Waiting until a file is available for reading with

Question

I'm watching a directory by calling ReadDirectoryChangesW synchronously. When a new file is available, I try to access it immediately with CreateFile with GENERIC_READ and FILE_SHARE_READ, but this gives me ERROR_SHARING_VIOLATION. The process that put the file in the watched directory does not finish writing by the time I try to read it.

Is there any way to reliably wait until the file is available for reading? I can put the method into a loop like the one below, but I'm hoping there's a better way.

while ((hFile = CreateFile (path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL)) == INVALID_HANDLE_VALUE)
{
    if (GetLastError() == ERROR_SHARING_VIOLATION)
        Sleep (500);
    else
        break; // some other error occurred
}

if (hFile == INVALID_HANDLE_VALUE)
{
    // deal with other error
    return 0;
}

ReadFile (...);

Answer 1

If you know something about how the file is created, maybe wait until the file stops growing for X seconds, or wait until a sentinel file is deleted. Or sense the state of the program which creates them.

Answer 2

There's no user-mode API for notifications on a closed file that I'm aware of. The loop you've proposed is really probably the best way. The only other thing you could do would be to watch for CloseFile in a filter driver ala Process Monitor, but yuck...

Answer 3

I don't think there is a notification for the kind of event you're looking for, but as an improvement, I'd suggest progressive delays. This way you will get fast response times for stuff like a drag/drop and won't hog the CPU with a tight loop if the user keeps the file open for an hour in Excel.

int delay= 10;
while ((hFile = CreateFile (path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL)) == INVALID_HANDLE_VALUE)
{
    if (GetLastError() == ERROR_SHARING_VIOLATION) {
        Sleep (delay);
        if (delay<5120) // max delay approx 5.Sec
            delay*= 2;
    }
    else
        break; // some other error occurred
}

Answer 4

As @Matt Davis said, there is unfortunately no user-mode API but there is a workaround that depending on your use-case (I've written mine below) may do just what you want.

What worked for me in the past was registering for FILE_NOTIFY_CHANGE_LAST_WRITE instead of FILE_NOTIFY_CHANGE_FILE_NAME when calling ReadDirectoryChangesW:

ZeroMemory(&overlapped, sizeof(OVERLAPPED));
overlapped.hEvent = hChangeEvent;

// ...    
ReadDirectoryChangesW(hSpoolPath,
                      eventBuffer,
                      EVENT_BUF_LENGTH,
                      FALSE,
                      FILE_NOTIFY_CHANGE_LAST_WRITE, // <----
                      NULL,
                      &overlapped,
                      NULL);
// ...
HANDLE events[2];

events[0] = hChangeEvent;
events[1] = hCancelEvent;

DWORD wRc = WaitForMultipleObjects(2, events, FALSE, DIRECTORY_WATCH_TIMEOUT);

The last write time gets updated as soon as the owning process closes the handle after creating the file and writing to it.

My use-case was one process that received HTTP-requests via TCP/IP and wrote the HTTP-body into a directory, where another process picked it up as soon as the receiving process was finished writing (and consequently closing the handle) it. The http-server was the only process that wrote to that directory, so I could rely on the create-write-close pattern.