我正在编写小应用程序，监视某些目录以查找新添加的文件。
我想把监控代码放在一个单独的线程中，所以我可以让主线程免费用于其他东西，并在需要时取消监控线程。
我能够正确编码所有内容，除了一件事：
我无法正确退出监控线程，因此这篇文章。
我在主线程中发信号通知一个事件对象，等待线程退出然后进行清理。
问题在于我使用ReadDirectoryChangesW，因为在我注释掉那段代码之后一切正常。
一旦发出事件句柄信号，ReadDirectoryChangesW就会阻止线程阻止它“捕获”事件并退出。如果我在目录中添加新文件“解锁”ReadDirectoryChangesW，则线程“捕获”该事件并退出。
为了进一步提供帮助，我在下面做了一个小MVCE，这说明了我到目前为止所说的内容。
#include <iostream>
#include <Windows.h>
#include <map>
struct SThreadParams
{
HANDLE hEvent;
HANDLE hDir;
int processDirectoryChanges(const char *buffer)
{
if (NULL == buffer) return -1;
DWORD offset = 0;
char fileName[MAX_PATH] = "";
FILE_NOTIFY_INFORMATION *fni = NULL;
do
{
fni = (FILE_NOTIFY_INFORMATION*)(&buffer[offset]);
// since we do not use UNICODE,
// we must convert fni->FileName from UNICODE to multibyte
int ret = ::WideCharToMultiByte(CP_ACP, 0, fni->FileName,
fni->FileNameLength / sizeof(WCHAR),
fileName, sizeof(fileName), NULL, NULL);
switch (fni->Action)
{
case FILE_ACTION_ADDED:
{
std::cout << "FILE_ACTION_ADDED " << fileName << std::endl;
}
break;
case FILE_ACTION_REMOVED:
{
std::cout << "FILE_ACTION_REMOVED " << fileName << std::endl;
}
break;
case FILE_ACTION_MODIFIED:
{
std::cout << "FILE_ACTION_MODIFIED " << fileName << std::endl;
}
break;
case FILE_ACTION_RENAMED_OLD_NAME:
{
std::cout << "FILE_ACTION_RENAMED_OLD_NAME " << fileName << std::endl;
}
break;
case FILE_ACTION_RENAMED_NEW_NAME:
{
std::cout << "FILE_ACTION_RENAMED_NEW_NAME " << fileName << std::endl;
}
break;
default:
break;
}
// clear string so we can reuse it
::memset(fileName, '\0', sizeof(fileName));
// advance to next entry
offset += fni->NextEntryOffset;
} while (fni->NextEntryOffset != 0);
return 0;
}
};
DWORD WINAPI thread(LPVOID arg)
{
SThreadParams p = *((SThreadParams *)arg);
OVERLAPPED ovl = { 0 };
DWORD bytesTransferred = 0, error = 0;
char buffer[1024];
if (NULL == (ovl.hEvent = ::CreateEvent(NULL, TRUE, FALSE, NULL)))
{
std::cout << "CreateEvent error = " << ::GetLastError() << std::endl;
return ::GetLastError();
};
do {
if (::ReadDirectoryChangesW(p.hDir, buffer, sizeof(buffer), FALSE,
FILE_NOTIFY_CHANGE_FILE_NAME,
NULL, &ovl, NULL))
{
if (::GetOverlappedResult(p.hDir, &ovl, &bytesTransferred, TRUE))
{
for (int i = 0; i < 5; ++i) std::cout << '=';
std::cout << std::endl;
if (-1 == p.processDirectoryChanges(buffer))
std::cout << "processDirectoryChanges error = " << std::endl;
}
else
{
bytesTransferred = 0;
std::cout << "GetOverlappedResult error = " << ::GetLastError() << std::endl;
}
if (0 == ::ResetEvent(ovl.hEvent))
{
std::cout << "ResetEvent error = " << ::GetLastError() << std::endl;
::CloseHandle(ovl.hEvent);
return ::GetLastError();
}
}
else
{
// we shall just output the error, and try again...
