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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 ex-
	      hausted.

       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  be-
       have  in	the same manner	as Linux.  POSIX.1-2008	changes	the specifica-
       tion of link(), making it implementation-dependent whether or not  old-
       path  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 3.74 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)

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

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