ReadDirectoryChangesW rejecting HANDLE accepted by CreateIoCompletionPort - c++

Question

I'm adding functionality to my (Qt-based) application to monitor an arbitrary folder on my Windows system for any activity recursively (something the Qt variant QFileSystemWatcher lacks). After opening the folder with CreatFileW(), I create a completion port to receive the overlapped I/O, and then I queue a read using ReadDirectoryChangesW().
I have placed all of this in the following "simple" Win32 console application to demonstrate (note that the "stdafx.h" header has been modified to include "windows.h", but is otherwise as the Visual Studio 2013 IDE generated it):
#include "stdafx.h"
#define MAX_BUFFER 4096
struct ThreadData
{;
DWORD winerr;
HANDLE handle;
unsigned int flags;
int recursive;
HANDLE completion_port;
CHAR buffer[MAX_BUFFER];
DWORD buffer_len;
OVERLAPPED overlapped;
};
int _tmain(int argc, _TCHAR* argv[])
{
DWORD winerr;
ThreadData td;
td.flags = FILE_NOTIFY_CHANGE_FILE_NAME|
FILE_NOTIFY_CHANGE_DIR_NAME|
FILE_NOTIFY_CHANGE_ATTRIBUTES|
FILE_NOTIFY_CHANGE_SIZE|
FILE_NOTIFY_CHANGE_LAST_WRITE|
FILE_NOTIFY_CHANGE_LAST_ACCESS|
FILE_NOTIFY_CHANGE_CREATION|
FILE_NOTIFY_CHANGE_SECURITY;
td.recursive = 1;
td.completion_port = INVALID_HANDLE_VALUE;
td.handle = INVALID_HANDLE_VALUE;
td.handle = CreateFileW(L"J:\\Font", // arbitrary folder
FILE_LIST_DIRECTORY, // required
FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
NULL,
OPEN_EXISTING,
// Use FILE_FLAG_OVERLAPPED for asynchronous operation with ReadDirectoryChangesW.
FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
NULL);
if(td.handle == INVALID_HANDLE_VALUE)
{
winerr = GetLastError();
CloseHandle(td->handle);
return 0;
}
td.completion_port = CreateIoCompletionPort(td.handle,
td.completion_port,
(ULONG_PTR)td,
0); // max num processors
if(td.completion_port == INVALID_HANDLE_VALUE)
{
winerr = GetLastError();
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}
BOOL rdc = ReadDirectoryChangesW(td.handle,
td.buffer, // read results
MAX_BUFFER,
td.recursive, // watch subdirectories
// NOTE: At least one flag is required!
td.flags, // see Notify Filters below
&td.buffer_len,
&td.overlapped,
NULL); // completion routine
if(rdc == 0)
{
winerr = GetLastError(); // "The handle is invalid. (0x6)"
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}
// Launch thread here to handle completions and trigger new ones
...
// Clean up when the thread is done
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}
The thing to note about this code is that it is modeled after a Python module ("watcher"), written in C, that provides similar functionality to a Python environment. I've used it in Python, and it works as expected with all of the same settings in this C++ fragment.
In the above code, CreateIoCompletionPort() accepts the HANDLE generated by CreateFileW(), but ReadDirectoryChangesW() does not. It returns 0, and GetLastError() is returning "The handle is invalid. (0x6)". I've tried this under both 32- and 64-bit compiles, just in case that made any difference (I was using the 64-bit version of Python). Also, the directory specified doesn't appear to matter: All directories I specify produce the same result, which suggests it's a problem with the settings somewhere.
Is there something in the CreateFileW() call that might cause the HANDLE to be valid for generating a completion port, but would give the ReadDirectoryChangesW() function heartburn?

Answer 1

You are not initializing the I/O Completion Port correctly, and you are not initializing the OVERLAPPED structure at all. ReadDirectoryChangesW() is failing because the OVERLAPPED::hEvent field contains an invalid event object handle. That is the invalid handle that the error code is referring to, not the directory handle.
Try this instead:
#include "stdafx.h"
#define MAX_BUFFER 4096
struct ThreadData
{
DWORD winerr;
HANDLE handle;
DWORD flags;
BOOL recursive;
HANDLE completion_port;
CHAR buffer[MAX_BUFFER];
DWORD buffer_len;
OVERLAPPED overlapped;
};
int _tmain(int argc, _TCHAR* argv[])
{
DWORD winerr;
ThreadData td;
td.flags = FILE_NOTIFY_CHANGE_FILE_NAME|
FILE_NOTIFY_CHANGE_DIR_NAME|
FILE_NOTIFY_CHANGE_ATTRIBUTES|
FILE_NOTIFY_CHANGE_SIZE|
FILE_NOTIFY_CHANGE_LAST_WRITE|
FILE_NOTIFY_CHANGE_LAST_ACCESS|
FILE_NOTIFY_CHANGE_CREATION|
FILE_NOTIFY_CHANGE_SECURITY;
td.recursive = TRUE;
td.completion_port = NULL;
td.handle = CreateFileW(L"J:\\Font", // arbitrary folder
FILE_LIST_DIRECTORY, // required
FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
NULL,
OPEN_EXISTING,
// Use FILE_FLAG_OVERLAPPED for asynchronous operation with ReadDirectoryChangesW.
FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
NULL);
if(td.handle == INVALID_HANDLE_VALUE)
{
winerr = GetLastError();
return 0;
}
td.completion_port = CreateIoCompletionPort(td.handle,
NULL,
(ULONG_PTR)&td,
0); // max num processors
if(td.completion_port == NULL)
{
winerr = GetLastError();
CloseHandle(td.handle);
return 0;
}
ZeroMemory(&td.overlapped, sizeof(td.overlapped)); // <-- add this!
// required if the thread uses GetOverlappedResult()...
// optional if the thread uses GetQueuedCompletionStatus()...
/*
td.overlapped.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if(td.overlapped.hEvent == NULL)
{
winerr = GetLastError();
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}
*/
BOOL rdc = ReadDirectoryChangesW(td.handle,
td.buffer, // read results
MAX_BUFFER,
td.recursive, // watch subdirectories
// NOTE: At least one flag is required!
td.flags, // see Notify Filters below
&td.buffer_len,
&td.overlapped,
NULL); // completion routine
if(rdc == FALSE)
{
winerr = GetLastError();
//CloseHandle(td.overlapped.hEvent);
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}
// Launch thread here to handle completions and trigger new ones
...
// Clean up when the thread is done
//CloseHandle(td.overlapped.hEvent);
CloseHandle(td.completion_port);
CloseHandle(td.handle);
return 0;
}

Answer 2

CreateIoCompletionPort returns NULL on error, not INVALID_HANDLE_VALUE. So your first error is on this line:
td.completion_port = INVALID_HANDLE_VALUE;
It must be this instead:
td.completion_port = NULL;
And this incorrect check:
if(td.completion_port == INVALID_HANDLE_VALUE)
Must be this instead:
if(td.completion_port == NULL)
You get NULL in td.completion_port after CreateIoCompletionPort because the initial value of td.completion_port is invalid. Also, you are incorrectly handling the error case (say try close invalid handles).