2023年6月22日发(作者：)

前几篇串口编程大致讲述了Windows下串口的大致操作，接下来分析流行的SerialPort类，它把Windows API封装好，方便开发利用

1、Win32下串口大致操作流程

(1)打开串口：CreateFile函数

(2)建立串口通信事件:CreateEvent函数

(3)初始化串口:SetCommState函数

(4)建立监视线程,即读写数据线程,因为我们不知道什么时候数据会到来,这里是一个异步事件

(5)写数据:WriteFile

(6)结束:关闭线程->停止WaitCommEvent->CloseHandle

Port类的数据结构

大致了解操作流程后,先看一下SerialPort类,均在代码注释了

数据成员:

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public:

int m_nWriteSize; //要写入串口的数据大小

HANDLE m_hComm; //串口句柄

protected:

// thread监视线程

CWinThread* m_Thread;

// synchronisation objects

//临界资源

CRITICAL_SECTION m_csCommunicationSync;

//监视线程运行标志

BOOL m_bThreadAlive;

// handles

/*事件句柄*/

HANDLE m_hWriteEvent;

HANDLE m_hShutdownEvent;

// There is a general shutdown when the port is closed.

//事件数组，包括一个写事件，接收事件，关闭事件

HANDLE m_hEventArray[3];

// structures

OVERLAPPED m_ov; //异步I/O模型

COMMTIMEOUTS m_CommTimeouts; //超时设置

DCB m_dcb; //设备控制块

// owner window

CWnd* m_pOwner;

// misc

UINT m_nPortNr;

char* m_szWriteBuffer; //写缓冲区

DWORD m_dwCommEvents; //

DWORD m_nWriteBufferSize; //写缓冲大小

函数成员：

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public:

/*******************Port Operation***********************/

// port initialisation

/*初始化串口*/

BOOL InitPort(CWnd* pPortOwner,

UINT portnr = 1,

UINT baud = 19200,

char parity = 'N',

UINT databits = 8,

UINT stopbits = 1,

DWORD dwCommEvents = EV_RXCHAR,

UINT writebuffersize = 1024);

//关闭端口

void ClosePort();

// start/stop comm watching

//控制串口监视线程

BOOL StartMonitoring(); //开启

BOOL RestartMonitoring(); //复位

BOOL StopMonitoring(); //停止

DWORD GetWriteBufferSize();//获取写缓冲大小

DWORD GetCommEvents(); //获取事件

DCB GetDCB(); //获取DCB

//写数据到串口

void WriteToPort(char* string);

void WriteToPort(char* string,int n);

void WriteToPort(LPCTSTR string);

void WriteToPort(LPCTSTR string,int n);

protected:

/***************** protected memberfunctions **********************/

void ProcessErrorMessage(char* ErrorText); //线程函数

static UINT CommThread(LPVOID pParam);

//接收字符

static void ReceiveChar(CSerialPort* port, COMSTAT comstat);

//写字符

static void WriteChar(CSerialPort* port);

3.串口操作

(1)初始化串口

流程：检查参数-->检测线程-->创建事件(监视线程)-->打开端口-->设置异步IO结构参数,详细见代码:

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/*初始化串口*/

BOOL CSerialPort::InitPort(CWnd* pPortOwner, // the owner (CWnd) of the port (receives

message)

UINT portnr, // portnumber (1..4)

UINT baud, // baudrate

char parity, // parity

UINT databits, // databits

UINT stopbits, // stopbits

DWORD dwCommEvents, // EV_RXCHAR, EV_CTS etc

UINT writebuffersize) // size to the writebuffer

{

assert(portnr > 0 && portnr < 5);

assert(pPortOwner != NULL);

// if the thread is alive: Kill

//线程在的话关断它

if (m_bThreadAlive)

{

do

{

SetEvent(m_hShutdownEvent);

} while (m_bThreadAlive);

TRACE("Thread endedn");

}

// create events

//创建事件

if (m_ != NULL)

ResetEvent(m_); else

m_ = CreateEvent(NULL, TRUE, FALSE, NULL);

if (m_hWriteEvent != NULL)

