本章我們將以工業控制和嵌入式系統中運用極為廣泛的串口通信為例講述多線程的典型應用。
而網絡通信也是多線程應用最廣泛的領域之一，所以本章的最後一節也將對多線程網絡通信進行簡短的描述。
1.串口通信
在工業控制系統中，工控機（一般都基於PC Windows平台）經常需要與單片機通過串口進行通信。因此，操作和使用PC的串口成為大多數單片機、嵌入式系統領域工程師必須具備的能力。
串口的使用需要通過三個步驟來完成的：
（1） 打開通信端口；
（2） 初始化串口，設置波特率、數據位、停止位、奇偶校驗等參數。為了給讀者一個直觀的印象，下圖從Windows的"控制面板－>系統－>設備管理器－>通信端口（COM1）"打開COM的設置窗口：
（3） 讀寫串口。
在WIN32平台下，對通信端口進行操作跟基本的文件操作一樣。
創建/打開COM資源
下列函數如果調用成功，則返回一個標識通信端口的句柄，否則返回-1：
HADLE CreateFile(PCTSTR lpFileName, //通信端口名，如"COM1"
WORD dwDesiredAccess, //對資源的訪問類型
WORD dwShareMode, //指定共享模式，COM不能共享，該參數為0
PSECURITY_ATTRIBUTES lpSecurityAttributes,
//安全描述符指針，可為NULL
WORD dwCreationDisposition, //創建方式
WORD dwFlagsAndAttributes, //文件屬性，可為NULL
HANDLE hTemplateFile //模板文件句柄，置為NULL
);
獲得/設置COM屬性
下列函數可以獲得COM口的設備控制塊，從而獲得相關參數：
BOOL WINAPI GetCommState(
HANDLE hFile, //標識通信端口的句柄
LPDCB lpDCB //指向一個設備控制塊(DCB結構)的指針
);
如果要調整通信端口的參數，則需要重新配置設備控制塊，再用WIN32 API SetCommState()函數進行設置：
BOOL SetCommState(
HANDLE hFile, //標識通信端口的句柄
LPDCB lpDCB //指向一個設備控制塊(DCB結構)的指針
);
DCB結構包含了串口的各項參數設置，如下：
typedef struct _DCB
{
// dcb
DWORD DCBlength; // sizeof(DCB)
DWORD BaudRate; // current baud rate
DWORD fBinary: 1; // binary mode, no EOF check
DWORD fParity: 1; // enable parity checking
DWORD fOutxCtsFlow: 1; // CTS output flow control
DWORD fOutxDsrFlow: 1; // DSR output flow control
DWORD fDtrControl: 2; // DTR flow control type
DWORD fDsrSensitivity: 1; // DSR sensitivity
DWORD fTXContinueOnXoff: 1; // XOFF continues Tx
DWORD fOutX: 1; // XON/XOFF out flow control
DWORD fInX: 1; // XON/XOFF in flow control
DWORD fErrorChar: 1; // enable error replacement
DWORD fNull: 1; // enable null stripping
DWORD fRtsControl: 2; // RTS flow control
DWORD fAbortOnError: 1; // abort reads/writes on error
DWORD fDummy2: 17; // reserved
WORD wReserved; // not currently used
WORD XonLim; // transmit XON threshold
WORD XoffLim; // transmit XOFF threshold
BYTE ByteSize; // number of bits/byte, 4-8
BYTE Parity; // 0-4=no,odd,even,mark,space
BYTE StopBits; // 0,1,2 = 1, 1.5, 2
char XonChar; // Tx and Rx XON character
char XoffChar; // Tx and Rx XOFF character
char ErrorChar; // error replacement character
char EofChar; // end of input character
char EvtChar; // received event character
WORD wReserved1; // reserved; do not use
} DCB;
讀寫串口
在讀寫串口之前，還要用PurgeComm()函數清空緩沖區，並用SetCommMask ()函數設置事件掩模來監視指定通信端口上的事件，其原型為：
BOOL SetCommMask(
HANDLE hFile, //標識通信端口的句柄
DWORD dwEvtMask //能夠使能的通信事件
);
串口上可能發生的事件如下表所示：
值 事件描述 EV_BREAK A break was detected on input. EV_CTS The CTS (clear-to-send) signal changed state. EV_DSR The DSR(data-set-ready) signal changed state. EV_ERR A line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY. EV_RING A ring indicator was detected. EV_RLSD The RLSD (receive-line-signal-detect) signal changed state. EV_RXCHAR A character was received and placed in the input buffer. EV_RXFLAG The event character was received and placed in the input buffer. The event character is specified in the device's DCB structure, which is applied to a serial port by using the SetCommState function. EV_TXEMPTY The last character in the output buffer was sent.
在設置好事件掩模後，我們就可以利用WaitCommEvent()函數來等待串口上發生事件，其函數原型為：
BOOL WaitCommEvent(
HANDLE hFile, //標識通信端口的句柄
LPDWORD lpEvtMask, //指向存放事件標識變量的指針
LPOVERLAPPED lpOverlapped, // 指向overlapped結構
);
我們可以在發生事件後，根據相應的事件類型，進行串口的讀寫操作：
BOOL ReadFile(HANDLE hFile, //標識通信端口的句柄
LPVOID lpBuffer, //輸入數據Buffer指針
DWORD nNumberOfBytesToRead, // 需要讀取的字節數
LPDWORD lpNumberOfBytesRead, //實際讀取的字節數指針
LPOVERLAPPED lpOverlapped //指向overlapped結構
);
BOOL WriteFile(HANDLE hFile, //標識通信端口的句柄
LPCVOID lpBuffer, //輸出數據Buffer指針
DWORD nNumberOfBytesToWrite, //需要寫的字節數
LPDWORD lpNumberOfBytesWritten, //實際寫入的字節數指針
LPOVERLAPPED lpOverlapped //指向overlapped結構
);
2.工程實例
下面我們用第1節所述API實現一個多線程的串口通信程序。這個例子工程（工程名為MultiThreadCom）的界面很簡單，如下圖所示：
它是一個多線程的應用程序，包括兩個工作者線程，分別處理串口1和串口2。為了簡化問題，我們讓連接兩個串口的電纜只包含RX、TX兩根連線（即不以硬件控制RS-232，串口上只會發生EV_TXEMPTY、EV_RXCHAR事件）。
在工程實例的BOOL CMultiThreadComApp::InitInstance()函數中，啟動並設置COM1和COM2，其源代碼為：
BOOL CMultiThreadComApp::InitInstance()
{
AfxEnableControlContainer();
//打開並設置COM1
hComm1=CreateFile("COM1", GENERIC_READ|GENERIC_WRITE, 0, NULL ,OPEN_EXISTING, 0,NULL);
if (hComm1==(HANDLE)-1)
{
AfxMessageBox("打開COM1失敗");
return false;
}
else
{
DCB wdcb;
GetCommState (hComm1,&wdcb);
wdcb.BaudRate=9600;
SetCommState (hComm1,&wdcb);
PurgeComm(hComm1,PURGE_TXCLEAR);
}
//打開並設置COM2
hComm2=CreateFile("COM2", GENERIC_READ|GENERIC_WRITE, 0, NULL ,OPEN_EXISTING, 0,NULL);
if (hComm2==(HANDLE)-1)
{
AfxMessageBox("打開COM2失敗");
return false;
}
else
{
DCB wdcb;
GetCommState (hComm2,&wdcb);
wdcb.BaudRate=9600;
SetCommState (hComm2,&wdcb);
PurgeComm(hComm2,PURGE_TXCLEAR);
}
CMultiThreadComDlg dlg;
m_pMainWnd = &dlg;
int nResponse = dlg.DoModal();
if (nResponse == IDOK)
{
// TODO: Place code here to handle when the dialog is
// dismissed with OK
}
else if (nResponse == IDCANCEL)
{
// TODO: Place code here to handle when the dialog is
// dismissed with Cancel
}
return FALSE;
}
此後我們在對話框CMultiThreadComDlg的初始化函數OnInitDialog中啟動兩個分別處理COM1和COM2的線程：
BOOL CMultiThreadComDlg::OnInitDialog()
{
CDialog::OnInitDialog();
// Add "About..." menu item to system menu.
// IDM_ABOUTBOX must be in the system command range.
ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
ASSERT(IDM_ABOUTBOX < 0xF000);
CMenu* pSysMenu = GetSystemMenu(FALSE);
if (pSysMenu != NULL)
{
CString strAboutMenu;
strAboutMenu.LoadString(IDS_ABOUTBOX);
if (!strAboutMenu.IsEmpty())
{
pSysMenu->AppendMenu(MF_SEPARATOR);
pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
}
}
// Set the icon for this dialog. The framework does this automatically
// when the application's main window is not a dialog
SetIcon(m_hIcon, TRUE); // Set big icon
SetIcon(m_hIcon, FALSE); // Set small icon
// TODO: Add extra initialization here
//啟動串口1處理線程
DWORD nThreadId1;
hCommThread1 = ::CreateThread((LPSECURITY_ATTRIBUTES)NULL, 0,
(LPTHREAD_START_ROUTINE)Com1ThreadProcess, AfxGetMainWnd()->m_hWnd, 0, &nThreadId1);
if (hCommThread1 == NULL)
{
AfxMessageBox("創建串口1處理線程失敗");
return false;
}
//啟動串口2處理線程
DWORD nThreadId2;
hCommThread2 = ::CreateThread((LPSECURITY_ATTRIBUTES)NULL, 0,
(LPTHREAD_START_ROUTINE)Com2ThreadProcess, AfxGetMainWnd()->m_hWnd, 0, &nThreadId2);
if (hCommThread2 == NULL)
{
AfxMessageBox("創建串口2處理線程失敗");
return false;
}
return TRUE; // return TRUE unless you set the focus to a control
}
兩個串口COM1和COM2對應的線程處理函數等待串口上發生事件，並根據事件類型和自身緩沖區是否有數據要發送進行相應的處理，其源代碼為：
DWORD WINAPI Com1ThreadProcess(HWND hWnd//主窗口句柄)
{
DWORD wEven;
char str[10]; //讀入數據
SetCommMask(hComm1, EV_RXCHAR | EV_TXEMPTY);
while (TRUE)
{
WaitCommEvent(hComm1, &wEven, NULL);
if(wEven = 0)
{
CloseHandle(hCommThread1);
hCommThread1 = NULL;
ExitThread(0);
}
else
{
switch (wEven)
{
case EV_TXEMPTY:
if (wTxPos < wTxLen)
{
//在串口1寫入數據
DWORD wCount; //寫入的字節數
WriteFile(hComm1, com1Data.TxBuf[wTxPos], 1, &wCount, NULL);
com1Data.wTxPos++;
}
break;
case EV_RXCHAR:
if (com1Data.wRxPos < com1Data.wRxLen)
{
//讀取串口數據, 處理收到的數據
DWORD wCount; //讀取的字節數
ReadFile(hComm1, com1Data.RxBuf[wRxPos], 1, &wCount, NULL);
com1Data.wRxPos++;
if(com1Data.wRxPos== com1Data.wRxLen);
::PostMessage(hWnd, COM_SENDCHAR, 0, 1);
}
break;
}
}
}
}
return TRUE;
}
DWORD WINAPI Com2ThreadProcess(HWND hWnd //主窗口句柄)
{
DWORD wEven;
char str[10]; //讀入數據
SetCommMask(hComm2, EV_RXCHAR | EV_TXEMPTY);
while (TRUE)
{
WaitCommEvent(hComm2, &wEven, NULL);
if (wEven = 0)
{
CloseHandle(hCommThread2);
hCommThread2 = NULL;
ExitThread(0);
}
else
{
switch (wEven)
{
case EV_TXEMPTY:
if (wTxPos < wTxLen)
{
//在串口2寫入數據
DWORD wCount; //寫入的字節數
WriteFile(hComm2, com2Data.TxBuf[wTxPos], 1, &wCount, NULL);
com2Data.wTxPos++;
}
break;
case EV_RXCHAR:
if (com2Data.wRxPos < com2Data.wRxLen)
{
//讀取串口數據, 處理收到的數據
DWORD wCount; //讀取的字節數
ReadFile(hComm2, com2Data.RxBuf[wRxPos], 1, &wCount, NULL);
