windows 为什么 ReadDirectoryChangesW 会忽略事件？
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Why does ReadDirectoryChangesW omit events?
提问by jpfollenius
I use ReadDirectoryChangesW to watch a specified directory and update indexing structures whenever a change is detected. I use the following code (roughly)
我使用 ReadDirectoryChangesW 来观察指定的目录并在检测到更改时更新索引结构。我使用以下代码（大致）
var
InfoPointer : PFileNotifyInformation;
NextOffset : DWORD;
...
while (not Terminated) do begin
if ReadDirectoryChangesW (FDirHandle, FBuffer, FBufferLength, True,
FFilter, @BytesRead, @FOverlap, nil) then
begin
WaitResult := WaitForMultipleObjects (2, @FEventArray, False, INFINITE);
if (WaitResult = waitFileChange) then
begin
InfoPointer := FBuffer;
repeat
NextOffset := InfoPointer.NextEntryOffset;
...
PByte (InfoPointer) := PByte (InfoPointer) + NextOffset;
until NextOffset = 0;
end;
end;
end;
Filter is
过滤器是
FFilter := FILE_NOTIFY_CHANGE_FILE_NAME or
FILE_NOTIFY_CHANGE_DIR_NAME or
FILE_NOTIFY_CHANGE_SIZE or
FILE_NOTIFY_CHANGE_LAST_WRITE;
and the directory handle is obtained like this:
和目录句柄是这样获得的：
FDirHandle := CreateFile (PChar (FDirectoryWatch.WatchedDirectory),
FILE_LIST_DIRECTORY or GENERIC_READ,
FILE_SHARE_READ or FILE_SHARE_WRITE or FILE_SHARE_DELETE,
nil, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS or
FILE_FLAG_OVERLAPPED, 0);
When I delete multiple files I get only one event and NextOffset is 0! And when I delete a directory I get only one event for the directory. What if I want one event for each file in the directory?
当我删除多个文件时，我只得到一个事件，而 NextOffset 为 0！当我删除一个目录时，我只会得到该目录的一个事件。如果我希望目录中的每个文件都有一个事件怎么办？
Any help would be appreciated.
任何帮助，将不胜感激。
回答by mghie
It seems to me that you are mixing the various ways to use ReadDirectoryChangesW(), you do both specify the FILE_FLAG_OVERLAPPEDflag when opening the directory and provide a pointer to the lpOverlappedparameter, meaning you want to wait on the event in the structure and handle the asynchronous I/O; and at the same time you call ReadDirectoryChangesW()in a loop in a worker thread. I would first try again with lpOverlappedset to nil, as you have a dedicated thread and can use the synchronous mode.
在我看来，您正在混合使用ReadDirectoryChangesW()的各种方法，您在打开目录时都指定了FILE_FLAG_OVERLAPPED标志并提供指向lpOverlapped参数的指针，这意味着您要等待结构中的事件并处理异步 I/O；同时在工作线程的循环中调用ReadDirectoryChangesW()。我会先将lpOverlapped设置为nil再试一次，因为您有一个专用线程并且可以使用同步模式。
In the documentation of the ReadDirectoryChangesW()API function the different ways to use it are described. Note that it is also possible that the buffer overflows, so change events can be lost anyway. Maybe you should rethink your strategy of relying solely on this function, comparing snapshots of directory contents could work as well.
在ReadDirectoryChangesW()API 函数的文档中，描述了使用它的不同方法。请注意，缓冲区也有可能溢出，因此无论如何更改事件都可能丢失。也许您应该重新考虑仅依赖此功能的策略，比较目录内容的快照也可以。
Edit:
编辑：
Your edited code looks better. In my tests however ReadDirectoryChangesW()did work as advertised, there were either several data entries in the returned buffer, or there were more than one buffer to process. This depends on timing, after hitting a breakpoint in Delphi I get several entries in one buffer.
