c# - 处理奇数场偏移

Question

我正在尝试将以下 C++ 结构移植到 C#。

#pragma pack(push,1)
struct FatBootSectorStruct {
    UCHAR  BS_jmpBoot[3];          // 0
    UCHAR  BS_OEMName[8];          // 3
    USHORT BPB_BytsPerSec;         // 11
    UCHAR  BPB_SecPerClus;         // 13
    USHORT BPB_RsvdSecCnt;         // 14
    UCHAR  BPB_NumFATs;            // 16
    USHORT BPB_RootEntCnt;         // 17
    USHORT BPB_TotSec16;           // 19
    UCHAR  BPB_Media;              // 21
    USHORT BPB_FATSz16;            // 22
    USHORT BPB_SecPerTrk;          // 24
    USHORT BPB_NumHeads;           // 26
    ULONG  BPB_HiddSec;            // 28
    ULONG  BPB_TotSec32;           // 32
    union {
        struct {
            UCHAR  BS_DrvNum;          // 36
            UCHAR  BS_Reserved1;       // 37
            UCHAR  BS_BootSig;         // 38
            ULONG  BS_VolID;           // 39
            UCHAR  BS_VolLab[11];      // 43
            UCHAR  BS_FilSysType[8];   // 54
            UCHAR  BS_Reserved2[448];  // 62
        } Fat16;
        struct {
            ULONG  BPB_FATSz32;        // 36
            USHORT BPB_ExtFlags;       // 40
            USHORT BPB_FSVer;          // 42
            ULONG  BPB_RootClus;       // 44
            USHORT BPB_FSInfo;         // 48
            USHORT BPB_BkBootSec;      // 50
            UCHAR  BPB_Reserved[12];   // 52
            UCHAR  BS_DrvNum;          // 64
            UCHAR  BS_Reserved1;       // 65
            UCHAR  BS_BootSig;         // 66
            ULONG  BS_VolID;           // 67
            UCHAR  BS_VolLab[11];      // 71
            UCHAR  BS_FilSysType[8];   // 82
            UCHAR  BPB_Reserved2[420]; // 90
        } Fat32;
    };
    USHORT Signature;              // 510
};

这是我的:

[StructLayout(LayoutKind.Explicit, Size = 512, Pack=1)]
internal struct FATBootSector
{
    [FieldOffset(0)]
    public byte[] BS_jmpBoot; // 0
    [FieldOffset(3)]
    public byte[] BS_OEMName; // 3
    [FieldOffset(11)]
    public ushort BPB_BytsPerSec; // 11
    [FieldOffset(13)]
    public byte BPB_SecPerClus; // 13
    [FieldOffset(14)]
    public ushort BPB_RsvdSecCnt; // 14
    [FieldOffset(16)]
    public byte BPB_NumFATs; // 16
    [FieldOffset(17)]
    public ushort BPB_RootEntCnt; // 17
    [FieldOffset(19)]
    public ushort BPB_TotSec16; // 19
    [FieldOffset(21)]
    public byte BPB_Media; // 21
    [FieldOffset(22)]
    public ushort BPB_FATSz16; // 22
    [FieldOffset(24)]
    public ushort BPB_SecPerTrk; // 24
    [FieldOffset(26)]
    public ushort BPB_NumHeads; // 26
    [FieldOffset(28)]
    public ulong BPB_HiddSec; // 28
    [FieldOffset(32)]
    public ulong BPB_TotSec32; // 32

    // FAT16
    [FieldOffset(36)]
    public byte FAT16_BS_DrvNum; // 36
    [FieldOffset(37)]
    public byte FAT16_BS_Reserved1; // 37
    [FieldOffset(38)]
    public byte FAT16_BS_BootSig; // 38
    [FieldOffset(39)]
    public ulong FAT16_BS_VolID; // 39
    [FieldOffset(43)]
    public byte[] FAT16_BS_VolLab; // 43
    [FieldOffset(54)]
    public byte[] FAT16_BS_FilSysType; // 54
    [FieldOffset(62)]
    public byte[] FAT16_BS_Reserved2; // 62

    // FAT32
    [FieldOffset(36)]
    public ulong FAT32_BPB_FATSz32; // 36
    [FieldOffset(40)]
    public ushort FAT32_BPB_ExtFlags; // 40
    [FieldOffset(42)]
    public ushort FAT32_BPB_FSVer; // 42
    [FieldOffset(44)]
    public ulong FAT32_BPB_RootClus; // 44
    [FieldOffset(48)]
    public ushort FAT32_BPB_FSInfo; // 48
    [FieldOffset(50)]
    public ushort FAT32_BPB_BkBootSec; // 50
    [FieldOffset(52)]
    public byte[] FAT32_BPB_Reserved; // 52
    [FieldOffset(64)]
    public byte FAT32_BS_DrvNum; // 64
    [FieldOffset(65)]
    public byte FAT32_BS_Reserved1; // 65
    [FieldOffset(66)]
    public byte FAT32_BS_BootSig; // 66
    [FieldOffset(67)]
    public byte FAT32_BS_VolID; // 67
    [FieldOffset(71)]
    public byte[] BS_VolLab; // 71
    [FieldOffset(82)]
    public byte[] FAT32_BS_FilSysType; // 82
    [FieldOffset(90)]
    public byte[] FAT32_BPB_Reserved2; // 90

    [FieldOffset(510)]
    public ushort Signature;

}

问题出在字段偏移量 0 和 3 上。当我尝试在 C# 中加载结构时，我得到 Could not load type 'FATBootSector' from assembly 'xxxx' because it contains an object field at offset 3 that that未正确对齐或被非对象字段重叠。。我遇到了this previous question on SO但这似乎没有帮助。

我也尝试过将其更改为如下所示的顺序结构:

[StructLayout(LayoutKind.Sequential, Size = 512, Pack=1, CharSet=CharSet.Ansi)]
internal struct FATBootSector
{

    [MarshalAs(UnmanagedType.U1)]
    public byte BS_jmpBoot0; // 0
    [MarshalAs(UnmanagedType.U1)]
    public byte BS_jmpBoot1; // 0
    [MarshalAs(UnmanagedType.U1)]
    public byte BS_jmpBoot2; // 0

    [MarshalAs(UnmanagedType.LPStr, SizeConst=8)]
    public string BS_OEMName; // 3
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_BytsPerSec; // 11
    [MarshalAs(UnmanagedType.U1)]
    public byte BPB_SecPerClus; // 13
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_RsvdSecCnt; // 14
    [MarshalAs(UnmanagedType.U1)]
    public byte BPB_NumFATs; // 16
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_RootEntCnt; // 17
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_TotSec16; // 19
    [MarshalAs(UnmanagedType.U1)]
    public byte BPB_Media; // 21
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_FATSz16; // 22
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_SecPerTrk; // 24
    [MarshalAs(UnmanagedType.U2)]
    public ushort BPB_NumHeads; // 26
    [MarshalAs(UnmanagedType.U4)]
    public ulong BPB_HiddSec; // 28
    [MarshalAs(UnmanagedType.U4)]
    public ulong BPB_TotSec32; // 32

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 474)]
    public byte[] FAT1632Info;

    [MarshalAs(UnmanagedType.U2)]
    public ushort Signature;

}

我正在使用以下代码来获取结构:

[DllImport("kernel32.dll", SetLastError = true)]
internal static extern bool ReadFile(IntPtr hFile, [Out] IntPtr lpBuffer, uint nNumberOfBytesToRead, out uint lpNumberOfBytesRead, [In] ref System.Threading.NativeOverlapped lpOverlapped);

public bool GetPartitionDetails()
{
        uint BytesRead;

        IntPtr BootSectorPtr = Marshal.AllocHGlobal(512);
        PInvoke.FATBootSector BootSector;

        System.Threading.NativeOverlapped Overlapped = new System.Threading.NativeOverlapped();

        bool ret = PInvoke.ReadFile(this.Handle, BootSectorPtr, (uint)512, out BytesRead, ref Overlapped);

        BootSector = (PInvoke.FATBootSector)Marshal.PtrToStructure(BootSectorPtr, typeof(PInvoke.FATBootSector)); // causes access violation

        return true;
}

我希望我不必这样做，但我能想到的唯一替代方法是使用 IntPtr 遍历内存。有什么想法吗？

Answer 1

这里有不少问题。首先，您在滥用 FieldOffset。当编译器无法为您布局结构时，您可以使用它。总是那是当你有 union 的时候。不要在这里使用 FieldOffset。当然不要预先指定尺寸。同样，让编译器来做。对照原生版本检查它。

然后你不正确地处理数组。您不需要 fixed，但您需要说明数组的长度。该结构的开头应如下所示:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
internal struct FATBootSector
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
    public byte[] BS_jmpBoot;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
    public byte[] BS_OEMName;
    ....
}

其余的问题都是由错误的类型转换引起的。 ULONG 在 C++ 中是一个 4 字节的无符号类型。这是 C# 中的 uint。您使用的是 8 字节宽的 ulong。

这是至少可以避免访问冲突的结构版本:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
internal struct FATBootSector
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
    public byte[] BS_jmpBoot; 
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
    public byte[] BS_OEMName; 
    public ushort BPB_BytsPerSec;
    public byte BPB_SecPerClus; 
    public ushort BPB_RsvdSecCnt;
    public byte BPB_NumFATs; 
    public ushort BPB_RootEntCnt;
    public ushort BPB_TotSec16; 
    public byte BPB_Media; 
    public ushort BPB_FATSz16;
    public ushort BPB_SecPerTrk;
    public ushort BPB_NumHeads;
    public uint BPB_HiddSec; 
    public uint BPB_TotSec32; 
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 474)]
    public byte[] FAT1632Info;
    public ushort Signature;
}

要转换 FAT16/FAT32 union ，您可能需要使用 fixed。或者有两种 FATBootSector 类型。一个用于 FAT16，一个用于 FAT32。