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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:
             _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 except a directory.  This will generally  not  work
            if  the file has a link count of zero (files created with O_TMP-
            FILE 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
            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/pro-
            tected_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  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
     behave  in the same manner as Linux.  POSIX.1-2008 changes the specifi-
     cation 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 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.