当一个文件1G以上的这种，使用内存文件映射会提高读写效率；
下边时段出自《windows核心编程》，读取一个大文件，然后统计里边字符出现次数的函数：
__int64 CountOs(void) {// Get system granularity SYSTEM_INFO sinf;GetSystemInfo(&sinf);// open the data fileHANDLE hFile = CreateFile(TEXT("C:\\1.TXT"), GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_SEQUENTIAL_SCAN, NULL);// create the file-mapping object.HANDLE hFileMapping = CreateFileMapping(hFile, NULL, PAGE_READONLY, 0, 0, NULL);DWORD dwFileSizeHight;__int64 qwFileSize = GetFileSize(hFile, &dwFileSizeHight);qwFileSize += (((__int64)dwFileSizeHight) << 32);// we no longer need access to the file object's handle. CloseHandle(hFile);__int64 qwFileOffset = 0, qwNumOf0s = 0;while (qwFileSize > 0){DWORD dwBytesInBlock = sinf.dwAllocationGranularity;if (qwFileSize < sinf.dwAllocationGranularity)dwBytesInBlock = (DWORD)qwFileSize;PBYTE pbFile = (PBYTE)MapViewOfFile(hFileMapping, FILE_MAP_READ, (DWORD)(qwFileOffset >> 32), (DWORD)(qwFileOffset & 0xFFFFFFFF), dwBytesInBlock);// count the number of 0s in this block.for (DWORD dwByte = 0; dwByte < dwBytesInBlock; dwByte++){if (pbFile[dwByte] == 'r')qwNumOf0s++;}// unmap the view; we don't want multiple views// in our address space. UnmapViewOfFile(pbFile);// skip to the next set of bytes in the file.qwFileOffset += dwBytesInBlock;qwFileSize -= dwBytesInBlock;}CloseHandle(hFileMapping);return qwNumOf0s; }
如果是往里边写数据就用 memcpy把数据考入pbFile指向的内存。还有就是把读标志该成写标志；
