O_TMPFILE (since Linux 3.11)
              Create an unnamed temporary file.  The pathname argument
              specifies a directory; an unnamed inode will be created in
              that directory's filesystem.  Anything written to the
              resulting file will be lost when the last file descriptor is
              closed, unless the file is given a name.

              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY
              and, optionally, O_EXCL.  If O_EXCL is not specified, then
              linkat(2) can be used to link the temporary file into the
              filesystem, making it permanent, using code like the

                  char path[PATH_MAX];
                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                                          S_IRUSR | S_IWUSR);

                  /* File I/O on 'fd'... */

                  snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",

              In this case, the open() mode argument determines the file
              permission mode, as with O_CREAT.

              Specifying O_EXCL in conjunction with O_TMPFILE prevents a
              temporary file from being linked into the filesystem in the
              above manner.  (Note that the meaning of O_EXCL in this case
              is different from the meaning of O_EXCL otherwise.)

              There are two main use cases for O_TMPFILE:

              *  Improved tmpfile(3) functionality: race-free creation of
                 temporary files that (1) are automatically deleted when
                 closed; (2) can never be reached via any pathname; (3) are
                 not subject to symlink attacks; and (4) do not require the
                 caller to devise unique names.

              *  Creating a file that is initially invisible, which is then
                 populated with data and adjusted to have appropriate
                 filesystem attributes (chown(2), chmod(2), fsetxattr(2),
                 etc.)  before being atomically linked into the filesystem
                 in a fully formed state (using linkat(2) as described

              O_TMPFILE requires support by the underlying filesystem; only
              a subset of Linux filesystems provide that support.  In the
              initial implementation, support was provided in the ext2,
              ext3, ext4, UDF, Minix, and shmem filesystems.  XFS support
              was added in Linux 3.15.

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