我正在异步调用ReadDirectoryChangesW
来监视后台线程中的目录更改。
这将打开目录(basePath
)并启动“读取”线程的方式:
m_hDIR = CreateFileW(
basePath,
FILE_LIST_DIRECTORY | GENERIC_READ,
FILE_SHARE_WRITE | FILE_SHARE_READ,
NULL,
OPEN_EXISTING,
FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
NULL);
if (m_hDIR == INVALID_HANDLE_VALUE)
throw CrException(CrWin32ErrorString());
//Start reading changes in background thread
m_Callback = std::move(a_Callback);
m_Reading = true;
m_ReadThread = std::thread(&CrDirectoryWatcher::StartRead, this);
这是
StartRead()
:(注意:m_Reading
是atomic<bool>
)void StartRead()
{
DWORD dwBytes = 0;
FILE_NOTIFY_INFORMATION fni{0};
OVERLAPPED o{0};
//Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
o.hEvent = CreateEvent(0, 0, 0, 0);
while(m_Reading)
{
if (!ReadDirectoryChangesW(m_hDIR,
&fni, sizeof(fni),
TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
&dwBytes, &o, NULL))
{
CrAssert(0, CrWin32ErrorString());
}
if (!GetOverlappedResult(m_hDIR, &o, &dwBytes, FALSE))
CrAssert(0, CrWin32ErrorString());
if (fni.Action != 0)
{
std::wstring fileName(fni.FileName, fni.FileNameLength);
m_Callback(fileName);
fni.Action = 0;
}
}
}
基本上,我正在“轮询”每帧的新更改。
现在,当我调用
GetOverlappedResult()
时,它将失败并产生以下错误:重叠的I / O事件未处于信号状态。
我想念什么吗?是
ReadDirectoryChangesW
被称为每个“刻度”吗?还是只是在检测到新变化时?注意:当我省略
OVERLAPPED
结构(和GetOverlappedResult
)时,它可以工作,但是会阻塞线程,直到读取更改为止。这会阻止我的应用程序正确终止。 (即我无法加入该线程) 最佳答案
调用GetOverlappedResult()
时,如果将bWait
参数设置为FALSE并且I / O操作尚未完成,则GetOverlappedResult()
失败,并显示ERROR_IO_INCOMPLETE
错误代码:
b等待[输入]
如果此参数为TRUE
,并且Internal
结构的lpOverlapped
成员为STATUS_PENDING
,则该函数在操作完成之前不会返回。如果此参数为FALSE
并且操作仍在进行中,则函数返回FALSE
,而GetLastError
函数返回ERROR_IO_INCOMPLETE
。
那不是致命的错误,因此只需忽略该错误并继续。
是的,请确保在ReadDirectoryChangesW()
报告上一个I / O操作首先完成之前,不要再次调用GetOverlappedResult()
。
如此说来,您的代码还有另一个问题。您的线程在堆栈上分配了一个FILE_NOTIFY_INFORMATION
实例。如果查看FILE_NOTIFY_INFORMATION
的定义,则其FileName
字段是可变长度的:
typedef struct _FILE_NOTIFY_INFORMATION {
DWORD NextEntryOffset;
DWORD Action;
DWORD FileNameLength;
WCHAR FileName[1];
} FILE_NOTIFY_INFORMATION, *PFILE_NOTIFY_INFORMATION;
文档名称
可变长度字段,包含相对于目录句柄的文件名。文件名采用Unicode字符格式,并且不以null结尾。
这意味着静态分配
FILE_NOTIFY_INFORMATION
将会太小,并且dwBytes
几乎总是为0,因为ReadDirectoryChangesW()
无法将完整的FILE_NOTIFY_INFORMATION
返回给您(除非FileName
恰好是长度为1个字符):首次调用
ReadDirectoryChangesW
时,系统会分配一个缓冲区来存储更改信息。该缓冲区与目录句柄相关联,直到它被关闭并且其大小在其生命周期内保持不变。在两次调用此函数之间发生的目录更改将添加到缓冲区中,然后在下一次调用时返回。如果缓冲区溢出,则将丢弃缓冲区的全部内容,lpBytesReturned
参数包含零,并且ReadDirectoryChangesW
函数将失败,错误代码为ERROR_NOTIFY_ENUM_DIR
。ERROR_NOTIFY_ENUM_DIR
1022(0x3FE)
通知更改请求正在完成,并且信息没有在调用者的缓冲区中返回。现在,调用者需要枚举文件以查找更改。
因此,您需要动态分配一个大字节缓冲区以接收
FILE_NOTIFY_INFORMATION
数据,然后只要GetOverlappedResult()
报告数据可用,就可以遍历该缓冲区。您的线程应该看起来像这样:
void StartRead()
{
DWORD dwBytes = 0;
std::vector<BYTE> buffer(1024*64);
OVERLAPPED o{0};
bool bPending = false;
//Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
o.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (!o.hEvent) {
CrAssert(0, CrWin32ErrorString());
}
while (m_Reading)
{
bPending = ReadDirectoryChangesW(m_hDIR,
&buffer[0], buffer.size(),
TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
&dwBytes, &o, NULL);
if (!bPending)
{
CrAssert(0, CrWin32ErrorString());
}
while (m_Reading)
{
if (GetOverlappedResult(m_hDIR, &o, &dwBytes, FALSE))
{
bPending = false;
if (dwBytes != 0)
{
FILE_NOTIFY_INFORMATION *fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(&buffer[0]);
do
{
if (fni->Action != 0)
{
std::wstring fileName(fni->FileName, fni->FileNameLength);
m_Callback(fileName);
}
if (fni->NextEntryOffset == 0)
break;
fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(reinterpret_cast<BYTE*>(fni) + fni->NextEntryOffset);
}
while (true);
}
break;
}
if (GetLastError() != ERROR_IO_INCOMPLETE) {
CrAssert(0, CrWin32ErrorString());
}
Sleep(10);
}
if (bPending)
{
CancelIo(m_hDIR);
GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE);
}
}
CloseHandle(o.hEvent);
}
另一种无需定期轮询I / O状态即可实现此目的的方法是摆脱
m_Reading
并使用等待事件。让操作系统在应调用GetOverlappedResult()
或终止时向线程发出信号,以使其可以在不忙于做某事的其余时间中休眠:m_hDIR = CreateFileW(
basePath,
FILE_LIST_DIRECTORY | GENERIC_READ,
FILE_SHARE_WRITE | FILE_SHARE_READ,
NULL,
OPEN_EXISTING,
FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
NULL);
if (m_hDIR == INVALID_HANDLE_VALUE)
throw CrException(CrWin32ErrorString());
m_TermEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (!m_TermEvent)
throw CrException(CrWin32ErrorString());
//Start reading changes in background thread
m_Callback = std::move(a_Callback);
m_ReadThread = std::thread(&CrDirectoryWatcher::StartRead, this);
...
SetEvent(m_TermEvent);
m_ReadThread.join();
void StartRead()
{
DWORD dwBytes = 0;
std::vector<BYTE> buffer(1024*64);
OVERLAPPED o{0};
bool bPending = false, bKeepRunning = true;
//Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
o.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (!o.hEvent) {
CrAssert(0, CrWin32ErrorString());
}
HANDLE h[2] = {o.hEvent, h_TermEvent};
do
{
bPending = ReadDirectoryChangesW(m_hDIR,
&buffer[0], buffer.size(),
TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
&dwBytes, &o, NULL);
if (!bPending)
{
CrAssert(0, CrWin32ErrorString());
}
switch (WaitForMultipleObjects(2, h, FALSE, INFINITE))
{
case WAIT_OBJECT_0:
{
if (!GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE)) {
CrAssert(0, CrWin32ErrorString());
}
bPending = false;
if (dwBytes == 0)
break;
FILE_NOTIFY_INFORMATION *fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(&buffer[0]);
do
{
if (fni->Action != 0)
{
std::wstring fileName(fni->FileName, fni->FileNameLength);
m_Callback(fileName);
}
if (fni->NextEntryOffset == 0)
break;
fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(reinterpret_cast<BYTE*>(fni) + fni->NextEntryOffset);
}
while (true);
break;
}
case WAIT_OBJECT_0+1:
bKeepRunning = false;
break;
case WAIT_FAILED:
CrAssert(0, CrWin32ErrorString());
break;
}
}
while (bKeepRunning);
if (bPending)
{
CancelIo(m_hDIR);
GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE);
}
CloseHandle(o.hEvent);
}
关于c++ - ReadDirectoryChangesW和GetOverlappedResult,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/43664998/