ReadDirectoryChangesW 仅将单个事件放入 FILE_NOTIFY_INFORMATION 缓冲区

Question

我有一个问题，ReadDirectoryChangesW 总是丢失事件。

我做了很多谷歌搜索，根据我的搜索，波纹管函数参数似乎是正确的，但没有人能确定。我开始这样看。

BOOL _watchRequestResult = false;
OVERLAPPED _ovl = { 0 };
_ovl.hEvent = ::CreateEventA(NULL, TRUE, FALSE, NULL);

_directoryHandle = ::CreateFileA("some path here", FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_DELETE | FILE_SHARE_WRITE,
  NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);

// This should be quite enough to fit multiple file events
static constexpr DWORD ResultDataLength = 10000;
// Byte size used for winapi calls and memcpy during move operation
static constexpr DWORD ResultDataByteSize = ResultDataLength * sizeof(FILE_NOTIFY_INFORMATION);
FILE_NOTIFY_INFORMATION _resultData[ResultDataLength] = { 0 };

_watchRequestResult = ::ReadDirectoryChangesW(
  _directoryHandle,
  (LPVOID)_resultData,
  ResultDataByteSize,
  TRUE,
  FILE_NOTIFY_CHANGE_FILE_NAME,
  NULL,
  &_ovl,
  NULL
);

在上面之后，我等待 _ovl.hEvent 使用 WaitForMultipleObjects。我使用多个对象，因为总是有事件告诉监视线程退出。

如果 ovl.hEvent 收到通知，我会这样做：

DWORD _ovlBytesReturned = 0;
// Imagine some struct that I use to pass the file info, not important how it looks
std::vector<MyFileInfoStruct> results;
if (::GetOverlappedResult(_directoryHandle, &_ovl, &_ovlBytesReturned, TRUE))
{
  int byteIndex = 0;
  bool previousWasRename = false;
  const int minSize = min(ResultDataLength, _ovlBytesReturned);
  while (byteIndex < minSize)
  {
    FILE_NOTIFY_INFORMATION* info = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(reinterpret_cast<char*>(&_resultData[0]) + byteIndex);
    byteIndex += info->NextEntryOffset;

    // read the stuff in the info
    results.push_back(MyFileInfoStruct::FromFileInfo(info));

    // If next entry index is 0, it means there is no next entry
    if (info->NextEntryOffset == 0)
    {
      break;
    }
  }
}
// if file is renamed, merge new name and old name to same result. However rename works to give me two FILE_NOTIFY_INFORMATION that both contain expected data
MergeResultRename(results)
// results is always 1 item long

此时我应该注意，info->NextEntryOffset 并不总是 0 - 如果我重命名一个文件，我会在 _resultData 中正确地得到两个条目，一个用于新文件名，一个用于旧文件名。

永远不会得到的是每个事件的多个文件更改。这是一个问题，整个代码看起来像这样（伪代码）

Let EVENTS be an array of HANDLE event objects
Let FILE_CHANGES be a buffer of file changes 
while(shouldBeWatching) 
{
  Wait for events from previous iteration stored in EVENTS array. Skip on first iteration.
  if(event has fired) 
  {
    if(event that fired is ovl.hEvent)
    {
      Put file changes from the event that fired into FILE_CHANGES array (seen in 2nd code sample above)
      Delete and close all handles related to the event:
         Close directory handle
         Close ovl.hEvent
    }
    else 
    {
      Close everything and quit thread.
    }
  }
  Start new request (seen above in 1st code sample)
  if(FILE_CHANGES is not empty) 
  {
    Process all info from FILE_CHANGES
  }
}

现在您可以看到，在处理 MyFileInfoStruct 数组之前，我重新启动了 ReadDirectoryChangesW 的请求。但问题是，如果复制了两个以上的文件，第二个文件在我处理前一个文件时被事件注册，但后续的更改将被忽略，直到我 "pick up" 最后一个更改并重新启动事件。

我可以通过让第二个线程执行 处理来自 FILE_CHANGES 部分的所有信息来部分解决这个问题。但这只会通过使整个 start request -> wait -> pick up[=58] 来减少错过事件的机会=] -> restart event 程序快一点。它实际上并没有提供 100% 的覆盖率，仍然有一段时间没有 ReadDirectoryChangesW 请求待处理。

