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I/O操作比较耗时，在硬盘频繁读写时尤其耗时，比如杀毒软件扫描，磁盘碎片整理，索引服务之类的。如果在运行效率有要求的环境下有大量I/O的话就不得不考虑如何让I/O不影响运行。

I/O操作比如说日志，可以输出到DbgView，也可以输出到文件。当有bug时，可以告诉我们问题出在哪儿。输出到DbgView与输出到文件各有利弊。输出到DbgView，好处是不影响程序运行，坏处是需要开DbgView(有时客户不方便开)，要考虑DbgView内存开销(这样的话，无法看到时间靠前的日志)，不能动DbgView(否则会堵塞程序，拖动进度条，保存文件，清除log之类的操作)。输出到文件，好处是要求少，坏处是有大量I/O操作，必须合理的设计保证不影响程序运行。

最先想到也是最容易想到的解决方案是，将Log丢到缓冲，创建一个子线程专门将这些Log输出到文件。我问自己能不能在现有的文件日志基础上做最少的改动以实现目的。

现有的日志函数

static unsigned short  EnableLog = 1;	//允许写日志static unsigned short  EnableDbgView = 0;	//允许写到debug viewTCHAR g_szFilePath[MAX_PATH];int MyIO(LPCSTR fmt, ...){	va_list ap;	char MsgBuf[512] = {0};		static char	szDate[50] = {0};	static char	szTime[50] = {0};	clock_t tStart = clock();	va_start( ap,fmt );	vsprintf( MsgBuf,fmt,ap );	va_end( ap );	if(MsgBuf[strlen(MsgBuf)-1] == '\n')  		MsgBuf[strlen(MsgBuf)-1] = '\0';	if(EnableLog)	{		SYSTEMTIME st;  		char szLog[1024] = {0}		GetLocalTime(&st);              // 取当前系统时间		sprintf_s(szTime, sizeof(szTime), "%02d:%02d:%02d.%03d", \			st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);				sprintf_s(szDate, sizeof(szDate), "%04d%02d%02d", st.wYear, st.wMonth, st.wDay);				sprintf_s();		int nBufLen = strlen(szBufferPerSec);		sprintf(szLog, "%s -> %s\r\n", szTime, MsgBuf);			TCHAR szFileName[MAX_PATH];		sprintf_s(szFileName, sizeof(szFileName), "%sMyIODemo_%s.txt", \			g_szFilePath, szDate);		HANDLE hLogFile = CreateFile(szFileName, 				GENERIC_WRITE,				FILE_SHARE_READ,//|FILE_SHARE_DELETE,				NULL,				OPEN_ALWAYS,//if not exist,creat new //CREATE_ALWAYS,				FILE_ATTRIBUTE_NORMAL,				0);		if(hLogFile  == INVALID_HANDLE_VALUE)		{			TRACE("Error!!!can't open %s to write", szFileName);			return -1;		}		SetFilePointer(hLogFile , NULL, NULL, FILE_END);				BOOL bRet = WriteFile(hLogFile , szLog, strlen(szLog), 			NULL, NULL);		if(!bRet)		{			TRACE( "Error!Could not write to file (error %d)\n", GetLastError());		}		if(clock()-tStart>2)		{							OutputDebugString("Error!log time-consumingly to file");		}		if(hLogFile )		{			CloseHandle(hLogFile );			hLogFile  = NULL;		}	}	if(EnableDbgView)		OutputDebugString(MsgBuf);	return 0;}

Consider a thread that issues an asynchronous I/O request to a device. This I/O request is passed to a device driver,which assumes the responsibility of actually performing the I/O.While the device driver waits for the device to respond, the application's thread is not suspended as it waits for the I/O request to complete. Instead, this thread continues executing and performs other usefull tasks.

