linkat

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
              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 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_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 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
              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
       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
       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.

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

COLOPHON
       This page is part of release 5.03 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                             2017-09-15                           LINK(2)