我在网上看了很多，发现经常提到两个解决方案：

使用单独的线程来处理文件更改（我已经这样做了）

增加 FILE_NOTIFY_INFORMATION[] 的大小。这对我不起作用，windows 只在那里放置一个事件

因此问题是：如何让 ReadDirectoryChangesW 和 GetOverlappedResult 继续在 FILE_NOTIFY_INFORMATION[] 缓冲区中添加文件更改，直到我 "pick up" 通过调用 GetOverlappedResult?这甚至可能吗？有没有人设法将多个结果放入一个缓冲区？

Answer 1

要重命名文件，需要执行两个操作：FILE_ACTION_RENAMED_OLD_NAME and FILE_ACTION_RENAMED_NEW_NAME. These two action events you can retrieve via once calling of ReadDirectoryChanges and GetOverlappedResult 正如您已经完成的那样。

I have a problem that ReadDirectoryChangesW keeps missing events.

要捕获复制两个文件和删除两个文件的事件，例如，首先我复制TESTA.txt和TESTB.txt 到目录 D:\testFolder，然后将它们都删除。我可以通过在 while 循环中调用 ReadDirectoryChanges and GetOverlappedResult 来获取所有事件。对于四次事件，这两个函数被调用了四次。

测试代码如下：

#include <windows.h>
#include <vector>

using namespace std;

typedef struct TEST_INFO {
    DWORD NextEntryOffset;
    DWORD Action;
    DWORD FileNameLength;
    WCHAR FileName[100];
}_TEST_INFO;

int main()
{
    BOOL _watchRequestResult = false;
    OVERLAPPED _ovl = { 0 };
    _ovl.hEvent = ::CreateEventA(NULL, TRUE, FALSE, NULL);

    HANDLE _directoryHandle = ::CreateFileA("d:\testFolder", FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_DELETE | FILE_SHARE_WRITE,
        NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);

    // This should be quite enough to fit multiple file events
    static constexpr DWORD ResultDataSize = 100;
    _TEST_INFO _resultData[ResultDataSize] = { 0 };

    while (true)
    {

        _watchRequestResult = ::ReadDirectoryChangesW(
            _directoryHandle,
            (LPVOID)_resultData,
            ResultDataSize * sizeof(_TEST_INFO),
            TRUE,
            FILE_NOTIFY_CHANGE_FILE_NAME,
            NULL,
            &_ovl,
            NULL
        );

        DWORD _ovlBytesReturned = 0;

        if (::GetOverlappedResult(_directoryHandle, &_ovl, &_ovlBytesReturned, TRUE))
        {
            int byteIndex = 0;

            while (TRUE)
            {
                _TEST_INFO* info = reinterpret_cast<_TEST_INFO*>(reinterpret_cast<char*>(&_resultData[0]) + byteIndex);
                byteIndex += info->NextEntryOffset;

                wprintf(L"File name: %s, ", info->FileName);
                printf("Action: ");
                switch (info->Action)
                {
                case FILE_ACTION_ADDED:
                    printf("Added \n");
                    break;
                case FILE_ACTION_REMOVED:
                    printf("Removed \n");
                    break;
                case FILE_ACTION_MODIFIED:
                    printf("Modified \n");
                    break;
                case FILE_ACTION_RENAMED_OLD_NAME:
                    printf("Rename old name \n");
                    break;
                case FILE_ACTION_RENAMED_NEW_NAME:
                    printf("Rename new name \n");
                    break;
                }

                // If next entry index is 0, it means there is no next entry
                if (info->NextEntryOffset == 0)
                {
                    break;
                }
            }
        }

    }

    getchar();
}

总结：

GetOverlappedResult函数：

The results reported by the GetOverlappedResult function are those of the specified handle's last overlapped operation.

所以重命名文件是重叠（重命名）操作，复制文件是重叠（复制）操作。但是复制两个文件是两个重叠（复制）操作。所以它需要调用 GetOverlappedResult 两次而不是一次。

ReadDirectoryChangesW 仅将单个事件放入 FILE_NOTIFY_INFORMATION 缓冲区

ReadDirectoryChangesW only places single event in the FILE_NOTIFY_INFORMATION buffer

c++

winapi

readdirectorychangesw

getoverlappedresult