Tom*_*ell 5 c++ usb serial-port usbserial visual-c++
我对串行I/O没有多少经验,但最近一直负责修复一些高度有缺陷的串行代码,因为原来的程序员已经离开了公司.
该应用程序是一个Windows程序,通过USB上运行的虚拟COMM端口串行与科学仪器通信.虚拟COMM端口USB驱动程序由FTDI提供,因为它们制造我们在仪器上使用的USB芯片.
串行代码位于非托管C++ DLL中,由我们的旧C++软件和新的C#/ .Net(WinForms)软件共享.
有两个主要问题:
许多XP系统都失败了
当第一个命令发送到仪器时,没有响应.当您发出下一个命令时,您将从第一个命令获得响应.
这是一个典型的使用场景(调用方法的完整源代码如下所示):
char szBuf [256];
CloseConnection ();
if (OpenConnection ())
{
ClearBuffer ();
// try to get a firmware version number
WriteChar ((char) 'V');
BOOL versionReadStatus1 = ReadString (szBuf, 100);
...
}
在发生故障的系统上,ReadString调用永远不会收到任何串行数据,并且会超时.但是如果我们发出另一个不同的命令,并再次调用ReadString,它将返回第一个命令的响应,而不是新命令!
但这只发生在Windows XP系统的大部分上 - 而且从不在Windows 7上.幸运的是,我们的XP开发机器运行良好,所以在我们开始进行beta测试之前我们没有看到问题.但我也可以通过在我的XP开发机上运行XP VM(VirtualBox)来重现这个问题.此外,只有在使用具有新C#版本的DLL时才会出现此问题 - 适用于旧的C++应用程序.
当我在调用ClearCommError之前将Sleep(21)添加到低级BytesInQue方法时,这似乎得到了解决,但这加剧了另一个问题--CPU使用率.睡眠时间少于21毫秒会使故障模式重新出现.
CPU使用率高
在进行串行I/O CPU时,使用率过高 - 通常高于90%.新的C#应用程序和旧的C++应用程序都会发生这种情况,但新应用程序的情况要糟糕得多.通常会使用户界面非常无响应,但并非总是如此.
这是我们的Port.cpp类的代码,它的所有可怕的荣耀.抱歉长度,但这是我正在使用的.最重要的方法可能是OpenConnection,ReadString,ReadChar和BytesInQue.
//
// Port.cpp: Implements the CPort class, which is
// the class that controls the serial port.
//
// Copyright (C) 1997-1998 Microsoft Corporation
// All rights reserved.
//
// This source code is only intended as a supplement to the
// Broadcast Architecture Programmer's Reference.
// For detailed information regarding Broadcast
// Architecture, see the reference.
//
#include <windows.h>
#include <stdio.h>
#include <assert.h>
#include "port.h"
// Construction code to initialize the port handle to null.
CPort::CPort()
{
m_hDevice = (HANDLE)0;
// default parameters
m_uPort = 1;
m_uBaud = 9600;
m_uDataBits = 8;
m_uParity = 0;
m_uStopBits = 0; // = 1 stop bit
m_chTerminator = '\n';
m_bCommportOpen = FALSE;
m_nTimeOut = 50;
m_nBlockSizeMax = 2048;
}
// Destruction code to close the connection if the port
// handle was valid.
CPort::~CPort()
{
if (m_hDevice)
CloseConnection();
}
// Open a serial communication port for writing short
// one-byte commands, that is, overlapped data transfer
// is not necessary.
BOOL CPort::OpenConnection()
{
char szPort[64];
m_bCommportOpen = FALSE;
// Build the COM port string as "COMx" where x is the port.
if (m_uPort > 9)
wsprintf(szPort, "\\\\.\\COM%d", m_uPort);
else
wsprintf(szPort, "COM%d", m_uPort);
// Open the serial port device.
m_hDevice = CreateFile(szPort,
GENERIC_WRITE | GENERIC_READ,
0,
NULL, // No security attributes
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
NULL);
if (m_hDevice == INVALID_HANDLE_VALUE)
{
SaveLastError ();
m_hDevice = (HANDLE)0;
return FALSE;
}
return SetupConnection(); // After the port is open, set it up.
} // end of OpenConnection()
// Configure the serial port with the given settings.
// The given settings enable the port to communicate
// with the remote control.
BOOL CPort::SetupConnection(void)
{
DCB dcb; // The DCB structure differs betwwen Win16 and Win32.
dcb.DCBlength = sizeof(DCB);
// Retrieve the DCB of the serial port.
BOOL bStatus = GetCommState(m_hDevice, (LPDCB)&dcb);
if (bStatus == 0)
{
SaveLastError ();
return FALSE;
}
// Assign the values that enable the port to communicate.
dcb.BaudRate = m_uBaud; // Baud rate
dcb.ByteSize = m_uDataBits; // Data bits per byte, 4-8
dcb.Parity = m_uParity; // Parity: 0-4 = no, odd, even, mark, space
dcb.StopBits = m_uStopBits; // 0,1,2 = 1, 1.5, 2
dcb.fBinary = TRUE; // Binary mode, no EOF check : Must use binary mode in NT
dcb.fParity = dcb.Parity == 0 ? FALSE : TRUE; // Enable parity checking
dcb.fOutX = FALSE; // XON/XOFF flow control used
dcb.fInX = FALSE; // XON/XOFF flow control used
dcb.fNull = FALSE; // Disable null stripping - want nulls
dcb.fOutxCtsFlow = FALSE;
dcb.fOutxDsrFlow = FALSE;
dcb.fDsrSensitivity = FALSE;
dcb.fDtrControl = DTR_CONTROL_ENABLE;
dcb.fRtsControl = RTS_CONTROL_DISABLE ;
// Configure the serial port with the assigned settings.
// Return TRUE if the SetCommState call was not equal to zero.
bStatus = SetCommState(m_hDevice, &dcb);
if (bStatus == 0)
{
SaveLastError ();
return FALSE;
}
DWORD dwSize;
COMMPROP *commprop;
DWORD dwError;
dwSize = sizeof(COMMPROP) + sizeof(MODEMDEVCAPS) ;
commprop = (COMMPROP *)malloc(dwSize);
memset(commprop, 0, dwSize);
if (!GetCommProperties(m_hDevice, commprop))
{
dwError = GetLastError();
}
m_bCommportOpen = TRUE;
return TRUE;
}
void CPort::SaveLastError ()
{
DWORD dwLastError = GetLastError ();
LPVOID lpMsgBuf;
FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
dwLastError,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), // Default language
(LPTSTR) &lpMsgBuf,
0,
NULL);
strcpy (m_szLastError,(LPTSTR)lpMsgBuf);
// Free the buffer.
LocalFree( lpMsgBuf );
}
void CPort::SetTimeOut (int nTimeOut)
{
m_nTimeOut = nTimeOut;
}
// Close the opened serial communication port.
void CPort::CloseConnection(void)
{
if (m_hDevice != NULL &&
m_hDevice != INVALID_HANDLE_VALUE)
{
FlushFileBuffers(m_hDevice);
CloseHandle(m_hDevice); ///that the port has been closed.
}
m_hDevice = (HANDLE)0;
// Set the device handle to NULL to confirm
m_bCommportOpen = FALSE;
}
int CPort::WriteChars(char * psz)
{
int nCharWritten = 0;
while (*psz)
{
nCharWritten +=WriteChar(*psz);
psz++;
}
return nCharWritten;
}
// Write a one-byte value (char) to the serial port.
