c++ - 视窗。在特定逻辑集群创建新文件

Question

相关的微软文档是： https ://msdn.microsoft.com/en-us/library/aa363911.aspx

但它描述了如何将现有的碎片文件移动到新的连续逻辑块。我想创建没有碎片的新文件。起初我通过 FSCTL_GET_VOLUME_BITMAP 获得免费的逻辑块。现在如何在具体的空闲逻辑块中写入文件？

Answer 1

使用ZwCreateFile - AllocationSize 参数。如果它存在文件系统，请尝试查找使用RtlFindClearBits运行的第一个连续空闲集群。这是解决方案

我做小测试

void PrintAllocSize(HANDLE hFile)
{
    IO_STATUS_BLOCK iosb;
    FILE_STANDARD_INFORMATION fsi;
    if (0 <= ZwQueryInformationFile(hFile, &iosb, &fsi, sizeof(fsi), FileStandardInformation))
    {
        DbgPrint("AllocationSize=%I64x, EndOfFile=%I64x\n", fsi.AllocationSize.QuadPart, fsi.EndOfFile.QuadPart);
    }

    STARTING_VCN_INPUT_BUFFER vcn = {};
    RETRIEVAL_POINTERS_BUFFER rpb;

    NTSTATUS status = ZwFsControlFile(hFile, 0, 0, 0, &iosb, FSCTL_GET_RETRIEVAL_POINTERS, &vcn, sizeof(vcn), &rpb, sizeof(rpb));

    switch (status)
    {
    case STATUS_SUCCESS:
    case STATUS_BUFFER_OVERFLOW:
        DbgPrint("ExtentCount=%x\n", rpb.ExtentCount);
        break;
    case STATUS_END_OF_FILE:
        DbgPrint("File Is Empty\n", rpb.ExtentCount);
        break;
    default:
        DbgPrint("ZwFsControlFile return %x\n", status);
    }
}

void DoTest(POBJECT_ATTRIBUTES poa)
{
    HANDLE hFile;
    IO_STATUS_BLOCK iosb;
    LARGE_INTEGER AllocationSize = { 0x100000 };
    if (0 <= ZwCreateFile(&hFile, FILE_APPEND_DATA|SYNCHRONIZE, poa, &iosb, &AllocationSize, 0, 0, FILE_SUPERSEDE, FILE_SYNCHRONOUS_IO_NONALERT, 0, 0))
    {
        PrintAllocSize(hFile);
        ZwClose(hFile);
    }

    DbgPrint("===============================\n");

    if (0 <= ZwOpenFile(&hFile, SYNCHRONIZE, poa, &iosb, 0, FILE_SYNCHRONOUS_IO_NONALERT))
    {
        PrintAllocSize(hFile);
        ZwClose(hFile);
    }

    ZwDeleteFile(poa);
}

和输出

AllocationSize=100000, EndOfFile=0
ExtentCount=1
===============================
AllocationSize=0, EndOfFile=0
File Is Empty

当我们创建不为零的文件时 AllocationSize - 文件系统保留空间，但文件大小仍为 0 - (AllocationSize=100000, EndOfFile=0, ExtentCount=1) 如果我们关闭句柄，而不写入数据 - 文件系统空闲分配的集群 ( AllocationSize=0，EndOfFile=0，文件为空）