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

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 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, 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_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.

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)).

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(2).

   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.

COLOPHON

       This page is part of release 4.04 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/.