内存映射文件

最新推荐文章于 2024-04-09 06:18:10 发布

转载最新推荐文章于 2024-04-09 06:18:10 发布 · 855 阅读

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本文介绍了一种方法来遍历大型文件并计算其中0字节的数量。通过使用Windows API函数，如GetSystemInfo、CreateFile、CreateFileMapping、GetFileSize及MapViewOfFile等，程序能够有效地处理大型文件。

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__int64 Count0s(void) {
//Views must always start on a multiple
//of the allocation granularity
SYSTEM_INFO sinf;
GetSystemInfo(&sinf);
//Open the data file.
HANDLE hFile = CreateFile(L"C://HugeFile.Big", 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 dwFileSizeHigh;
__int64 qwFileSize = GetFileSize(hFile, &dwFileSizeHigh);
qwFileSize += (((__int64) dwFileSizeHigh) << 32);
//We no longer need access to the file object's handle.
CloseHandle(hFile);
__int64 qwFileOffset = 0, qwNumOf0s = 0;
while (qwFileSize > 0)
{
  // Determine the number of bytes to be mapped in this view
  DWORD dwBytesInBlock = sinf.dwAllocationGranularity;
  if(qwFileSize < sinf.dwAllocationGranularity)
   dwBytesInBlock =(DWORD) qwFileSize;
  PBYTE pbFile = (PBYTE) MapViewOfFile(hFileMapping, FILE_MAP_READ, (DWORD)(qwFileOffset >> 32),
   // Starting byte
   (DWORD)(qwFileOffset & 0xFFFFFFFF),
   // in file
   dwBytesInBlock);
  // # of bytes to map
  // Count the number of Js in this block.
  for(DWORD dwByte = 0; dwByte < dwBytesInBlock; dwByte++)
  {
   if(pbFile[dwByte] == 0)
    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);
}