C＃联盟结构编组

Question

我正在尝试将Video4Linux集成到我的托管应用程序中。 事实上，我已经宣布了所有必需的结构和相对的ioctl 。 在这个问题中，我提出了两个ioctl ： SetFormat和GetFormat ; 虽然前者运作良好（就像我实际使用的那样），但后者却让我记忆力不好。

GetFormat ioctl实际上正在执行，但是一旦应用程序访问ioctl参数（调试器或我的应用程序本身），它总是与以下堆栈崩溃：

System.NullReferenceException: ...
at System.String.mempy4(...) in <filename unknown>:0
at System.String.memcpy(...) in <filename unknown>:0
at Derm.Platform.Video4Linux.ControlGetFormat(...) in <...>
...

我对p /调用ioctl进行了一些调查，再一次，我无法理解为什么我的应用程序崩溃了。 我怀疑它是由于结构内存布局，但我无法解释为什么SetFormat工作而GetFormat不工作（因为他们使用相同的参数）。

我必须P / Invoke接收/返回结构v4l2_format的ioctl例程：

struct v4l2_format {
        enum v4l2_buf_type type;
        union {
                struct v4l2_pix_format          pix;     /* V4L2_BUF_TYPE_VIDEO_CAPTURE */
                struct v4l2_pix_format_mplane   pix_mp;  /* V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE */
                struct v4l2_window              win;     /* V4L2_BUF_TYPE_VIDEO_OVERLAY */
                struct v4l2_vbi_format          vbi;     /* V4L2_BUF_TYPE_VBI_CAPTURE */
                struct v4l2_sliced_vbi_format   sliced;  /* V4L2_BUF_TYPE_SLICED_VBI_CAPTURE */
                __u8    raw_data[200];                   /* user-defined */
        } fmt;
};

struct v4l2_pix_format {
    __u32                   width;
    __u32                   height;
    __u32                   pixelformat;
    enum v4l2_field         field;
    __u32                   bytesperline;   /* for padding, zero if unused */
    __u32                   sizeimage;
    enum v4l2_colorspace    colorspace;
    __u32                   priv;           /* private data, depends on pixelformat */
};

我使用以下表格来描述结构v4l2_format

[StructLayout(LayoutKind.Explicit, Pack = 4, CharSet = CharSet.Ansi)]
public struct Format
{
    public Format(BufferType bufferType)
    {
        Type = bufferType;
        Pixmap = new PixmapFormat();
        RawData = new byte[RawDataLength];
    }

    public Format(BufferType bufferType, PixmapFormat pixmapFormat)
    {
        Type = bufferType;
        Pixmap = pixmapFormat;
        RawData = new byte[RawDataLength];
    }

    [FieldOffset(0)]
    public BufferType Type;

    [FieldOffset(4)]
    public PixmapFormat Pixmap;

    [FieldOffset(4)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=RawDataLength)]
    public byte[] RawData;

    public const int RawDataLength = 200;
}

[StructLayout(LayoutKind.Sequential, Pack = 4)]
public struct PixmapFormat
{
    public PixmapFormat(uint width, uint height, PixelFormatCode pixelFormat)
    {
        Width = width;
        Height = height;
        PixelFormat = pixelFormat;
        Field = Video4Linux.BufferField.Any;
        BytesPerLine = 0;
        SizeImage = 0;
        Colorspace = Video4Linux.PixelColorspace.None;
        Private = 0;
    }

    public UInt32 Width;

    public UInt32 Height;

    public PixelFormatCode PixelFormat;

    public BufferField Field;

    public UInt32 BytesPerLine;

    public UInt32 SizeImage;

    public PixelColorspace Colorspace;

    public UInt32 Private;
}

最后，这是调用ioctl的方法：

public static void ControlGetFormat(IntPtr fd, BufferType pixmapType, out PixmapFormat pixmapFormat)
{
    if (fd == IntPtr.Zero)
        throw new ArgumentException("invalid file descriptor", "fd");

    Format format = new Format(pixmapType);

    int result = IoCtrlGetFormat(fd, GetFormat.ControlCode, ref format);

    if (result < 0)
        ThrowExceptionOnError();

    pixmapFormat = format.Pixmap;
}

private static readonly IoCtrlRequest GetFormat = new IoCtrlRequest(IoCtrlDirection.Read | IoCtrlDirection.Write, 'V', 4, typeof(Format));

[DllImport("libc", EntryPoint = "ioctl", SetLastError = true)]
private static extern int IoCtrlGetFormat(IntPtr fd, Int32 code, ref Format argument);

public static void ControlSetFormat(IntPtr fd, BufferType pixmapType, ref PixmapFormat pixmapFormat)
{
    if (fd == IntPtr.Zero)
        throw new ArgumentException("invalid file descriptor", "fd");

    PixmapFormat pixmapFormatCopy = pixmapFormat;
    Format format = new Format(pixmapType, pixmapFormatCopy);

    int result = IoCtrlSetFormat(fd, SetFormat.ControlCode, ref format);

    if (result < 0)
        ThrowExceptionOnError();

    pixmapFormat = format.Pixmap;
}

private static readonly IoCtrlRequest SetFormat = new IoCtrlRequest(IoCtrlDirection.Read | IoCtrlDirection.Write, 'V', 5, typeof(Format));

[DllImport("libc", EntryPoint = "ioctl", SetLastError = true)]
private static extern int IoCtrlSetFormat(IntPtr fd, Int32 code, ref Format argument);

Answer 1

通过将结构大小强制为实际大小（在我的情况下为204 ）解决了这个问题，并删除了数组字段RawData （由David Heffernan建议）。

确实：

[StructLayout(LayoutKind.Explicit, Pack = 4, CharSet = CharSet.Ansi, Size = 204)]
public struct Format
{
    public Format(BufferType bufferType)
    {
        Type = bufferType;
        Pixmap = new PixmapFormat();
    }

    public Format(BufferType bufferType, PixmapFormat pixmapFormat)
    {
        Type = bufferType;
        Pixmap = pixmapFormat;
    }

    [FieldOffset(0)]
    public BufferType Type;

    [FieldOffset(4)]
    public PixmapFormat Pixmap;
}

当然， Marshal.SizeOf(typeof(Format)) == 204 。