      End If

    Catch exc As InvalidOperationException
      MessageBox.Show("InvalidOperationException when reading from Serial Port COM" & -1 * DeviceContext, Application.ProductName)
      ExceptionOccurred = True
    Catch exc As TimeoutException
      MessageBox.Show("TimeoutException when reading from Serial Port COM" & -1 * DeviceContext, Application.ProductName)
      ExceptionOccurred = True
    End Try

  End If

  If ExceptionOccurred Then
    Return 1
  Else
    Return 0
  End If

End Function