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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 relative to two directory file descriptors

SYNOPSIS

       #include <unistd.h>

       int link(const char *path1, const char *path2);
       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 file descriptor was opened without O_SEARCH, the function shall check whether
       directory searches are permitted using the current permissions of the directory underlying
       the file descriptor. If the file descriptor was opened with 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.

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:

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.1‐2008 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,  2013  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013  by  the
       Institute  of  Electrical  and  Electronics  Engineers,  Inc and The Open Group.  (This is
       POSIX.1-2008 with the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .