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LINK(3P)		   POSIX Programmer's Manual		      LINK(3P)

PROLOG
       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 - link to a file

SYNOPSIS
       #include <unistd.h>

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

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.

       Upon  successful	 completion, link() shall mark for update the st_ctime
       field of the file. Also, the st_ctime and st_mtime fields of the direc‐
       tory 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.

RETURN VALUE
       Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
       returned and errno set to indicate the error.

ERRORS
       The link() function 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  permis‐
	      sion  to	access	the  existing file and this is required by the
	      implementation.

       EEXIST The path2 argument resolves to an existing file 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 the path1 or path2 argument exceeds {PATH_MAX} or
	      a pathname component 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 is not a directory.

       EPERM  The file named by path1 is a directory and  either  the  calling
	      process  does not have appropriate privileges or the implementa‐
	      tion 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 dif‐
	      ferent file systems and  the  implementation  does  not  support
	      links between file systems.

       EXDEV  path1 refers to a named STREAM.

       The link() function may fail if:

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

       ENAMETOOLONG
	      As a result of encountering a symbolic link in resolution of the
	      path1  or path2 argument, the length of the substituted pathname
	      string exceeded {PATH_MAX}.

       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.

RATIONALE
       Linking to a directory is restricted to the superuser in most  histori‐
       cal  implementations  because  this capability may produce loops in the
       file hierarchy or otherwise corrupt the file system.   This  volume  of
       IEEE Std 1003.1-2001  continues	that  philosophy by prohibiting link()
       and unlink() from doing this. Other functions could do it if the imple‐
       mentor 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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       symlink(),    unlink(),	  the	 Base	 Definitions	 volume	    of
       IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003	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 .

IEEE/The Open Group		     2003			      LINK(3P)
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