写入系统日志失败:提示.997，重叠I/O操作正在进行中

假设我读入一个文本.写入到系统日志.开始开可以正常运行.但是多写几次就会提示"997，重叠I/O操作正在进行中"
请问一下是什么原因??
BOOL syslog(DWORD dwID,char*str,WORD wType)//参数:事件id;事件附加信息;事件类型
{
HANDLE hd=RegisterEventSource(NULL,"TestDLL");//指定/打开事件源
char* buff[1];
int res;
CFile MyFile;
LPVOID pBuffer;
int nLen =0;
if (MyFile.Open("C:\\log.txt",MyFile.modeRead))
{
//这个log.txt内容比较多.
CString str = "";
nLen= MyFile.GetLength();
pBuffer = new LPVOID[nLen+1];
ZeroMemory(pBuffer ,nLen+1);
MyFile.Read(pBuffer,nLen);
buff[0]=(char*)pBuffer;
MyFile.Close();
}

if(hd)
{
res=ReportEvent(hd,wType,0,dwID,NULL,1,0,(LPCTSTR*)buff,NULL);//写日志
delete pBuffer;
DeregisterEventSource(hd);//关闭事件源
if(res)
return TRUE;
else
{
int ErrId = GetLastError();//获得错误码
CString str;
LPSTR lpBuffer;
FormatMessage ( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_FROM_SYSTEM,NULL,
ErrId, LANG_NEUTRAL,(LPTSTR) & lpBuffer,0 ,NULL );
if(lpBuffer)
{
str.Format("Fail to write Windows event log,\r\n%d:%s",ErrId,lpBuffer);
}
LocalFree (lpBuffer);
AfxMessageBox(str);
}
}
return FALSE;
}