ResetEvent(m_hWriteEvent);

else

m_hWriteEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

if (m_hShutdownEvent != NULL)

ResetEvent(m_hShutdownEvent);

else

m_hShutdownEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

// initialize the event objects

//事件数组初始化，设定优先级别

m_hEventArray[0] = m_hShutdownEvent; // highest priority

m_hEventArray[1] = m_;

m_hEventArray[2] = m_hWriteEvent;

// initialize critical section

//初始化一个临界资源对象

InitializeCriticalSection(&m_csCommunicationSync);

// set buffersize for writing and save the owner

m_pOwner = pPortOwner;

if (m_szWriteBuffer != NULL)

delete [] m_szWriteBuffer;

m_szWriteBuffer = new char[writebuffersize];

m_nPortNr = portnr;

m_nWriteBufferSize = writebuffersize;

m_dwCommEvents = dwCommEvents;

BOOL bResult = FALSE;

char *szPort = new char[50];

char *szBaud = new char[50];

// now it critical!

/*********************************************

多个线程操作相同的数据时，一般是需要按顺序访问的，否则会引导数据错乱，

无法控制数据，变成随机变量。为解决这个问题，就需要引入互斥变量，让每个

线程都按顺序地访问变量。这样就需要使用EnterCriticalSection和LeaveCriticalSection函数。

**********************************************************************/

//进入临界区

EnterCriticalSection(&m_csCommunicationSync);

// if the port is already opened: close it

//端口已经打开的就关闭它

if (m_hComm != NULL)

{

CloseHandle(m_hComm);

m_hComm = NULL;

}

// prepare port strings

//串口参数

sprintf(szPort, "COM%d", portnr);

sprintf(szBaud, "baud=%d parity=%c data=%d stop=%d", baud, parity, databits, stopbits);

// get a handle to the port

/****************************************************************

*通信程序在CreateFile处指定串口设备及相关的操作属性，再返回一个句柄，

*该句柄将被用于后续的通信操作，并贯穿整个通信过程串口打开后，其属性

*被设置为默认值，根据具体需要，通过调用GetCommState(hComm,&&dcb)读取

*当前串口设备控制块DCB设置，修改后通过SetCommState(hComm,&&dcb)将其写

*入。运用ReadFile()与WriteFile()这两个API函数实现串口读写操作，若为异

*步通信方式，两函数中最后一个参数为指向OVERLAPPED结构的非空指针，在读

*写函数返回值为FALSE的情况下，调用GetLastError()函数，返回值为ERROR_IO_PENDING，

*表明I/O操作悬挂，即操作转入后台继续执行。此时，可以用WaitForSingleObject()

*来等待结束信号并设置最长等待时间

*****************************************************************/

m_hComm = CreateFile(szPort, // communication port string

(COMX)串口号

GENERIC_READ | GENERIC_WRITE, // read/write types 可以读写

0, // comm devices must be

opened with exclusive access独占方式打开串口

NULL, // no security attributes 无安全属性

OPEN_EXISTING, // comm devices must use

OPEN_EXISTING

FILE_FLAG_OVERLAPPED, // Async I/O 异步I/O

0); // template must be 0 for

comm devices

//如果创建不成功，错误处理

if (m_hComm == INVALID_HANDLE_VALUE)

{

// port not found

delete [] szPort;

delete [] szBaud;

return FALSE;

}

// set the timeout values

//设置超时上限(异步IO)

m_tervalTimeout = 1000;

m_talTimeoutMultiplier = 1000;

m_talTimeoutConstant = 1000;

m_otalTimeoutMultiplier = 1000;

m_otalTimeoutConstant = 1000;