std::cout << "ReadDirectoryChangesW error = " << ::GetLastError() << std::endl;
}
error = ::WaitForSingleObject(p.hEvent, 2000);
} while (WAIT_TIMEOUT == error);
::CloseHandle(ovl.hEvent);
return 0;
}
int main()
{
SThreadParams s;
s.hDir = ::CreateFile(SOME_DIRECTORY,
FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
if (INVALID_HANDLE_VALUE == s.hDir)
{
std::cout << "CreateFile error = " << ::GetLastError() << std::endl;
return 1;
}
s.hEvent = ::CreateEvent(NULL, TRUE, FALSE, NULL);
if (NULL == s.hEvent)
{
std::cout << "CreateEvent error = " << ::GetLastError() << std::endl;
::CloseHandle(s.hDir);
return 1;
}
HANDLE hThread = ::CreateThread(NULL, 0, thread, (LPVOID)&s, 0, NULL);
if (NULL == hThread)
{
std::cout << "CreateThread error = " << ::GetLastError() << std::endl;
::CloseHandle(s.hDir);
::CloseHandle(s.hEvent);
return 1;
}
std::cout << "press any key to close program..." << std::endl;
std::cin.get();
if (0 == ::CancelIoEx(s.hDir, NULL))
{
std::cout << "CancelIoEx error = " << ::GetLastError() << std::endl;
::CloseHandle(s.hDir);
::CloseHandle(s.hEvent);
return 1;
}
if (0 == ::SetEvent(s.hEvent))
{
std::cout << "SetEvent error = " << ::GetLastError() << std::endl;
::CloseHandle(s.hDir);
::CloseHandle(s.hEvent);
return 1;
}
// wait for thread to exit
DWORD error = ::WaitForSingleObject(hThread, INFINITE);
std::cout << "Thread exited with error code = " << error << std::endl;
::CloseHandle(s.hEvent);
::CloseHandle(s.hDir);
::CloseHandle(hThread);
return 0;
}
我已将OVERLAPPED结构从线程移出到传递给线程的结构中。然后我将OVERLAPPED.hEvent设置为强制“取消阻止”ReadDirectoryChangesW。这似乎有效，但让我感到害怕，因为我认为它不安全/容易出错，因为它没有记录。
我尝试使用完成例程，但由于我是新手，所以没有成功。我能够收到通知，但第一次通过后，缓冲区的内容（填充ReadDirectoryChangesW的内容）无法正确读取。我仍然试图独自完成这项工作，但可以使用帮助。
我可以使用I / O完成端口，但由于我只监视一个目录，我认为这有点过分。如果我弄错了，请告诉我如何使用I / O完成端口，我很乐意尝试一下。
鉴于上面的MVCE，你能指导我如何修改线程过程中的代码，使其正常退出（没有ReadDirectoryChangesW阻塞）。
我有一种感觉，我将不得不使用完成例程。在这种情况下，我会谦虚地要求一些伪代码或书面说明，因为这是我第一次使用它们。
每当我觉得自己取得了进步时，我会相应地更新这篇文章及相关数据。
如果需要进一步的信息/说明，请发表评论，我会回复。
谢谢，
问候。
答案 0 :(得分：0)
存在3种与文件进行异步操作的方法：
你选择了最差的变种。我会在你的地方使用IoCompletionPort。在这种情况下，您不需要创建事件，线程，调用GetOverlappedResult，不需要任何循环..
所有需要调用BindIoCompletionCallback（或RtlSetIoCompletionCallback）的文件和所有内容！
关于取消 - 在XP中不存在CancelIoEx（“我正在尝试支持Windows XP”
）
但您只需关闭目录句柄 - 在这种情况下，IO将被STATUS_NOTIFY_CLEANUP取消。所以代码看起来像这样：
RUNDOWN_REF_EVENT g_rundown; // Run-Down Protection
class SPYDATA :
#ifdef _USE_NT_VERSION_
IO_STATUS_BLOCK
#else
OVERLAPPED
#endif
{
HANDLE _hFile;
LONG _dwRef;
union {
FILE_NOTIFY_INFORMATION _fni;
UCHAR _buf[PAGE_SIZE];
};
void DumpDirectoryChanges()
{
union {
PVOID buf;
PBYTE pb;
PFILE_NOTIFY_INFORMATION pfni;
};
buf = _buf;
for (;;)
{
DbgPrint("%x <%.*S>\n", pfni->Action, pfni->FileNameLength >> 1, pfni->FileName);
ULONG NextEntryOffset = pfni->NextEntryOffset;
if (!NextEntryOffset)
{
break;
}
pb += NextEntryOffset;
}
}
#ifdef _USE_NT_VERSION_
static VOID WINAPI _OvCompRoutine(
_In_ NTSTATUS dwErrorCode,
_In_ ULONG_PTR dwNumberOfBytesTransfered,
_Inout_ PIO_STATUS_BLOCK Iosb
)
{
static_cast<SPYDATA*>(Iosb)->OvCompRoutine(dwErrorCode, (ULONG)dwNumberOfBytesTransfered);
}
#else
static VOID WINAPI _OvCompRoutine(
_In_ DWORD dwErrorCode, // really this is NTSTATUS
