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:
	       _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 except a directory.  This will generally not work
	      if the file has a link count of zero (files created with	O_TMP-
	      FILE 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.  For example, on an ext4(5) filesystem that does
	      not  employ  the	dir_index  feature, the limit on the number of
	      hard links to a file is 65,000; on btrfs(5), the limit is 65,535
	      links.

       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)).

       EPERM  oldpath	 is    marked	 immutable   or	  append-only.	  (See
	      ioctl_iflags(2).)

       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().

   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 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			       LINK(2)