// configure

/*分别调用Windows API设置串口参数*/

if (SetCommTimeouts(m_hComm, &m_CommTimeouts)) //设置超时

{

/*******************************************************

若对端口数据的响应时间要求较严格，可采用事件驱动方式。

事件驱动方式通过设置事件通知，当所希望的事件发生时，Windows

发出该事件已发生的通知，这与DOS环境下的中断方式很相似。Windows

定义了9种串口通信事件，较常用的有以下三种：

EV_RXCHAR:接收到一个字节，并放入输入缓冲区；

EV_TXEMPTY:输出缓冲区中的最后一个字符，发送出去；

EV_RXFLAG:接收到事件字符(DCB结构中EvtChar成员)，放入输入缓冲区

在用SetCommMask()指定了有用的事件后，应用程序可调用WaitCommEvent()来等待事

件的发生。SetCommMask(hComm,0)可使WaitCommEvent()中止。

**************************************************************/

if (SetCommMask(m_hComm, dwCommEvents)) //设置通信事件

{

if (GetCommState(m_hComm, &m_dcb)) //获取当前DCB参数

{

m_r = 'q'; //设置字件字符 m_ntrol = RTS_CONTROL_ENABLE; // set RTS bit high!

if (BuildCommDCB(szBaud, &m_dcb)) //填写DCB结构

{

if (SetCommState(m_hComm, &m_dcb)) //配置DCB

; // continue

else

ProcessErrorMessage("SetCommState()");

}

else

ProcessErrorMessage("BuildCommDCB()");

}

else

ProcessErrorMessage("GetCommState()");

}

else

ProcessErrorMessage("SetCommMask()");

}

else

ProcessErrorMessage("SetCommTimeouts()");

delete [] szPort;

delete [] szBaud;

// flush the port

//终止读写并清空接收和发送

PurgeComm(m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT |

PURGE_TXABORT);

// release critical section

//出临界区

LeaveCriticalSection(&m_csCommunicationSync);

TRACE("Initialisation for communicationport %d Startmonitor to

communicate.n", portnr);

return TRUE;

}

(2)监视线程的控制

先看比较简单的线程控制吧，主要有开启线程，复位和停止

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//开始监视串口 BOOL CSerialPort::StartMonitoring()

{

if (!(m_Thread = AfxBeginThread(CommThread, this)))

return FALSE;

TRACE("Thread startedn");

return TRUE;

}

//

// Restart the comm thread

//

//重启监视线程（挂起重启）

BOOL CSerialPort::RestartMonitoring()

{

TRACE("Thread resumedn");

m_Thread->ResumeThread();

return TRUE;

}

//

// Suspend the comm thread

//

//挂起监视线程

BOOL CSerialPort::StopMonitoring()

{

TRACE("Thread suspendedn");

m_Thread->SuspendThread();

return TRUE;

}

(3)监视线程

我们把读写串口的操作全部交给监视线程，现在简单看一下监视线程的大致流程：

检查串口-->进入循环{WaitCommEvent(不阻塞询问)询问事件-->如果有事件来到-->到相应处理(关闭读写)}

详细代码如下:

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//监视串口函数

UINT CSerialPort::CommThread(LPVOID pParam)

{ // Cast the void pointer passed to the thread back to

// a pointer of CSerialPort class

CSerialPort *port = (CSerialPort*)pParam;

// Set the status variable in the dialog class to

// TRUE to indicate the thread is running.

port->m_bThreadAlive = TRUE;

// Misc. variables

DWORD BytesTransfered = 0;

DWORD Event = 0;

DWORD CommEvent = 0;

DWORD dwError = 0;

COMSTAT comstat;

BOOL bResult = TRUE;

// Clear comm buffers at startup

//检查串口是否打开

if (port->m_hComm) // check if the port is opened

PurgeComm(port->m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR |

PURGE_RXABORT | PURGE_TXABORT);

// begin forever loop. This loop will run as long as the thread is alive.

//不断读取数据

for (;;)

{

// Make a call to WaitCommEvent(). This call will return immediatly

// because our port was created as an async port (FILE_FLAG_OVERLAPPED

// and an m_OverlappedStructerlapped structure specified). This call will cause the

// m_OverlappedStructerlapped element m_, which is part of

the m_hEventArray to

// be placed in a non-signeled state if there are no bytes available to be read,

// or to a signeled state if there are bytes available. If this event handle

// is set to the non-signeled state, it will be set to signeled when a

// character arrives at the port.

// we do this for each port!