int CPort::WriteChar(char c)
{
DWORD dwBytesInOutQue = BytesInOutQue ();
if (dwBytesInOutQue > m_dwLargestBytesInOutQue)
m_dwLargestBytesInOutQue = dwBytesInOutQue;
static char szBuf[2];
szBuf[0] = c;
szBuf[1] = '\0';
DWORD dwBytesWritten;
DWORD dwTimeOut = m_nTimeOut; // 500 milli seconds
DWORD start, now;
start = GetTickCount();
do
{
now = GetTickCount();
if ((now - start) > dwTimeOut )
{
strcpy (m_szLastError, "Timed Out");
return 0;
}
WriteFile(m_hDevice, szBuf, 1, &dwBytesWritten, NULL);
}
while (dwBytesWritten == 0);
OutputDebugString(TEXT(strcat(szBuf, "\r\n")));
return dwBytesWritten;
}
int CPort::WriteChars(char * psz, int n)
{
DWORD dwBytesWritten;
WriteFile(m_hDevice, psz, n, &dwBytesWritten, NULL);
return dwBytesWritten;
}
// Return number of bytes in RX queue
DWORD CPort::BytesInQue ()
{
COMSTAT ComStat ;
DWORD dwErrorFlags;
DWORD dwLength;
// check number of bytes in queue
ClearCommError(m_hDevice, &dwErrorFlags, &ComStat ) ;
dwLength = ComStat.cbInQue;
return dwLength;
}
DWORD CPort::BytesInOutQue ()
{
COMSTAT ComStat ;
DWORD dwErrorFlags;
DWORD dwLength;
// check number of bytes in queue
ClearCommError(m_hDevice, &dwErrorFlags, &ComStat );
dwLength = ComStat.cbOutQue ;
return dwLength;
}
int CPort::ReadChars (char* szBuf, int nMaxChars)
{
if (BytesInQue () == 0)
return 0;
DWORD dwBytesRead;
ReadFile(m_hDevice, szBuf, nMaxChars, &dwBytesRead, NULL);
return (dwBytesRead);
}
// Read a one-byte value (char) from the serial port.
int CPort::ReadChar (char& c)
{
static char szBuf[2];
szBuf[0] = '\0';
szBuf[1] = '\0';
if (BytesInQue () == 0)
return 0;
DWORD dwBytesRead;
ReadFile(m_hDevice, szBuf, 1, &dwBytesRead, NULL);
c = *szBuf;
if (dwBytesRead == 0)
return 0;
return dwBytesRead;
}
BOOL CPort::ReadString (char *szStrBuf , int nMaxLength)
{
char str [256];
char str2 [256];
DWORD dwTimeOut = m_nTimeOut;
DWORD start, now;
int nBytesRead;
int nTotalBytesRead = 0;
char c = ' ';
static char szCharBuf [2];
szCharBuf [0]= '\0';
szCharBuf [1]= '\0';
szStrBuf [0] = '\0';
start = GetTickCount();
while (c != m_chTerminator)
{
nBytesRead = ReadChar (c);
nTotalBytesRead += nBytesRead;
if (nBytesRead == 1 && c != '\r' && c != '\n')
{
*szCharBuf = c;
strncat (szStrBuf,szCharBuf,1);
if (strlen (szStrBuf) == nMaxLength)
return TRUE;
// restart timer for next char
start = GetTickCount();
}
// check for time out
now = GetTickCount();
if ((now - start) > dwTimeOut )
{
strcpy (m_szLastError, "Timed Out");
return FALSE;
}
}
return TRUE;
}
int CPort::WaitForQueToFill (int nBytesToWaitFor)
{
DWORD start = GetTickCount();
do
{
if (BytesInQue () >= nBytesToWaitFor)
break;
if (GetTickCount() - start > m_nTimeOut)
return 0;
} while (1);
return BytesInQue ();
}
int CPort::BlockRead (char * pcInputBuffer, int nBytesToRead)
{
int nBytesRead = 0;
int charactersRead;
while (nBytesToRead >= m_nBlockSizeMax)
{
if (WaitForQueToFill (m_nBlockSizeMax) < m_nBlockSizeMax)
return nBytesRead;
charactersRead = ReadChars (pcInputBuffer, m_nBlockSizeMax);
pcInputBuffer += charactersRead;
nBytesRead += charactersRead;
nBytesToRead -= charactersRead;
}
if (nBytesToRead > 0)
{
if (WaitForQueToFill (nBytesToRead) < nBytesToRead)
return nBytesRead;
charactersRead = ReadChars (pcInputBuffer, nBytesToRead);
nBytesRead += charactersRead;
nBytesToRead -= charactersRead;
}
return nBytesRead;
}
根据我的测试和阅读,我在这段代码中看到了几个可疑的东西:
从未设置COMMTIMEOUTS.MS文档说"如果你没有设置超时值,就会出现不可预测的结果".但我尝试设置这个,但没有帮助.
