WaitForSingleObject with FindFirstChangeNotification not behaving as expected [duplicate] - c++

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This question already has answers here:
FindFirstChangeNotification is notifying about changes twice
(2 answers)
Closed 1 year ago.
I have to find out if there is a new file in a directory on Windows. Following this MSDN example (Obtaining Directory Change Notifications), I came up with the following test program:
#include <iostream>
#include <Windows.h>
#include <vector>
#include <string>
std::string FindNewFile(std::vector<std::string>& vsNewFileList, std::vector<std::string>& vsOldFileList)
{
std::string sNewFileName;
int nScore = 0;
for (auto& newFile : vsNewFileList)
{
nScore = 0;
for (auto& oldFile : vsOldFileList)
if(!newFile.compare(oldFile))
nScore++;
if (nScore!=1)
{
sNewFileName = newFile;
break;
}
}
return sNewFileName;
}
void GetCurrentFilesInDir(std::string sDir, std::vector<std::string>& vsFileList)
{
WIN32_FIND_DATA ffd;
sDir += "\\*";
std::wstring wStr = std::wstring(sDir.begin(), sDir.end());
LPCWSTR lpcwsDir = (LPCWSTR)wStr.c_str();
HANDLE hFind = FindFirstFile(lpcwsDir, &ffd);
if (hFind == INVALID_HANDLE_VALUE)
{
std::cout << "Nope\n";
return;
}
vsFileList.clear();
do
{
int nSize = WideCharToMultiByte(CP_ACP, 0, ffd.cFileName, -1, 0, 0, 0, 0);
char* pcStr = new char[nSize];
WideCharToMultiByte(CP_ACP, 0, ffd.cFileName, -1, pcStr, nSize, 0, 0);
//std::cout << pcStr << "\n";
vsFileList.push_back(std::string(pcStr));
delete[] pcStr;
} while (FindNextFile(hFind, &ffd) != 0);
}
int main()
{
// watch the foo directory for new files
std::string sDir = "C:\\foo";
std::vector<std::string> vsOldFileList, vsNewFileList;
GetCurrentFilesInDir(sDir, vsOldFileList);
std::wstring wStr = std::wstring(sDir.begin(), sDir.end());
LPCWSTR lpcwsDir = (LPCWSTR)wStr.c_str();
DWORD dwWaitStatus;
HANDLE dwChangeHandle;
dwChangeHandle = FindFirstChangeNotification(
lpcwsDir,
FALSE,
FILE_NOTIFY_CHANGE_FILE_NAME);
while (TRUE)
{
// returns multiple times before and after new file appears!!
dwWaitStatus = WaitForSingleObject(dwChangeHandle, INFINITE);
switch(dwWaitStatus)
{
case WAIT_OBJECT_0:
GetCurrentFilesInDir(sDir, vsNewFileList);
std::string sNewFileName = FindNewFile(vsNewFileList, vsOldFileList);
std::cout << sNewFileName << "\n";
GetCurrentFilesInDir(sDir, vsOldFileList);
FindNextChangeNotification(dwChangeHandle);
break;
}
}
}
The problem is that, when I save a new file in C:\foo (for instance, using Notepad++ to "Save As" an open .txt file in C:\foo), the call to WaitForSingleObject() in the while loop will return 0 multiple times. Since my FindNewFile() method returns an empty string if there is no new file in the directory, I will get output like:
a.txt
or:
b.txt
Or even:
c.txt
c.txt
Can someone explain what I am missing here?

Answer 1

Using FindNextChangeNotification can not tell you what actually happened, and the operation of the file may involve multiple changes.
You can try to use ReadDirectoryChangesW and here is a sample:
#include <windows.h>
#include <iostream>
using namespace std;
wstring getname(FILE_NOTIFY_INFORMATION* tmp)
{
wstring s = L"";
for (int i = 0; i < tmp->FileNameLength / 2; i++)
s += tmp->FileName[i];
return s;
}
int main(int argc, const char* argv[])
{
HANDLE hDir;
char notify[1024];
DWORD cbBytes;
LPTSTR path;
FILE_NOTIFY_INFORMATION* pnotify = (FILE_NOTIFY_INFORMATION*)notify;
FILE_NOTIFY_INFORMATION* tmp;
// GetCurrentDirectory(MAX_PATH,path.GetBuffer(MAX_PATH+1));
path = (LPTSTR)L"D:\\test";
hDir = CreateFile(path, FILE_LIST_DIRECTORY,
FILE_SHARE_READ |
FILE_SHARE_WRITE |
FILE_SHARE_DELETE, NULL,
OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS |
FILE_FLAG_OVERLAPPED, NULL);
wcout << L"===CreateFile complete===" << endl;
if (hDir == INVALID_HANDLE_VALUE)
{
wcout << L"invalid handle value" << endl;
return -1;
}
FILE_NOTIFY_INFORMATION buffer[1024];
FILE_NOTIFY_INFORMATION* pbuffer;
while (TRUE)
{
wcout << L"waiting..." << endl;
WaitForSingleObject(hDir, INFINITE);
if (ReadDirectoryChangesW(hDir, &buffer, sizeof(buffer),
TRUE, FILE_NOTIFY_CHANGE_FILE_NAME,
&cbBytes, NULL, NULL))
{
pbuffer = buffer;
do {
tmp = pbuffer;
switch (tmp->Action)
{
case FILE_ACTION_ADDED:
wcout << L"Directory/File added - " << getname(tmp) << endl;
break;
case FILE_ACTION_REMOVED:
wcout << L"Directory/File removed - " << getname(tmp) << endl;
break;
case FILE_ACTION_MODIFIED:
wcout << L"Directory/File modfied - " << getname(tmp) << endl;
break;
case FILE_ACTION_RENAMED_OLD_NAME:
wcout << L"Directory/File old name - " << getname(tmp) << endl;
break;
case FILE_ACTION_RENAMED_NEW_NAME:
wcout << L"Directory/File new name - " << getname(tmp) << endl;
break;
default:
wcout << L"unknown action\n" << endl;
break;
}
pbuffer += pbuffer->NextEntryOffset;
} while (pbuffer->NextEntryOffset);
}
else
{
wcout << "readChangesW failed now return" << endl;
return -1;
}
}
}
When you do the Save As operation, you will find:
Therefore, multiple file operations were triggered when actually saving as, and you also performed multiple comparisons when processing new file comparisons, so empty characters were output.
More reference: FindFirstChangeNotification is notifying about changes twice