/********************************************************************

WaitCommEvent函数第3个参数1pOverlapped可以是一个OVERLAPPED结构的变量指针

，也可以是NULL，当用NULL时，表示该函数是同步的，否则表示该函数是异步的。

调用WaitCommEvent时，如果异步操作不能立即完成，会立即返回FALSE，系统在

WaitCommEvent返回前将OVERLAPPED结构成员hEvent设为无信号状态，等到产生通信

事件时，系统将其置有信号

***********************************************************************/

bResult = WaitCommEvent(port->m_hComm, &Event, &port->m_ov);//表示该函数是异步的

if (!bResult)

{

//如果WaitCommEvent返回Error为NULL，则查询错误信息

// If WaitCommEvent() returns FALSE, process the last error to determin

// the reason..

switch (dwError = GetLastError())

{

case ERROR_IO_PENDING: //正常情况，没有字符可读

{

// This is a normal return value if there are no bytes

// to read at the port.

// Do nothing and continue

break;

}

case 87: //系统错误

{

// Under Windows NT, this value is returned for some reason.

// I have not investigated why, but it is also a valid reply

// Also do nothing and continue.

break;

}

default: //发生其他错误

{

// All other error codes indicate a serious error has

// occured. Process this error.

port->ProcessErrorMessage("WaitCommEvent()");

break;

}

}

}

else //WaitCommEvent()能正确返回

{

// If WaitCommEvent() returns TRUE, check to be sure there are

// actually bytes in the buffer to read. //

// If you are reading more than one byte at a time from the buffer

// (which this program does not do) you will have the situation occur

// where the first byte to arrive will cause the WaitForMultipleObjects()

// function to stop waiting. The WaitForMultipleObjects() function

// resets the event handle in m_ to the non-signelead state

// as it returns.

//

// If in the time between the reset of this event and the call to

// ReadFile() more bytes arrive, the m_ handle will be set

again

// to the signeled state. When the call to ReadFile() occurs, it will

// read all of the bytes from the buffer, and the program will

// loop back around to WaitCommEvent().

//

// At this point you will be in the situation where m_ is

set,

// but there are no bytes available to read. If you proceed and call

// ReadFile(), it will return immediatly due to the async port setup, but

// GetOverlappedResults() will not return until the next character arrives.

//

// It is not desirable for the GetOverlappedResults() function to be in

// this state. The thread shutdown event (event 0) and the WriteFile()

// event (Event2) will not work if the thread is blocked by GetOverlappedResults().

//

// The solution to this is to check the buffer with a call to ClearCommError().

// This call will reset the event handle, and if there are no bytes to read

// we can loop back through WaitCommEvent() again, then proceed.

// If there are really bytes to read, do nothing and proceed.

bResult = ClearCommError(port->m_hComm, &dwError, &comstat);

if (e == 0)

continue;

} // end if bResult

//主等待函数，会阻塞线程

// Main wait function. This function will normally block the thread

// until one of nine events occur that require action.

//等待3个事件：关断/读/写，有一个事件发生就返回

Event = WaitForMultipleObjects(3, //3个事件

port->m_hEventArray,//事件数组

FALSE, //有一个事件发生就返回

INFINITE); //超时时间

switch (Event)

{

case 0:

{

// Shutdown event. This is event zero so it will be

// the higest priority and be serviced first.

//关断事件，关闭串口

CloseHandle(port->m_hComm);

port->m_hComm=NULL;

port->m_bThreadAlive = FALSE;

// Kill this thread. break is not needed, but makes me feel better.

AfxEndThread(100);

break;

}

case 1: // read event 将定义的各种消息发送出去

{

//接收

GetCommMask(port->m_hComm, &CommEvent);

if (CommEvent & EV_RXCHAR)

// Receive character event from port.

