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       link, linkat - make a new name for a file


       #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)):

           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:


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

       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".

       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().


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


       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

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

       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

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

              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

              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

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

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


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


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

       linkat(): POSIX.1-2008.


       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

   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.


       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.


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


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