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 (con-
       sult 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 sup-
       ports 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.

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 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}.

       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
              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 argument.

       ENAMETOOLONG
              As a result of encountering a symbolic link in resolution of the path1 or path2 argument, the length of  the  sub-
              stituted 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() func-
       tion 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 historical implementations because this capability may pro-
       duce 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 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.

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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                      LINK(3P)