com2Data.wRxPos++;
if(com2Data.wRxPos== com2Data.wRxLen);
::PostMessage(hWnd, COM_SENDCHAR, 0, 1);
}
break;
}
}
}
return TRUE;
}
線程控制函數中所操作的com1Data和com2Data是與串口對應的數據結構struct tagSerialPort的實例，這個數據結構是：
typedef struct tagSerialPort
{
BYTE RxBuf[SPRX_BUFLEN];//接收Buffer
WORD wRxPos; //當前接收字節位置
WORD wRxLen; //要接收的字節數
BYTE TxBuf[SPTX_BUFLEN];//發送Buffer
WORD wTxPos; //當前發送字節位置
WORD wTxLen; //要發送的字節數
}SerialPort, * LPSerialPort;
3.多線程串口類
使用多線程串口通信更方便的途徑是編寫一個多線程的串口類，例如Remon Spekreijse編寫了一個CSerialPort串口類。仔細分析這個類的源代碼，將十分有助於我們對先前所學多線程及同步知識的理解。
3.1類的定義
#ifndef __SERIALPORT_H__
#define __SERIALPORT_H__
#define WM_COMM_BREAK_DETECTED WM_USER+1 // A break was detected on input.
#define WM_COMM_CTS_DETECTED WM_USER+2 // The CTS (clear-to-send) signal changed state.
#define WM_COMM_DSR_DETECTED WM_USER+3 // The DSR (data-set-ready) signal changed state.
#define WM_COMM_ERR_DETECTED WM_USER+4 // A line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY.
#define WM_COMM_RING_DETECTED WM_USER+5 // A ring indicator was detected.
#define WM_COMM_RLSD_DETECTED WM_USER+6 // The RLSD (receive-line-signal-detect) signal changed state.
#define WM_COMM_RXCHAR WM_USER+7 // A character was received and placed in the input buffer.
#define WM_COMM_RXFLAG_DETECTED WM_USER+8 // The event character was received and placed in the input buffer.
#define WM_COMM_TXEMPTY_DETECTED WM_USER+9 // The last character in the output buffer was sent.
class CSerialPort
{
public:
// contruction and destruction
CSerialPort();
virtual ~CSerialPort();
// port initialisation
BOOL InitPort(CWnd* pPortOwner, UINT portnr = 1, UINT baud = 19200, char parity = 'N', UINT databits = 8, UINT stopsbits = 1, DWORD dwCommEvents = EV_RXCHAR | EV_CTS, UINT nBufferSize = 512);
// start/stop comm watching
BOOL StartMonitoring();
BOOL RestartMonitoring();
BOOL StopMonitoring();
DWORD GetWriteBufferSize();
DWORD GetCommEvents();
DCB GetDCB();
void WriteToPort(char* string);
protected:
// protected memberfunctions
void ProcessErrorMessage(char* ErrorText);
static UINT CommThread(LPVOID pParam);
static void ReceiveChar(CSerialPort* port, COMSTAT comstat);
static void WriteChar(CSerialPort* port);
// thread
CWinThread* m_Thread;
// synchronisation objects
CRITICAL_SECTION m_csCommunicationSync;
BOOL m_bThreadAlive;
// handles
HANDLE m_hShutdownEvent;
HANDLE m_hComm;
HANDLE m_hWriteEvent;
// Event array.
// One element is used for each event. There are two event handles for each port.
// A Write event and a receive character event which is located in the overlapped structure (m_ov.hEvent).
// There is a general shutdown when the port is closed.
