The title pretty much says it, on Windows, can I somehow simulate multiple working directories in a multithreaded application by using something similar to openat?
See also this question.
1 Answer
Yes there is, and it's called NtCreateFile() (https://msdn.microsoft.com/en-us/library/bb432380(v=vs.85).aspx) :)
openat() takes an open fd to a base directory from which path operations start. Similarly, you can supply a HANDLE to the ObjectAttributes.RootDirectory of NtCreateFile() to have whatever that directory's path is to be used as a base for relative path operations.
If going direct to the NT kernel API is too much for you and you want to remain within Win32, you can retrieve the current path of the base directory HANDLE easily enough (see https://msdn.microsoft.com/en-us/library/windows/desktop/aa366789(v=vs.85).aspx). If you open the directory without FILE_SHARE_DELETE permissions, nobody will be able to rename it and therefore the path retrieved will never move so long as you keep the HANDLE open.
You then stitch together the retrieved path with the relative path using normal string concatenation.
The NT kernel API approach is the only solution on Windows which allows the base directory HANDLE to be arbitrarily renamed by third party processes and that not cause races in your code. i.e. it's the only equivalent to POSIX openat(). I agree it's very unfortunate that Win32 doesn't make available this facility - in fact, Win32 doesn't make available atomic renames which is a very useful POSIX facility indeed to let you avoid lock files and is something the NT kernel API provides too.
2 Comments
Solomon Ucko
The NT kernel provides lots of stuff for the sake of its historical POSIX subsystem: en.wikipedia.org/wiki/Microsoft_POSIX_subsystem
Lothar
Maybe it does, but without proper documentation and example codes who is going to use it. I still had no time to figure out NtCreateFile and that after 8 years where i could use it for good but still blocking other processes file actions. But MS can't even document shit on their brand new WinUI 3/Windows App SDK.
openatequivalent, anyway. Could you update your question to be specific about what you really want?openatcall on Windows, you'll need to use _get_osfhandle to get a native file handle from a file descriptor, and GetFinalPathNameByHandle to retrieve the pathname. Concatenate the filename and open the file.