link(2) — Linux manual page

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

LINK(2)                 Linux Programmer's Manual                LINK(2)

NAME         top

       link, linkat - make a new name for a file

SYNOPSIS         top

       #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         top

       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         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       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 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         top

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

CONFORMING TO         top

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

       linkat(): POSIX.1-2008.

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                        LINK(2)

Pages that refer to this page: link(1)ln(1)fcntl(2)open(2)rename(2)symlink(2)syscalls(2)unlink(2)remove(3)capabilities(7)inode(7)inotify(7)signal-safety(7)symlink(7)