How to check if a file is already open by another process in C?

Question

I see that standard C has no way of telling if a file is already opened in another process. So the answer should contain several examples for each platform. I need that chec

Answer 1

Getting the open_files information is DIFFICULT, it's like pulling teeth, and if you don't have an immediate need for it you shouldn't be asking for "several examples for each platform" just for the hell of it. Just my opinion, of course.

Linux and many Unix systems have a system utility called lsof which finds open file handles and stuff. The way it does so is by accessing /dev/kmem, which is a pseudo-file containing a copy of "live" kernel memory, i.e. the working storage of the operating system kernel. There are tables of open files in there, naturally, and the memory structure is open-source and documented, so it's just a matter of a lot of busywork for lsof to go in there, find the information and format it for the user.

Documentation for the deep innards of Windows, on the other hand, is practically nonexistent, and I'm not aware that the data structures are somehow exposed to the outside. I'm no Windows expert, but unless the Windows API explicitly offers this kind of information it may simply not be available.

Whatever is available is probably being used by Mark Russinovich's SysInternals utilities; the first one that comes to mind is FileMon. Looking at those may give you some clues. Update: I've just been informed that SysInternals Handles.exe is even closer to what you want.

If you manage to figure that out, good; otherwise you may be interested in catching file open/close operations as they happen: The Windows API offers a generous handful of so-called Hooks: http://msdn.microsoft.com/en-us/library/ms997537.aspx. Hooks allow you to request notification when certain things happen in the system. I believe there's one that will tell you when a program –systemwide– opens a file. So you can make your own list of files opened for the duration you're listening to your hooks. I don't know for sure but I suspect this may be what FileMon does.

The Windows API, including the hook functions, can be accessed from C. Systemwide hooks will require you to create a DLL to be loaded alongside your program.

Hope these hints help you get started.