Waiting until a file is available for reading with Win32

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

There's no user-mode API for notifictions 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 2

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 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
}