HANDLE m_hEventArray[3];
// structures
OVERLAPPED m_ov;
COMMTIMEOUTS m_CommTimeouts;
DCB m_dcb;
// owner window
CWnd* m_pOwner;
// misc
UINT m_nPortNr;
char* m_szWriteBuffer;
DWORD m_dwCommEvents;
DWORD m_nWriteBufferSize;
};
#endif __SERIALPORT_H__
3.2類的實現
3.2.1構造函數與析構函數
進行相關變量的賦初值及內存恢復：
CSerialPort::CSerialPort()
{
m_hComm = NULL;
// initialize overlapped structure members to zero
m_ov.Offset = 0;
m_ov.OffsetHigh = 0;
// create events
m_ov.hEvent = NULL;
m_hWriteEvent = NULL;
m_hShutdownEvent = NULL;
m_szWriteBuffer = NULL;
m_bThreadAlive = FALSE;
}
//
// Delete dynamic memory
//
CSerialPort::~CSerialPort()
{
do
{
SetEvent(m_hShutdownEvent);
}
while (m_bThreadAlive);
TRACE("Thread ended\n");
delete []m_szWriteBuffer;
}
3.2.2核心函數：初始化串口
在初始化串口函數中，將打開串口，設置相關參數，並創建串口相關的用戶控制事件，初始化臨界區（Critical Section），以成隊的EnterCriticalSection()、LeaveCriticalSection()函數進行資源的排它性訪問：
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
if (m_bThreadAlive)
{
do
{
SetEvent(m_hShutdownEvent);
}
while (m_bThreadAlive);
TRACE("Thread ended\n");
}
// create events
if (m_ov.hEvent != NULL)
ResetEvent(m_ov.hEvent);
m_ov.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (m_hWriteEvent != NULL)
ResetEvent(m_hWriteEvent);
m_hWriteEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (m_hShutdownEvent != NULL)
ResetEvent(m_hShutdownEvent);
m_hShutdownEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
// initialize the event objects
m_hEventArray[0] = m_hShutdownEvent; // highest priority
m_hEventArray[1] = m_ov.hEvent;
m_hEventArray[2] = m_hWriteEvent;
// initialize critical section
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!
EnterCriticalSection(&m_csCommunicationSync);
// if the port is already opened: close it
if (m_hComm != NULL)
{
CloseHandle(m_hComm);
m_hComm = NULL;
}
// prepare port strings
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
m_hComm = CreateFile(szPort, // communication port string (COMX)
GENERIC_READ | GENERIC_WRITE, // read/write types
0, // comm devices must be opened with exclusive access
NULL, // no security attributes
OPEN_EXISTING, // comm devices must use OPEN_EXISTING
FILE_FLAG_OVERLAPPED, // Async I/O
0); // template must be 0 for comm devices
if (m_hComm == INVALID_HANDLE_VALUE)
{
// port not found
delete []szPort;
delete []szBaud;
return FALSE;
}
// set the timeout values
m_CommTimeouts.ReadIntervalTimeout = 1000;
m_CommTimeouts.ReadTotalTimeoutMultiplier = 1000;
m_CommTimeouts.ReadTotalTimeoutConstant = 1000;
m_CommTimeouts.WriteTotalTimeoutMultiplier = 1000;
m_CommTimeouts.WriteTotalTimeoutConstant = 1000;
// configure
if (SetCommTimeouts(m_hComm, &m_CommTimeouts))
{
if (SetCommMask(m_hComm, dwCommEvents))
{
if (GetCommState(m_hComm, &m_dcb))
{
m_dcb.fRtsControl = RTS_CONTROL_ENABLE; // set RTS bit high!
if (BuildCommDCB(szBaud, &m_dcb))
{
if (SetCommState(m_hComm, &m_dcb))
;
// normal operation... continue
else
ProcessErrorMessage("SetCommState()");
}
else
ProcessErrorMessage("BuildCommDCB()");
}
else
ProcessErrorMessage("GetCommState()");
}
else
ProcessErrorMessage("SetCommMask()");
}
else
ProcessErrorMessage("SetCommTimeouts()");
delete []szPort;
delete []szBaud;
// flush the port
PurgeComm(m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT);
// release critical section
LeaveCriticalSection(&m_csCommunicationSync);
TRACE("Initialisation for communicationport %d completed.\nUse Startmonitor to communicate.\n", portnr);
return TRUE;
}
3.3.3核心函數：串口線程控制函數
串口線程處理函數是整個類中最核心的部分，它主要完成兩類工作：
（1）利用WaitCommEvent函數對串口上發生的事件進行獲取並根據事件的不同類型進行相應的處理；
（2）利用WaitForMultipleObjects函數對串口相關的用戶控制事件進行等待並做相應處理。
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
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.
