LINK(2)                             Linux Programmer's Manual                             LINK(2)



NAME
       link, linkat - make a new name for a file

SYNOPSIS
       #include <unistd.h>

       int link(const char *oldpath, const char *newpath);

       #include <fcntl.h>           /* Definition of AT_* constants */
       #include <unistd.h>

       int linkat(int olddirfd, const char *oldpath,
                  int newdirfd, const char *newpath, int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       linkat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION
       link() creates a new link (also known as a hard link) to an existing file.

       If newpath exists, it will not be overwritten.

       This  new  name  may be used exactly as the old one for any operation; both names refer to
       the same file (and so have the same permissions and ownership) and  it  is  impossible  to
       tell which name was the "original".

   linkat()
       The  linkat()  system call operates in exactly the same way as link(), except for the dif‐
       ferences described here.

       If the pathname given in oldpath is relative, then  it  is  interpreted  relative  to  the
       directory referred to by the file descriptor olddirfd (rather than relative to the current
       working directory of the calling process, as is done by link() for a relative pathname).

       If oldpath is relative and olddirfd is the special value AT_FDCWD, then oldpath is  inter‐
       preted relative to the current working directory of the calling process (like link()).

       If oldpath is absolute, then olddirfd is ignored.

       The interpretation of newpath is as for oldpath, except that a relative pathname is inter‐
       preted relative to the directory referred to by the file descriptor newdirfd.

       The following values can be bitwise ORed in flags:

       AT_EMPTY_PATH (since Linux 2.6.39)
              If oldpath is an empty string, create a link to the  file  referenced  by  olddirfd
              (which  may  have been obtained using the open(2) O_PATH flag).  In this case, old‐
              dirfd can refer to any type of file, not just a directory.  This will generally not
              work if the file has a link count of zero (files created with O_TMPFILE and without
              O_EXCL are an exception).  The caller must have the CAP_DAC_READ_SEARCH  capability
              in  order  to  use  this  flag.  This flag is Linux-specific; define _GNU_SOURCE to
              obtain its definition.

       AT_SYMLINK_FOLLOW (since Linux 2.6.18)
              By default, linkat(), does not dereference oldpath if it is a symbolic  link  (like
              link()).   The flag AT_SYMLINK_FOLLOW can be specified in flags to cause oldpath to
              be dereferenced if it is a symbolic link.  If procfs is mounted, this can  be  used
              as an alternative to AT_EMPTY_PATH, like this:

                  linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
                         newname, AT_SYMLINK_FOLLOW);

       Before kernel 2.6.18, the flags argument was unused, and had to be specified as 0.

       See openat(2) for an explanation of the need for linkat().

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EACCES Write access to the directory containing newpath is denied, or search permission is
              denied for one of the directories in the path prefix of oldpath or  newpath.   (See
              also path_resolution(7).)

       EDQUOT The user's quota of disk blocks on the filesystem has been exhausted.

       EEXIST newpath already exists.

       EFAULT oldpath or newpath points outside your accessible address space.

       EIO    An I/O error occurred.

       ELOOP  Too many symbolic links were encountered in resolving oldpath or newpath.

       EMLINK The file referred to by oldpath already has the maximum number of links to it.

       ENAMETOOLONG
              oldpath or newpath was too long.

       ENOENT A  directory  component  in oldpath or newpath does not exist or is a dangling sym‐
              bolic link.

       ENOMEM Insufficient kernel memory was available.

       ENOSPC The device containing the file has no room for the new directory entry.

       ENOTDIR
              A component used as a directory in oldpath or newpath is not, in fact, a directory.

       EPERM  oldpath is a directory.

       EPERM  The filesystem containing oldpath and newpath does not support the creation of hard
              links.

       EPERM (since Linux 3.6)
              The  caller  does  not  have permission to create a hard link to this file (see the
              description of /proc/sys/fs/protected_hardlinks in proc(5)).

       EROFS  The file is on a read-only filesystem.

       EXDEV  oldpath and newpath are not on the  same  mounted  filesystem.   (Linux  permits  a
              filesystem  to  be mounted at multiple points, but link() does not work across dif‐
              ferent mount points, even if the same filesystem is mounted on both.)

       The following additional errors can occur for linkat():

       EBADF  olddirfd or newdirfd is not a valid file descriptor.

       EINVAL An invalid flag value was specified in flags.

       ENOENT AT_EMPTY_PATH  was  specified  in  flags,  but  the  caller  did   not   have   the
              CAP_DAC_READ_SEARCH capability.

       ENOENT An  attempt  was  made to link to the /proc/self/fd/NN file corresponding to a file
              descriptor created with

                  open(path, O_TMPFILE | O_EXCL, mode);

              See open(2).

       ENOENT oldpath is a relative pathname and olddirfd refers to a  directory  that  has  been
              deleted,  or newpath is a relative pathname and newdirfd refers to a directory that
              has been deleted.

       ENOTDIR
              oldpath is relative and olddirfd is a file descriptor referring  to  a  file  other
              than a directory; or similar for newpath and newdirfd

       EPERM  AT_EMPTY_PATH  was  specified  in  flags,  oldpath is an empty string, and olddirfd
              refers to a directory.

VERSIONS
       linkat() was added to Linux in kernel 2.6.16; library support was added to glibc  in  ver‐
       sion 2.4.

CONFORMING TO
       link(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.

       linkat(): POSIX.1-2008.

NOTES
       Hard  links,  as  created  by  link(), cannot span filesystems.  Use symlink(2) if this is
       required.

       POSIX.1-2001 says that link() should dereference oldpath if it is a symbolic  link.   How‐
       ever,  since kernel 2.0, Linux does not do so: if oldpath is a symbolic link, then newpath
       is created as a (hard) link to the same symbolic link file (i.e., newpath becomes  a  sym‐
       bolic link to the same file that oldpath refers to).  Some other implementations behave in
       the same manner as Linux.  POSIX.1-2008 changes the specification  of  link(),  making  it
       implementation-dependent  whether or not oldpath is dereferenced if it is a symbolic link.
       For precise control over the treatment  of  symbolic  links  when  creating  a  link,  use
       linkat(2).

   Glibc notes
       On  older  kernels where linkat() is unavailable, the glibc wrapper function falls back to
       the use of link(), unless the AT_SYMLINK_FOLLOW is specified.  When  oldpath  and  newpath
       are  relative  pathnames,  glibc  constructs  pathnames  based  on  the  symbolic links in
       /proc/self/fd that correspond to the olddirfd and newdirfd arguments.

BUGS
       On NFS filesystems, the return code may be wrong in case the NFS server performs the  link
       creation and dies before it can say so.  Use stat(2) to find out if the link got created.

SEE ALSO
       ln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolution(7), symlink(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-08-19                                    LINK(2)