NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       link, linkat - make a new name for a file

SYNOPSIS         top

       #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         top

       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 differences 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 interpreted 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 interpreted 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, olddirfd 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         top

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

ERRORS         top

       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 symbolic 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 different 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         top

       linkat() was added to Linux in kernel 2.6.16; library support was
       added to glibc in version 2.4.

CONFORMING TO         top

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

       linkat(): POSIX.1-2008.

NOTES         top

       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.  However, 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 symbolic 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         top

       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         top

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

COLOPHON         top

       This page is part of release 3.75 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)