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_OverlappedStruct.hEvent, 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!
bResult = WaitCommEvent(port->m_hComm, &Event, &port->m_ov);
if (!bResult)
{
// If WaitCommEvent() returns FALSE, process the last error to determin
// the reason..
switch (dwError = GetLastError())
{
case ERROR_IO_PENDING:
{
// This is a normal return value if there are no bytes
// to read at the port.
// Do nothing and continue
break;
}
case 87:
{
// 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:
{
// All other error codes indicate a serious error has
// occured. Process this error.
port->ProcessErrorMessage("WaitCommEvent()");
break;
}
}
}
else
{
// 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_OverlappedStruct.hEvent 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_OverlappedStruct.hEvent 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_OverlappedStruct.hEvent 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 (comstat.cbInQue == 0)
continue;
} // end if bResult
// Main wait function. This function will normally block the thread
// until one of nine events occur that require action.
Event = WaitForMultipleObjects(3, port->m_hEventArray, FALSE, INFINITE);
switch (Event)
{
case 0:
{
// Shutdown event. This is event zero so it will be
// the higest priority and be serviced first.
port->m_bThreadAlive = FALSE;
// Kill this thread. break is not needed, but makes me feel better.
AfxEndThread(100);
break;
}
case 1:
// read event
{
GetCommMask(port->m_hComm, &CommEvent);
if (CommEvent &EV_CTS)
::SendMessage(port->m_pOwner->m_hWnd, WM_COMM_CTS_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);
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_RXCHAR)
// Receive character event from port.
ReceiveChar(port, comstat);
break;
}
case 2:
// write event
{
// Write character event from port
WriteChar(port);
break;
}
} // end switch
} // close forever loop
return 0;
}
下列三個函數用於對串口線程進行啟動、掛起和恢復：
//
// start comm watching
//
BOOL CSerialPort::StartMonitoring()
{
if (!(m_Thread = AfxBeginThread(CommThread, this)))
return FALSE;
TRACE("Thread started\n");
return TRUE;
}
//
// Restart the comm thread
//
BOOL CSerialPort::RestartMonitoring()
{
TRACE("Thread resumed\n");
m_Thread->ResumeThread();
return TRUE;
}
//
// Suspend the comm thread
//
BOOL CSerialPort::StopMonitoring()
{
TRACE("Thread suspended\n");
m_Thread->SuspendThread();
return TRUE;
}
3.3.4讀寫串口
下面一組函數是用戶對串口進行讀寫操作的接口：
//
// Write a character.
//
void CSerialPort::WriteChar(CSerialPort *port)
{
BOOL bWrite = TRUE;
BOOL bResult = TRUE;
DWORD BytesSent = 0;
ResetEvent(port->m_hWriteEvent);
// Gain ownership of the critical section
EnterCriticalSection(&port->m_csCommunicationSync);
if (bWrite)
{
// Initailize variables
port->m_ov.Offset = 0;
port->m_ov.OffsetHigh = 0;
// Clear buffer
PurgeComm(port->m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT);
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
&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 != strlen((char*)port->m_szWriteBuffer))
{
TRACE("WARNING: WriteFile() error.. Bytes Sent: %d; Message Length: %d\n",
BytesSent, strlen((char*)port->m_szWriteBuffer));
}
}
//
// Character received. Inform the owner
//
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.
