Create a hard link from a file handle on Unix?

Question

If I\'ve got a handle to an open file, is it possible to create a hard link to that file after all references to it have been removed from the filesystem?

For example, s

Answer 1

The newly released linux 3.11 offers a solution to this problem with the new O_TMPFILE open(2) flag. With this flag you can create an "invisible" file (i.e. an inode with no hardlinks) in some file system (specified by a directory in that file system). Then, after the file is fully set up, you can create a hardlink using linkat. It works like this:

fd = open("/tmp", O_TMPFILE | O_RDWR, 0600);
// write something to the file here
// fchown()/fchmod() it
linkat(fd, "", AT_FDCWD, "/tmp/test", AT_EMPTY_PATH);

Note that aside from the >=3.11 kernel requirement, this also requires support from the underlying file system (I tried the above snippet on ext3 and it worked, but it did not seem to work on btrfs).