我正在尝试将 Video4Linux 集成到我的托管应用程序中。事实上，我已经声明了所有必需的结构和相关的ioctl。在这个问题中，我提出了两个ioctl：SetFormat和GetFormat；虽然前者运行良好（就像我实际使用的其他十几个一样），但后者却让我有不好的记忆行为。
ioctl 实际上正在执行，GetFormat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），它总是会因以下堆栈而崩溃：
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/invoking ioctl进行了一些调查，但我再次无法理解为什么我的应用程序崩溃了。我怀疑这是因为结构内存布局，但我无法解释为什么SetFormat工作GetFormat而不工作（因为它们使用相同的参数）。
我必须 P/Invoke 一个ioctl接收/返回结构的例程v4l2_format：
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 */
};
我已经v4l2_format使用以下形式对结构进行了转换
[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，这里是调用 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);