Ubuntu Manpage: link, linkat - make a new name for a file

NAME

       link, linkat - make a new name for a file

LIBRARY

       Standard C library (libc, -lc)

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 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 except 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 Linux 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 to indicate  the
       error.

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_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. 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/protected_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 mounts, even if the same filesystem is mounted on both.)

The following additional errors can occur for linkat():

EBADF oldpath (newpath) is relative but olddirfd (newdirfd) is neither AT_FDCWD nor 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 An attempt was made to link to a /proc/self/fd/NN file corresponding to a file that
has been deleted.

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

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

STANDARDS

       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 symlink(2) if this is
       required.

       POSIX.1-2001 says that link() should  dereference  oldpath  if  it  is  a  symbolic  link.
       However,  since  Linux  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().

   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

       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.