P調用Marshal.PtrToStructure上的空引用異常 <T> （ptr）

Question

嘗試用Mono調用linux / libc的read（）方法時，我們得到一種奇怪的行為。

[16:05:17.258][UNHANDLED EXCEPTION][BEGIN]
[16:05:18.463]System.NullReferenceException: Object reference not set to an instance of an object
  at (wrapper unknown) PI.SDK.UI.HW.PAX.ProlinKeyboardManager+InputEvent:PtrToStructure (intptr,object)
  at (wrapper managed-to-native) System.Runtime.InteropServices.Marshal:PtrToStructure (intptr,System.Type)
  at System.Runtime.InteropServices.Marshal.PtrToStructure[T] (IntPtr ptr)  in :0 
  at PI.SDK.UI.HW.PAX.ProlinKeyboardManager.StartListening ()  in :0 
  at PI.SDK.PAX.Playground.Program+c.b__37_0 ()  in :0 
  at System.Threading.ThreadHelper.ThreadStart_Context (System.Object state)  in :0 
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx)  in :0 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx)  in :0 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state)  in :0 
  at System.Threading.ThreadHelper.ThreadStart ()  in :0

問題似乎出在Marshal.PtrToStructure<T>(IntPtr)但我看不到它在哪里...

repro案例的托管/ C＃部分如下：

[StructLayout(LayoutKind.Sequential)]
private struct InputEvent
{
    public int Seconds;
    public int Microseconds;
    public ushort type;
    public ushort code;
    public int value;
}

[DllImport(LIBC, EntryPoint = "read", CallingConvention = CallingConvention.Cdecl)]
private static extern int NativeRead(int fd, ref IntPtr buf, int nbytes);

public void StartListening()
{
    var kbDeviceName = "/dev/keypad";
    int fd = -1;
    fd = NativeOpen(kbDeviceName, 2);
    Console.WriteLine($"[KBD] {fd}");

    if (fd < 0)
        throw new InvalidOperationException($"Unable to open Keyboard device {kbDeviceName}. Return code {fd}.");

    var ev = new InputEvent[64];
    var size = Marshal.SizeOf<InputEvent>();
    Console.WriteLine($"[EventSize] {size}");
    var totalSize = size * 64;
    Console.WriteLine($"[TotalEventSize] {totalSize}");

    var ptr = IntPtr.Zero;

    while (true)
    {
        var rd = NativeRead(fd, ref ptr, totalSize);
        Console.WriteLine($"[KBD][Read] {rd}");
        Console.WriteLine($"[PTR] {ptr}");
        if (rd > size)
        {
            var eventNum = rd / size;
            Console.WriteLine($"[Events] {eventNum}");
            for (int i = 0; i < eventNum; i++)
            {
                var entry = Marshal.PtrToStructure<InputEvent>(ptr);
                Console.WriteLine($"code: {entry.code} | type: {entry.type} | value: {entry.value}");
                ptr = new IntPtr(ptr.ToInt32() + size);
            }
        }
    }
}

這些Console.Writeline的輸出是：

[KBD] 5
[EventSize] 16
[TotalEventSize] 1024
[KBD][Read] 32
[PTR] 1460304317
[Events] 2
[16:05:17.258][UNHANDLED EXCEPTION][BEGIN]
... THE EXCEPTION GOES HERE ...

這是我們嘗試復制的C本機示例：

int i;
int eventNum = 0;
struct input_event ev[64];

int size = sizeof(struct input_event);
int rd = 0;

memset(ev, 0, sizeof(ev));
rd = read(gfd, ev, sizeof(ev)); 
eventNum = rd / size;
for (i = 0; i < eventNum; ++i)
{
    /* EV_KEY means type is key (not mouse, etc) */
    if (ev[i].type == EV_KEY && ev[i].value == 1)
    {
        return ev[i].code;
    }
}

有人可以指出我應該在哪里出錯嗎？

謝謝！ 真的感謝任何幫助。 古登堡

Answer 1

兩個問題：

1）NativeRead的第二個參數不應是對指針的引用。 相反，它應該是指針本身。 請驗證PInvoke聲明，並將“ ref IntPtr”更改為“ IntPtr”。 例：

private static extern int NativeRead(int fd, IntPtr buf, int nbytes);

2）read需要指向緩沖區的指針，並且此代碼傳遞0。您應該為緩沖區分配本機內存。 例：

var ptr = Marshal.AllocHGlobal(totalSize);

循環中還有另一個問題：循環中遞增的指針與用於進一步讀取的指針相同。 相反，應始終使用緩沖區指針進行讀取。 這應該解決它：

  var buffer_ptr = Marshal.AllocHGlobal(totalSize);

  while (true)
  {
    var rd = NativeRead(fd, buffer_ptr, totalSize);
    Console.WriteLine($"[KBD][Read] {rd}");
    Console.WriteLine($"[PTR] {buffer_ptr}");
    if (rd > size)
    {
      var eventNum = rd / size;
      Console.WriteLine($"[Events] {eventNum}");
      var event_ptr = buffer_ptr;
      for (int i = 0; i < eventNum; i++)
      {
        var entry = Marshal.PtrToStructure<InputEvent>(event_ptr);
        Console.WriteLine($"code: {entry.code} | type: {entry.type} | value: {entry.value}");
        event_ptr = IntPtr.Add(event_ptr, size);
      }
    }
  }

最好的祝福！