Wait for file to be unlocked - Windows

Question

I'm writing a TFTP server program for university, which needs exclusive access to the files it opens for reading. Thus it can be configured that if a file is locked by another process that it waits for the file to become unlocked.

Is there any way on Win32 to wait for a file become unlocked without creating a handle for it first?

The reason I ask, is that if another process calls CreateFile() with a dwShareMode that is incompatible to the one my process uses, I won't even be able to get a file handle to use for waiting on the lock using LockFileEx().

Thanks for your help in advance!

Answer 1

As tools from MS like OH and Process Explorer do it, it is definitely possible to get all the handles opened by a process. From there to wait on what you'd like the road is still long, but it is a beginning :)

If you have no success with the Win32 API, one place to look at is for sure the NT Native API http://en.wikipedia.org/wiki/Native_API

You can start from here http://msdn.microsoft.com/en-us/library/windows/desktop/ms724509%28v=vs.85%29.aspx and see if it works with the SystemProcessInformation flag. Look also here for a start http://nsylvain.blogspot.com/2007/09/how-list-all-open-handles.html

The native API is poorly documented, but you can find resources online (like here http://www.osronline.com/article.cfm?id=91)

As a disclaimer, I should add that the Native API is somehow "internal", and therefore subject to change on future versions. Some functions, however, are exposed also publicly in the DDK, at kernel level, so the likelihood of these functions to change is low.

Good luck!

Answer 2

If you take a look at the Stack Overflow questions What Win32 API can be used to find the process that has a given file open? and SYSTEM_HANDLE_INFORMATION structure, you will find links to code that can be used to enumerate processes and all open handles of each running process. This information can be used to obtain a HANDLE to the process that has the file open as well as its HANDLE for the file. You would then use DuplicateHandle() to create a copy of the file HANDLE, but in the TFTP process' handle table. The duplicated HANDLE could then be used by the TFTP process with LockFileEx().

This solution relies on an internal function, NtQuerySystemInformation(), and an undocumented system information class value that can be used to enumerate open handles. Note that this feature of NtQuerySystemInformation() "may be altered or unavailable in future versions of Windows". You might want to use an SEH handler to guard against access violations were that to happen.