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

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

       link - link to a file

       #include <unistd.h>

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

       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 nam-
       ing 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 implementa-
       tion 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 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.

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

       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 permission 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}.

              The length of the path1 or path2 argument exceeds {PATH_MAX} or a  pathname  component  is  longer  than

       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.

              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  privi-
              leges 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 link() function may fail if:

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

              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.

   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 pass-
       word 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);

       Some implementations do allow links between file systems.

       Linking to a directory is restricted to the superuser in most historical implementations because this  capabil-
       ity  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  func-
       tions 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 explic-
       itly allow this optional behavior.

       The exception for cross-file system links is intended to apply only to links that are  programmatically  indis-
       tinguishable from "hard" links.


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

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The original Standard can be
       obtained online at .

IEEE/The Open Group                  2003                             LINK(3P)