您编辑的代码看起来更好。然而，在我的测试中ReadDirectoryChangesW()确实如宣传的那样工作，返回的缓冲区中有多个数据条目，或者有多个缓冲区需要处理。这取决于时间，在 Delphi 中遇到断点后，我在一个缓冲区中获得了多个条目。
For completeness I attach the test code, implemented using Delphi 5:
为了完整起见，我附上了使用 Delphi 5 实现的测试代码：
type
TWatcherThread = class(TThread)
private
fChangeHandle: THandle;
fDirHandle: THandle;
fShutdownHandle: THandle;
protected
procedure Execute; override;
public
constructor Create(ADirectoryToWatch: string);
destructor Destroy; override;
procedure Shutdown;
end;
constructor TWatcherThread.Create(ADirectoryToWatch: string);
const
FILE_LIST_DIRECTORY = 1;
begin
inherited Create(TRUE);
fChangeHandle := CreateEvent(nil, FALSE, FALSE, nil);
fDirHandle := CreateFile(PChar(ADirectoryToWatch),
FILE_LIST_DIRECTORY or GENERIC_READ,
FILE_SHARE_READ or FILE_SHARE_WRITE or FILE_SHARE_DELETE,
nil, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS or FILE_FLAG_OVERLAPPED, 0);
fShutdownHandle := CreateEvent(nil, FALSE, FALSE, nil);
Resume;
end;
destructor TWatcherThread.Destroy;
begin
if fDirHandle <> INVALID_HANDLE_VALUE then
CloseHandle(fDirHandle);
if fChangeHandle <> 0 then
CloseHandle(fChangeHandle);
if fShutdownHandle <> 0 then
CloseHandle(fShutdownHandle);
inherited Destroy;
end;
procedure TWatcherThread.Execute;
type
PFileNotifyInformation = ^TFileNotifyInformation;
TFileNotifyInformation = record
NextEntryOffset: DWORD;
Action: DWORD;
FileNameLength: DWORD;
FileName: WideChar;
end;
const
BufferLength = 65536;
var
Filter, BytesRead: DWORD;
InfoPointer: PFileNotifyInformation;
Offset, NextOffset: DWORD;
Buffer: array[0..BufferLength - 1] of byte;
Overlap: TOverlapped;
Events: array[0..1] of THandle;
WaitResult: DWORD;
FileName, s: string;
begin
if fDirHandle <> INVALID_HANDLE_VALUE then begin
Filter := FILE_NOTIFY_CHANGE_FILE_NAME or FILE_NOTIFY_CHANGE_DIR_NAME
or FILE_NOTIFY_CHANGE_SIZE or FILE_NOTIFY_CHANGE_LAST_WRITE;
FillChar(Overlap, SizeOf(TOverlapped), 0);
Overlap.hEvent := fChangeHandle;
Events[0] := fChangeHandle;
Events[1] := fShutdownHandle;
while not Terminated do begin
if ReadDirectoryChangesW (fDirHandle, @Buffer[0], BufferLength, TRUE,
Filter, @BytesRead, @Overlap, nil)
then begin
WaitResult := WaitForMultipleObjects(2, @Events[0], FALSE, INFINITE);
if WaitResult = WAIT_OBJECT_0 then begin
InfoPointer := @Buffer[0];
Offset := 0;
repeat
NextOffset := InfoPointer.NextEntryOffset;
FileName := WideCharLenToString(@InfoPointer.FileName,
InfoPointer.FileNameLength);
SetLength(FileName, StrLen(PChar(FileName)));
s := Format('[%d] Action: %.8xh, File: "%s"',
[Offset, InfoPointer.Action, FileName]);
OutputDebugString(PChar(s));
PByte(InfoPointer) := PByte(DWORD(InfoPointer) + NextOffset);
Offset := Offset + NextOffset;
until NextOffset = 0;
end;
end;
end;
end;
end;
procedure TWatcherThread.Shutdown;
begin
Terminate;
if fShutdownHandle <> 0 then
SetEvent(fShutdownHandle);
end;
////////////////////////////////////////////////////////////////////////////////
procedure TForm1.FormCreate(Sender: TObject);
begin
fThread := TWatcherThread.Create('D:\Temp');
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
if fThread <> nil then begin
TWatcherThread(fThread).Shutdown;
fThread.Free;
end;
end;
Deleting a directory does indeed only return one change for it, nothing for the files contained in it. But it does make sense, as you are watching the handle of the parent directory only. If you need notifications for subdirectories you probably need to watch them as well.
删除一个目录确实只返回一个更改，其中包含的文件没有任何更改。但这确实是有道理的，因为您只是在观察父目录的句柄。如果您需要子目录的通知，您可能还需要监视它们。
回答by Pieter van Wyk
We've had the same problem with losing events, especially if a lot of changes happens at the same time, ie. 500 files are copied to the monitored directory.
我们在丢失事件方面遇到了同样的问题，特别是如果同时发生很多变化，即。500 个文件被复制到监控目录。
In the end we found Cromisand use the Directory watch. We have never looked back again.
最后我们找到了Cromis并使用Directory watch。我们再也没有回头。
