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       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       link, linkat — link one file to another file

SYNOPSIS

       #include <unistd.h>

       int link(const char *path1, const char *path2);

       #include <fcntl.h>

       int linkat(int fd1, const char *path1, int fd2,
           const char *path2, int flag);

DESCRIPTION

       The  link()  function  shall  create  a  new link (directory entry) for the existing file,
       path1.

       The path1 argument points to a pathname naming an existing file. The path2 argument points
       to  a  pathname  naming  the  new directory entry to be created. The link() function shall
       atomically create a new link for the existing file and the link count of the file shall be
       incremented by one.

       If  path1  names  a  directory,  link()  shall  fail  unless  the  process has appropriate
       privileges and the implementation supports using link() on directories.

       If path1 names a symbolic link, it is implementation-defined whether  link()  follows  the
       symbolic link, or creates a new link to the symbolic link itself.

       Upon  successful  completion,  link()  shall  mark  for update the last file status change
       timestamp of the file. Also, the last  data  modification  and  last  file  status  change
       timestamps of the directory that contains the new entry shall be marked for update.

       If  link()  fails,  no  link  shall be created and the link count of the file shall remain
       unchanged.

       The implementation may require that the calling  process  has  permission  to  access  the
       existing file.

       The  linkat()  function  shall  be  equivalent to the link() function except that symbolic
       links shall be handled as specified by the value of flag (see below)  and  except  in  the
       case  where either path1 or path2 or both are relative paths. In this case a relative path
       path1 is interpreted relative to the directory associated with  the  file  descriptor  fd1
       instead  of  the current working directory and similarly for path2 and the file descriptor
       fd2.  If the access mode of the open file description associated with the file  descriptor
       is  not  O_SEARCH, the function shall check whether directory searches are permitted using
       the current permissions of the directory underlying the file  descriptor.  If  the  access
       mode is O_SEARCH, the function shall not perform the check.

       Values  for  flag  are  constructed  by a bitwise-inclusive OR of flags from the following
       list, defined in <fcntl.h>:

       AT_SYMLINK_FOLLOW
             If path1 names a symbolic link, a new link for the target of the  symbolic  link  is
             created.

       If  linkat() is passed the special value AT_FDCWD in the fd1 or fd2 parameter, the current
       working directory shall be used for the respective path argument. If both fd1 and fd2 have
       value  AT_FDCWD, the behavior shall be identical to a call to link(), except that symbolic
       links shall be handled as specified by the value of flag.

       If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the path1 argument names a
       symbolic link, a new link is created for the symbolic link path1 and not its target.

RETURN VALUE

       Upon  successful  completion,  these  functions shall return 0. Otherwise, these functions
       shall return -1 and set errno to indicate the error.

ERRORS

These functions shall fail if:

EACCES A component of either path prefix denies search permission, or the requested link
requires writing in a directory that denies write permission, or the calling
process does not have permission to access the existing file and this is required
by the implementation.

EEXIST The path2 argument resolves to an existing directory entry or refers to a symbolic
link.

ELOOP A loop exists in symbolic links encountered during resolution of the path1 or path2
argument.

EMLINK The number of links to the file named by path1 would exceed {LINK_MAX}.

ENAMETOOLONG
The length of a component of a pathname is longer than {NAME_MAX}.

ENOENT A component of either path prefix does not exist; the file named by path1 does not
exist; or path1 or path2 points to an empty string.

ENOENT or ENOTDIR
The path1 argument names an existing non-directory file, and the path2 argument
contains at least one non-<slash> character and ends with one or more trailing
<slash> characters. If path2 without the trailing <slash> characters would name an
existing file, an [ENOENT] error shall not occur.

ENOSPC The directory to contain the link cannot be extended.

ENOTDIR
A component of either path prefix names an existing file that is neither a
directory nor a symbolic link to a directory, or the path1 argument contains at
least one non-<slash> character and ends with one or more trailing <slash>
characters and the last pathname component names an existing file that is neither a
directory nor a symbolic link to a directory, or the path1 argument names an
existing non-directory file and the path2 argument names a nonexistent file,
contains at least one non-<slash> character, and ends with one or more trailing
<slash> characters.

EPERM The file named by path1 is a directory and either the calling process does not have
appropriate privileges or the implementation prohibits using link() on directories.

EROFS The requested link requires writing in a directory on a read-only file system.