但是采用了异步I/O之后，实际测试过程中，CreateFile有时候需要16ms，而WriteFile经常超过2ms，甚至超过100ms。因此我考虑文件句柄放到全局，一开始的时候创建，退出程序前销毁，同时将日志先丢到缓冲，每隔1s再一次性写到文件中。

改进后的日志函数

static unsigned short  EnableLog = 1;		//允许写日志static unsigned short  EnableDbgView = 0;	//允许写到debug viewstatic HANDLE g_hLogFile = NULL;static OVERLAPPED g_laped = {0};TCHAR g_szFilePath[MAX_PATH];int MyIO(LPCSTR fmt, ...){	va_list ap;	SYSTEMTIME _SysTime;	char MsgBuf[512] = {0};		bool bFlush = false;	static clock_t tLastLog = clock()-1;	static clock_t tLast = clock()-1;	clock_t tStart = clock();	char szToday[50] = {0};	static char	szDate[50] = {0};	static char	szTime[50] = {0};	static char szBufferPerSec[1024*1024] = {0};	static int nCurFileSize = 1024*1024;	va_start( ap,fmt );	vsprintf( MsgBuf,fmt,ap );	va_end( ap );	if(MsgBuf[strlen(MsgBuf)-1] == '\n')  		MsgBuf[strlen(MsgBuf)-1] = '\0';	if(EnableLog)	{		SYSTEMTIME st;  		if(tStart>tLast)		{			GetLocalTime(&st);              // 取当前系统时间			sprintf_s(szTime, sizeof(szTime), "%02d:%02d:%02d.%03d", \					st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);			if(tStart-tLastLog>1000)			{				bFlush = true;					tLastLog = tStart;			}					tLast = tStart;		}				sprintf_s(szToday, sizeof(szToday), "%04d%02d%02d", st.wYear, st.wMonth, st.wDay);		if(strcmp(szToday, szDate))		{			if(g_hLogFile!=NULL)			{				CloseHandle(g_hLogFile);				g_hLogFile = NULL;			}			strcpy(szDate, szToday);		}		int nBufLen = strlen(szBufferPerSec);		sprintf(szBufferPerSec+strlen(szBufferPerSec)/*, sizeof(szBufferPerSec)*/, "%s -> %s\r\n", szTime, MsgBuf);			if(bFlush)		{			if(g_hLogFile == NULL)			{				TCHAR szFileName[MAX_PATH];				sprintf_s(szFileName, sizeof(szFileName), "%sMyIODemo_%s.txt", \					g_szFilePath, szDate);				g_hLogFile = CreateFile(szFileName, 						GENERIC_WRITE,						FILE_SHARE_READ,//|FILE_SHARE_DELETE,						NULL,						OPEN_ALWAYS,//if not exist,creat new //CREATE_ALWAYS,						FILE_FLAG_OVERLAPPED/*FILE_ATTRIBUTE_NORMAL*/,						0);				if(g_hLogFile == INVALID_HANDLE_VALUE)				{					TRACE("Error!!!can't open %s to write", szFileName);					return -1;				}				SetFilePointer(g_hLogFile, nCurFileSize, NULL, FILE_BEGIN);				SetEndOfFile(g_hLogFile);			}		                        //!sld! 2011.9.13 快满的时候，文件尺寸翻倍。如果不加这个的话，一旦写入超过文件尺寸，最后文件Log将没有内容，全是NULL。			if((int)(nCurFileSize - g_laped.Offset - strlen(szBufferPerSec))<=0)			{				nCurFileSize = nCurFileSize*2;				SetFilePointer(g_hLogFile, nCurFileSize, NULL, FILE_BEGIN);				SetEndOfFile(g_hLogFile);			}			BOOL bRet = WriteFile(g_hLogFile, szBufferPerSec, strlen(szBufferPerSec), 				NULL, &g_laped);			if(!bRet)			{				DWORD dwErrNo = GetLastError();				if(dwErrNo!=ERROR_IO_PENDING)				{					char szErr[1024];					sprintf_s(szErr, sizeof(szErr), "Error!Could not write to file (error %d)\n", dwErrNo);      					OutputDebugString(szErr);				}			}			if(clock()-tStart>2)			{								OutputDebugString("Error!log time-consumingly to file");			}			g_laped.Offset+=strlen(szBufferPerSec);			memset(szBufferPerSec, 0, sizeof(szBufferPerSec));					}	}	if(EnableDbgView)		OutputDebugString(MsgBuf);	return 0;}

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