_In_ DWORD dwNumberOfBytesTransfered,
_Inout_ LPOVERLAPPED lpOverlapped
)
{
static_cast<SPYDATA*>(lpOverlapped)->OvCompRoutine(dwErrorCode, dwNumberOfBytesTransfered);
}
#endif
VOID OvCompRoutine(NTSTATUS status, DWORD dwNumberOfBytesTransfered)
{
DbgPrint("[%x,%x]\n", status, dwNumberOfBytesTransfered);
if (0 <= status)
{
if (status != STATUS_NOTIFY_CLEANUP)
{
if (dwNumberOfBytesTransfered) DumpDirectoryChanges();
DoRead();
}
else
{
DbgPrint("\n---- NOTIFY_CLEANUP -----\n");
}
}
Release();
g_rundown.ReleaseRundownProtection();
}
~SPYDATA()
{
Cancel();
}
public:
void DoRead()
{
if (g_rundown.AcquireRundownProtection())
{
AddRef();
#ifdef _USE_NT_VERSION_
NTSTATUS status = ZwNotifyChangeDirectoryFile(_hFile, 0, 0, this, this, &_fni, sizeof(_buf), FILE_NOTIFY_VALID_MASK, TRUE);
if (NT_ERROR(status))
{
OvCompRoutine(status, 0);
}
#else
if (!ReadDirectoryChangesW(_hFile, _buf, sizeof(_buf), TRUE, FILE_NOTIFY_VALID_MASK, (PDWORD)&InternalHigh, this, 0))
{
OvCompRoutine(RtlGetLastNtStatus(), 0);
}
#endif
}
}
SPYDATA()
{
_hFile = 0;// ! not INVALID_HANDLE_VALUE because use ntapi for open file
_dwRef = 1;
#ifndef _USE_NT_VERSION_
RtlZeroMemory(static_cast<OVERLAPPED*>(this), sizeof(OVERLAPPED));
#endif
}
void AddRef()
{
InterlockedIncrement(&_dwRef);
}
void Release()
{
if (!InterlockedDecrement(&_dwRef))
{
delete this;
}
}
BOOL Create(POBJECT_ATTRIBUTES poa)
{
IO_STATUS_BLOCK iosb;
NTSTATUS status = ZwOpenFile(&_hFile, FILE_GENERIC_READ, poa, &iosb, FILE_SHARE_VALID_FLAGS, FILE_DIRECTORY_FILE);
if (0 <= status)
{
return
#ifdef _USE_NT_VERSION_
0 <= RtlSetIoCompletionCallback(_hFile, _OvCompRoutine, 0);
#else
BindIoCompletionCallback(_hFile, _OvCompRoutine, 0);
#endif
}
return FALSE;
}
void Cancel()
{
if (HANDLE hFile = InterlockedExchangePointer(&_hFile, 0))
{
NtClose(hFile);
}
}
};
void DemoF()
{
if (g_rundown.Create())
{
STATIC_OBJECT_ATTRIBUTES(oa, "\\systemroot\\tmp");//SOME_DIRECTORY
if (SPYDATA* p = new SPYDATA)
{
if (p->Create(&oa))
{
p->DoRead();
}
MessageBoxW(0, L"wait close program...", L"", MB_OK);
p->Cancel();
p->Release();
}
g_rundown.ReleaseRundownProtection();
g_rundown.WaitForRundown();
}
}
等待所有IO完成后我使用Run-Down Protection。遗憾的是，这不是在用户模式下实现的，但并不是很难自己实现这个非常有用的功能。我的实施：
class __declspec(novtable) RUNDOWN_REF
{
LONG _LockCount;
protected:
virtual void RundownCompleted() = 0;
public:
RUNDOWN_REF()
{
_LockCount = 1;
}
BOOL AcquireRundownProtection()
{
LONG LockCount = _LockCount, prevLockCount;
do
{
if (!LockCount)
{
return FALSE;
}
LockCount = InterlockedCompareExchange(&_LockCount, LockCount + 1, prevLockCount = LockCount);
} while (LockCount != prevLockCount);
return TRUE;
}
void ReleaseRundownProtection()
{
if (!InterlockedDecrement(&_LockCount))
{
RundownCompleted();
}
}
};
class RUNDOWN_REF_EVENT : public RUNDOWN_REF
{
HANDLE _hEvent;
virtual void RundownCompleted()
{
SetEvent(_hEvent);
}
public:
BOOL Create()
{
return (_hEvent = CreateEvent(0, TRUE, FALSE, 0)) != 0;
}
RUNDOWN_REF_EVENT()
{
_hEvent = 0;
}
~RUNDOWN_REF_EVENT()
{
if (_hEvent) CloseHandle(_hEvent);
}
void WaitForRundown()
{
if (WaitForSingleObject(_hEvent, INFINITE) != WAIT_OBJECT_0) __debugbreak();
}
};