Question

Please note this is not duplicate of File r/w locking and unlink. (The difference – platform. Operations of files like locking and deletion have totally different semantics, thus the sultion would be different).

I have following problem. I want to create a file system based session storage where each session data is stored in simple file named with session ids.

I want following API: write(sid,data,timeout), read(sid,data,timeout), remove(sid)
where sid==file name, Also I want to have some kind of GC that may remove all timed-out sessions.

Quite simple task if you work with single process but absolutly not trivial when working with multiple processes or even over shared folders.

The simplest solution I thought about was:

write/read:
   hanlde=CreateFile
   LockFile(handle)
   read/write data
   UnlockFile(handle)
   CloseHanlde(handle)

GC (for each file in directory)
   hanlde=CreateFile
   LockFile(handle)
   check if timeout occured
     DeleteFile
   UnlockFile(handle)
   CloseHanlde(handle)

But AFIAK I can’t call DeleteFile on opended locked file (unlike in Unix where file locking is
not mandatory and unlink is allowed for opened files.

But if I put DeleteFile outside of Locking loop bad scenario may happen

GC - CreateFile/LockFile/Unlock/CloseHandle,
write - oCreateFile/LockFile/WriteUpdatedData/Unlock/CloseHandle
GC - DeleteFile

Does anybody have an idea how such issue may be solved? Are there any tricks that allow
combine file locking and file removal or make operation on file atomic (Win32)?

Notes:

• I don’t want to use Database,
• I look for a solution for Win32 API for NT 5.01 and above

Thanks.

Answer 1

I don’t really understand how this is supposed to work. However, deleting a file that’s opened by another process is possible. The process that creates the file has to use the FILE_SHARE_DELETE flag for the dwShareMode argument of CreateFile(). A subsequent DeleteFile() call will succeed. The file doesn’t actually get removed from the file system until the last handle on it is closed.