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

Answer 1

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.