如果不立即获取数据,许多方法(例如ReadString)将进入紧密循环并使用重复读取来敲击端口.这似乎可以解释高CPU使用率.
使用GetTickCount(),许多方法都有自己的超时处理.这不是COMMTIMEOUTS的用途吗？
在新的C#(WinForms)程序中,所有这些串行例程都是从主线程直接从MultiMediaTimer事件中调用的.也许应该在不同的线程中运行？
BytesInQue方法似乎是一个瓶颈.如果我在CPU使用率很高时断开调试器,那通常就是程序停止的地方.此外,在调用ClearCommError之前向此方法添加Sleep(21)似乎可以解决XP问题,但会加剧CPU使用率问题.
代码似乎不必要地复杂化.
我的问题
任何人都可以解释为什么这只适用于少数XP系统上的C#程序？
关于如何重写这个的任何建议？指向优秀的示例代码的指针将是最受欢迎的.
该课程存在一些严重问题,而且微软的版权就更糟糕了.
这门课没什么特别的.它让我想知道为什么它甚至存在,除了作为适配器而不是Create/Read/WriteFile.如果您在.NET Framework中使用SerialPort类,则甚至不需要此类.
您的CPU使用率是因为代码在等待设备有足够的可用数据时进入无限循环.代码也可以说while(1);如果你必须坚持使用Win32和C++,你可以在调用CreateFile时查看完成端口并设置OVERLAPPED标志.这样,您可以在单独的工作线程中等待数据.
与多个COM端口通信时需要小心.自从我完成C++以来已经很长时间了,但我相信Read和Write方法中的静态缓冲区szBuff对于该类的所有实例都是静态的.这意味着如果您"同时"对两个不同的COM端口调用Read,则会产生意外结果.
至于某些XP机器上的问题,如果在每次读/写后检查GetLastError并记录结果,你肯定会发现问题.它应该检查GetLastError,因为它有时并不总是一个"错误",而是来自子系统的请求做其他事情以获得你想要的结果.
如果设置COMMTIMEOUTS正确,你可以摆脱整个循环阻塞.如果在执行读取之前存在特定的Read操作超时超时SetCommTimeouts.
我设置ReadIntervalTimeout为max timeout以确保Read不会比m_nTimeOut更快地返回.如果任何两个字节之间的时间过去,该值将导致Read返回.如果它被设置为2毫秒并且第一个字节进入t,第二个进入t + 1,第三个进入t + 4,则ReadFile仅返回前两个字节,因为超过了字节之间的间隔.ReadTotalTimeoutConstant确保您永远不会等待超过m_nTimeOut,无论如何.
maxWait = BytesToRead * ReadTotalTimeoutMultiplier + ReadTotalTimeoutConstant.从而(BytesToRead * 0) + m_nTimeout = m_nTimeout
BOOL CPort::SetupConnection(void)
{
// Snip...
COMMTIMEOUTS comTimeOut;
comTimeOut.ReadIntervalTimeout = m_nTimeOut; // Ensure's we wait the max timeout
comTimeOut.ReadTotalTimeoutMultiplier = 0;
comTimeOut.ReadTotalTimeoutConstant = m_nTimeOut;
comTimeOut.WriteTotalTimeoutMultiplier = 0;
comTimeOut.WriteTotalTimeoutConstant = m_nTimeOut;
SetCommTimeouts(m_hDevice,&comTimeOut);
}
// If return value != nBytesToRead check check GetLastError()
// Most likely Read timed out.
int CPort::BlockRead (char * pcInputBuffer, int nBytesToRead)
{
DWORD dwBytesRead;
if (FALSE == ReadFile(
m_hDevice,
pcInputBuffer,
nBytesToRead,
&dwBytesRead,
NULL))
{
// Check GetLastError
return dwBytesRead;
}
return dwBytesRead;
}
我不知道这是否完全正确,但它应该给你一个想法.如果您的程序依赖于同步的东西,请删除ReadChar和ReadString方法并使用它.也要注意设置高时间输出.通信速度很快,只需几毫秒.