ReceiveChar(port, comstat);

if (CommEvent & EV_CTS)

::SendMessage(port->m_pOwner->m_hWnd,

WM_COMM_CTS_DETECTED, (WPARAM) 0, (LPARAM) port->m_nPortNr);

if (CommEvent & EV_BREAK)

::SendMessage(port->m_pOwner->m_hWnd,

WM_COMM_BREAK_DETECTED, (WPARAM) 0, (LPARAM) port->m_nPortNr);

if (CommEvent & EV_ERR)

::SendMessage(port->m_pOwner->m_hWnd,

WM_COMM_ERR_DETECTED, (WPARAM) 0, (LPARAM) port->m_nPortNr);

if (CommEvent & EV_RING)

::SendMessage(port->m_pOwner->m_hWnd,

WM_COMM_RING_DETECTED, (WPARAM) 0, (LPARAM) port->m_nPortNr);

if (CommEvent & EV_RXFLAG)

::SendMessage(port->m_pOwner->m_hWnd,

WM_COMM_RXFLAG_DETECTED, (WPARAM) 0, (LPARAM) port->m_nPortNr);

break;

}

case 2: // write event发送数据 {

// Write character event from port

//写

WriteChar(port);

break;

}

} // end switch

} // close forever loop

return 0;

}

(4)读取数据操作

读取数据是一个异步操作，当有数据发来时，会触发读事件m_，监视线程捕捉到事件后并获知是读事件，进入相关读处理，这里调用函数ReceiveChar

，ReceiveChar中调用ReadFile函数将串口数据读到Buffer缓冲中，相关代码如下：

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//接收数据

void CSerialPort::ReceiveChar(CSerialPort* port, COMSTAT comstat)

{

BOOL bRead = TRUE;

BOOL bResult = TRUE;

DWORD dwError = 0;

DWORD BytesRead = 0;

unsigned char RXBuff;

for (;;)

{

// Gain ownership of the comm port critical section.

// This process guarantees no other part of this program

// is using the port object.

EnterCriticalSection(&port->m_csCommunicationSync);

// ClearCommError() will update the COMSTAT structure and

// clear any other errors.

//更新COMSTAT

bResult = ClearCommError(port->m_hComm, &dwError, &comstat);

LeaveCriticalSection(&port->m_csCommunicationSync);

// start forever loop. I use this type of loop because I

// do not know at runtime how many loops this will have to

// run. My solution is to start a forever loop and to

// break out of it when I have processed all of the

// data available. Be careful with this approach and

// be sure your loop will exit.

// My reasons for this are not as clear in this sample

// as it is in my production code, but I have found this

// solutiion to be the most efficient way to do this.

//所有字符均被读出，中断循环

if (e == 0)

{

// break out when all bytes have been read

break;

}

EnterCriticalSection(&port->m_csCommunicationSync);

if (bRead)

{

//串口读出，读出缓冲区中字节

bResult = ReadFile(port->m_hComm, // Handle to COMM port

&RXBuff, // RX Buffer Pointer

1, // Read one byte

&BytesRead, // Stores number of bytes read

&port->m_ov); // pointer to the m_ov structure

// deal with the error code

//若返回错误，错误处理

if (!bResult)

{

switch (dwError = GetLastError())

{

case ERROR_IO_PENDING:

{

// asynchronous i/o is still in progress

// Proceed on to GetOverlappedResults();

//异步IO仍在进行

bRead = FALSE;

break;

}

default:

{ //其他错误处理 // Another error has occured. Process this error.

port->ProcessErrorMessage("ReadFile()");

break;

}

}

}

else

{ //ReadFile返回TRUE

// ReadFile() returned complete. It is not necessary to call

GetOverlappedResults()

bRead = TRUE;

}

} // close if (bRead)

//异步IO操作仍在进行，需要调用GetOverlappedResult查询

if (!bRead)

{

bRead = TRUE;

bResult = GetOverlappedResult(port->m_hComm, // Handle to COMM port

&port->m_ov, // Overlapped structure

&BytesRead, // Stores number of

bytes read

TRUE); // Wait flag

// deal with the error code

if (!bResult)

{

port->ProcessErrorMessage("GetOverlappedResults() in ReadFile()");

}

} // close if (!bRead)

LeaveCriticalSection(&port->m_csCommunicationSync);

// notify parent that a byte was received

::SendMessage((port->m_pOwner)->m_hWnd, WM_COMM_RXCHAR, (WPARAM)

RXBuff, (LPARAM) port->m_nPortNr);

} // end forever loop

}

(5)写数据

也是由监视线程操作，不过触发事件交给主线程来触发，函数是WriteToPort

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// Write a string to the port

//

void CSerialPort::WriteToPort(char* string)