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.
if (comstat.cbInQue == 0)
{
// break out when all bytes have been read
break;
}
EnterCriticalSection(&port->m_csCommunicationSync);
if (bRead)
{
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
if (!bResult)
{
switch (dwError = GetLastError())
{
case ERROR_IO_PENDING:
{
// asynchronous i/o is still in progress
// Proceed on to GetOverlappedResults();
bRead = FALSE;
break;
}
default:
{
// Another error has occured. Process this error.
port->ProcessErrorMessage("ReadFile()");
break;
}
}
}
else
{
// ReadFile() returned complete. It is not necessary to call GetOverlappedResults()
bRead = TRUE;
}
} // close if (bRead)
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
}
//
// Write a string to the port
//
void CSerialPort::WriteToPort(char *string)
{
assert(m_hComm != 0);
memset(m_szWriteBuffer, 0, sizeof(m_szWriteBuffer));
strcpy(m_szWriteBuffer, string);
// set event for write
SetEvent(m_hWriteEvent);
}
//
// Return the output buffer size
//
DWORD CSerialPort::GetWriteBufferSize()
{
return m_nWriteBufferSize;
}
3.3.5控制接口
應用程序員使用下列一組public函數可以獲取串口的DCB及串口上發生的事件：
//
// Return the device control block
//
DCB CSerialPort::GetDCB()
{
return m_dcb;
}
//
// Return the communication event masks
//
DWORD CSerialPort::GetCommEvents()
{
return m_dwCommEvents;
}
3.3.6錯誤處理
//
// If there is a error, give the right message
//
void CSerialPort::ProcessErrorMessage(char *ErrorText)
{
char *Temp = new char[200];
LPVOID lpMsgBuf;
FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
NULL, GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
// Default language
(LPTSTR) &lpMsgBuf, 0, NULL);
sprintf(Temp,
"WARNING: %s Failed with the following error:\n%s\nPort: %d\n", (char*)
ErrorText, lpMsgBuf, m_nPortNr);
MessageBox(NULL, Temp, "Application Error", MB_ICONSTOP);
LocalFree(lpMsgBuf);
delete []Temp;
}
仔細分析Remon Spekreijse的CSerialPort類對我們理解多線程及其同步機制是大有益處的，從http://codeguru.earthweb.com/network/serialport.shtml我們可以獲取CSerialPort類的介紹與工程實例。另外，電子工業出版社《Visual C++/Turbo C串口通信編程實踐》一書的作者龔建偉也編寫了一個使用CSerialPort類的例子，可以從http://www.gjwtech.com/scomm/sc2serialportclass.htm獲得詳情。
4.多線程網絡通信
在網絡通信中使用多線程主要有兩種途徑，即主監控線程和線程池。
4.1主監控線程
這種方式指的是程序中使用一個主線程監控某特定端口，一旦在這個端口上發生連接請求，則主監控線程動態使用CreateThread派生出新的子線程處理該請求。主線程在派生子線程後不再對子線程加以控制和調度，而由子線程獨自和客戶方發生連接並處理異常。
使用這種方法的優點是：
（1）可以較快地實現原型設計，尤其在用戶數目較少、連接保持時間較長時有表現較好；
（2）主線程不與子線程發生通信，在一定程度上減少了系統資源的消耗。
其缺點是：
（1）生成和終止子線程的開銷比較大；
（2）對遠端用戶的控制較弱。
這種多線程方式總的特點是"動態生成，靜態調度"。
4.2線程池
這種方式指的是主線程在初始化時靜態地生成一定數量的懸掛子線程，放置於線程池中。隨後，主線程將對這些懸掛子線程進行動態調度。一旦客戶發出連接請求，主線程將從線程池中查找一個懸掛的子線程：
（1）如果找到，主線程將該連接分配給這個被發現的子線程。子線程從主線程處接管該連接，並與用戶通信。當連接結束時，該子線程將自動懸掛，並進人線程池等待再次被調度；
（2）如果當前已沒有可用的子線程，主線程將通告發起連接的客戶。
使用這種方法進行設計的優點是：
（1）主線程可以更好地對派生的子線程進行控制和調度；
（2）對遠程用戶的監控和管理能力較強。
雖然主線程對子線程的調度要消耗一定的資源，但是與主監控線程方式中派生和終止線程所要耗費的資源相比，要少很多。因此，使用該種方法設計和實現的系統在客戶端連接和終止變更頻繁時有上佳表現。
這種多線程方式總的特點是"靜態生成，動態調度"。
