Creating a filename for an opened file descriptor

Question

I have a file on the filesystem. I'm opening the file with the open(2) function to get the file descriptor to that file.

Now I remove the file. But I still have the file descriptor, so I can read and write to that file without problems, because the filesystem will not remove the file allocation of my file until last file descriptor is closed.

But after I remove the file, and while I still hold the file descriptor, can I somehow re-create (re-bind) the filename to that file descriptor? So the file would appear again on the filesystem, so it won't be removed when I close the file descriptor? (all I have is an opened file descriptor and nothing else).

I'm mostly interested if this can be done on macOS (on Linux/glibc it seems to be possible to do by using linkat with the AT_EMPTY_PATH flag).

Answer 1

As you mention, linkat on Linux allows this to be done using AT_EMPTY_PATH, but only for privileged processes. (There’s some discrepancy between the man page and the current implementation; see the commit introducing "flink" and the commit reverting it for details.) It can also be done through /proc.

The macOS version of linkat doesn’t support this, and I’m not aware of any other way of achieving this.

Answer 2

It seems that fclonefileat(2) on macOS 10.12 and later should be able to do the job.