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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)