EXDEV The link named by path2 and the file named by path1 are on different file systems
and the implementation does not support links between file systems.

EXDEV path1 refers to a named STREAM.

The linkat() function shall fail if:

EACCES The access mode of the open file description associated with fd1 or fd2 is not
O_SEARCH and the permissions of the directory underlying fd1 or fd2, respectively,
do not permit directory searches.

EBADF The path1 or path2 argument does not specify an absolute path and the fd1 or fd2
argument, respectively, is neither AT_FDCWD nor a valid file descriptor open for
reading or searching.

ENOTDIR
The path1 or path2 argument is not an absolute path and fd1 or fd2, respectively,
is a file descriptor associated with a non-directory file.

These functions may fail if:

ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the
path1 or path2 argument.

ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic
link produced an intermediate result with a length that exceeds {PATH_MAX}.

The linkat() function may fail if:

EINVAL The value of the flag argument is not valid.

The following sections are informative.

EXAMPLES

   Creating a Link to a File
       The  following  example  shows  how  to  create  a  link to a file named /home/cnd/mod1 by
       creating a new directory entry named /modules/pass1.

           #include <unistd.h>

           char *path1 = "/home/cnd/mod1";
           char *path2 = "/modules/pass1";
           int   status;
           ...
           status = link (path1, path2);

   Creating a Link to a File Within a Program
       In the following program example, the link() function links the /etc/passwd file  (defined
       as  PASSWDFILE)  to a file named /etc/opasswd (defined as SAVEFILE), which is used to save
       the current password file. Then, after removing the  current  password  file  (defined  as
       PASSWDFILE),  the new password file is saved as the current password file using the link()
       function again.

           #include <unistd.h>

           #define LOCKFILE "/etc/ptmp"
           #define PASSWDFILE "/etc/passwd"
           #define SAVEFILE "/etc/opasswd"
           ...
           /* Save current password file */
           link (PASSWDFILE, SAVEFILE);

           /* Remove current password file. */
           unlink (PASSWDFILE);

           /* Save new password file as current password file. */
           link (LOCKFILE,PASSWDFILE);

APPLICATION USAGE

       Some implementations do allow links between file systems.

       If path1 refers to a symbolic  link,  application  developers  should  use  linkat()  with
       appropriate flags to select whether or not the symbolic link should be resolved.

RATIONALE

       Linking  to  a directory is restricted to the superuser in most historical implementations
       because this capability may produce loops in the file hierarchy or otherwise  corrupt  the
       file  system.  This volume of POSIX.1‐2017 continues that philosophy by prohibiting link()
       and unlink() from doing this. Other functions could do it if the implementor designed such
       an extension.

       Some  historical implementations allow linking of files on different file systems. Wording
       was added to explicitly allow this optional behavior.

       The exception for cross-file system links is intended to apply  only  to  links  that  are
       programmatically indistinguishable from ``hard'' links.

       The  purpose  of  the  linkat()  function  is  to link files in directories other than the
       current working directory without exposure to race conditions. Any part of the path  of  a
       file  could be changed in parallel to a call to link(), resulting in unspecified behavior.
       By opening a file descriptor for the directory of both the existing file  and  the  target
       location  and using the linkat() function it can be guaranteed that the both filenames are
       in the desired directories.

       The AT_SYMLINK_FOLLOW flag allows for implementing both common  behaviors  of  the  link()
       function.  The  POSIX  specification requires that if path1 is a symbolic link, a new link
       for the target of the symbolic  link  is  created.  Many  systems  by  default  or  as  an
       alternative  provide  a  mechanism to avoid the implicit symbolic link lookup and create a
       new link for the symbolic link itself.

       Earlier versions of this standard specified only the link() function, and required  it  to
       behave  like  linkat()  with the AT_SYMLINK_FOLLOW flag. However, historical practice from
       SVR4 and Linux kernels had link() behaving like linkat() with no flags, and  many  systems
       that  attempted  to  provide  a conforming link() function did so in a way that was rarely
       used, and when it was used did not conform to the standard (e.g., by not being atomic,  or
       by  dereferencing  the  symbolic  link  incorrectly). Since applications could not rely on
       link() following links in practice, the linkat() function  was  added  taking  a  flag  to
       specify the desired behavior for the application.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
       the Institute of Electrical and Electronics Engineers, Inc and The  Open  Group.   In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .