LINK(2)



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 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  refer-
              enced  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
       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_resolu-
              tion(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/pro-
              tected_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  corre-
              sponding 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 direc-
              tory 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 version 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 sym-
       link(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 specifi-
       cation 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 func-
       tion falls back to the use of link(), unless the  AT_SYMLINK_FOLLOW  is
       specified.  When oldpath and newpath are relative pathnames, glibc con-
       structs 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_resolu-
       tion(7), symlink(7)

COLOPHON
       This page is part of release 3.71 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)

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