when reading a sparse file, the free space of disk decreases.

Situation :
1,creating a empty Sparse file on the given volume (system xp or server 2003) and specify the 10G size of the file.

2,Like reading a normal file ,codes as follow:

void CFileCheckDlg::OnButtonCreate() 
{
        // TODO: Add your control notification handler code here
        UpdateData(TRUE);
        HANDLE hFile = CreateFile(m_strSrcFile,
                GENERIC_READ|GENERIC_WRITE,
                FILE_SHARE_READ|FILE_SHARE_WRITE,
                NULL,
                CREATE_ALWAYS,
                FILE_ATTRIBUTE_NORMAL,
                0);

        if ( INVALID_HANDLE_VALUE  == hFile)
        {
                LOGFILE("CreateFile failure,Error=%d",GetLastError());
                return;
        }

        DWORD dwCtrlCode = 0x900c4; // FSCTL_SET_SPARSE=0x900c4 标记为稀疏文件
        DWORD dwBytesReturned;
        BOOL bRet = ::DeviceIoControl(hFile, dwCtrlCode, NULL, 0, NULL, 0, &dwBytesReturned, NULL);
        if (!bRet)
        {
                LOGFILE("sparse file set error!,dwError=%d",GetLastError());
                return;
        }

        LARGE_INTEGER uli;
        uli.QuadPart = m_dwSparseSize;
        uli.QuadPart = uli.QuadPart<<20;
        SetFilePointer(hFile,uli.LowPart,&uli.HighPart,FILE_BEGIN);
        SetEndOfFile(hFile);
        CloseHandle(hFile);
}

void CFileCheckDlg::ReadSparseFile()
{
HANDLE hFile = CreateFile(m_strFileName,
		GENERIC_READ,
		FILE_SHARE_READ|FILE_SHARE_WRITE,
		NULL,
		OPEN_EXISTING,
		FILE_ATTRIBUTE_NORMAL,	//FILE_ATTRIBUTE_ARCHIVE FILE_ATTRIBUTE_NORMAL  FILE_FLAG_BACKUP_SEMANTICS
		0);
	if ( INVALID_HANDLE_VALUE  == hFile)
	    return FALSE;
	
	ULARGE_INTEGER uli;
	uli.LowPart = GetFileSize(hFile,&uli.HighPart);
	DWORD dwSize = 1<<15;
	DWORD dwCount = (DWORD)((uli.QuadPart/dwSize)+(uli.QuadPart%dwSize?1:0));
	PBYTE pBuf = new BYTE[dwSize];
	for (DWORD i=0; i<dwcount;>	{
		DWORD dwReal=0;
		BOOL bRead = ReadFile(hFile,pBuf,dwSize,&dwReal,NULL);
		if (!bRead)
		{
			CloseHandle(hFile);
			return FALSE;	
		}
		DWORD dwPos = (i+1)*100/dwCount;
		m_cProgress.SetPos(dwPos);
	}
	
	delete []pBuf;
	CloseHandle(hFile); 
}

3,Sometimes, i find the free space of my disk decreases 10G,and even the ReadFile Function return error "disk full" ,
4,So far, i didn`t know why, i am glad to hear your tips. Thanks

Tags: C++