{

assert(m_hComm != 0);

//写进写缓冲区

memset(m_szWriteBuffer, 0, sizeof(m_szWriteBuffer));

strcpy(m_szWriteBuffer, string);

m_nWriteSize=strlen(string);

// set event for write

SetEvent(m_hWriteEvent);

}

线程调用的函数WriteChar，把缓冲里的数据写到串口中，期间调用WriteFile

详细代码：

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//写数据

void CSerialPort::WriteChar(CSerialPort* port)

{

BOOL bWrite = TRUE;

BOOL bResult = TRUE;

DWORD BytesSent = 0;

ResetEvent(port->m_hWriteEvent); //复位写事件句柄

// Gain ownership of the critical section

EnterCriticalSection(&port->m_csCommunicationSync);

if (bWrite)

{

// Initailize variables

port->m_ = 0;

port->m_High = 0;

// Clear buffer

PurgeComm(port->m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR

PURGE_RXABORT | PURGE_TXABORT);

//串口写入

| bResult = WriteFile(port->m_hComm, // Handle to

COMM Port

port->m_szWriteBuffer, // Pointer to

message buffer in calling finction

// strlen((char*)port->m_szWriteBuffer), // Length of message

to send

port->m_nWriteSize, // Length of message to send

&BytesSent, // Where to

store the number of bytes sent

&port->m_ov);

Overlapped structure

// deal with any error codes

if (!bResult)

{

DWORD dwError = GetLastError();

switch (dwError)

{

case ERROR_IO_PENDING:

{

// continue to GetOverlappedResults()

BytesSent = 0;

bWrite = FALSE;

break;

}

default:

{

// all other error codes

port->ProcessErrorMessage("WriteFile()");

}

}

}

else

{

LeaveCriticalSection(&port->m_csCommunicationSync);

}

} // end if(bWrite)

if (!bWrite)

{

bWrite = TRUE;

bResult = GetOverlappedResult(port->m_hComm, // Handle to COMM port

&port->m_ov, // Overlapped structure

//

&BytesSent, // Stores number of bytes sent

TRUE); // Wait flag

LeaveCriticalSection(&port->m_csCommunicationSync);

// deal with the error code

// if (!bResult)

{

// port->ProcessErrorMessage("GetOverlappedResults() in WriteFile()");

}

} // end if (!bWrite)

//Verify that the data size send equals what we tried to send

if (BytesSent != port->m_nWriteSize) // Length of message to send)

{

TRACE("WARNING: WriteFile() error.. Bytes Sent: %d; Message Length: %dn",

BytesSent, strlen((char*)port->m_szWriteBuffer));

}

// ::SendMessage((port->m_pOwner)->m_hWnd, WM_COMM_TXEMPTY_DETECTED,

(WPARAM) RXBuff, (LPARAM) port->m_nPortNr);

// ::SendMessage((port->m_pOwner)->m_hWnd,

WM_COMM_TXEMPTY_DETECTED,0,(LPARAM) port->m_nPortNr);

}

(6)其他操作

其他比如获取DCB，关闭等，比较简单，不做分析

代码如下：

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//

// Return the device control block

//

DCB CSerialPort::GetDCB()

{

return m_dcb;

}

//

// Return the communication event masks

//

DWORD CSerialPort::GetCommEvents()

{ return m_dwCommEvents;

}

//

// Return the output buffer size

//

DWORD CSerialPort::GetWriteBufferSize()

{

return m_nWriteBufferSize;

}

//修改了多次ClosePort()函数，最终确定以下方式，可以很好执行关闭

void CSerialPort::ClosePort()

{

while (m_bThreadAlive)

{

SetCommMask(m_hComm, 0);

SetEvent(m_hShutdownEvent);

}

if (m_)

{

CloseHandle(m_);

}

if (m_hWriteEvent)

{

CloseHandle(m_hWriteEvent);

}

if (m_hShutdownEvent)

{

CloseHandle(m_hShutdownEvent);

}

m_ = NULL;

m_hWriteEvent = NULL;

m_hShutdownEvent = NULL;

if (m_hComm != NULL)

{

CloseHandle(m_hComm);

m_hComm = NULL;

}

}