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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
       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 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
       historical 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
       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, 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)

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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