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



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

NAME
       link, linkat -- link one file to another file

SYNOPSIS
       #include <unistd.h>

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

       #include <fcntl.h>

       int linkat(int fd1, const char *path1, int fd2,
           const char *path2, int flag);

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.

       If path1 names a symbolic link, it is implementation-defined whether
       link() follows the symbolic link, or creates a new link to the symbolic
       link itself.

       Upon successful completion, link() shall mark for update the last file
       status change timestamp of the file. Also, the last data modification
       and last file status change timestamps 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.

       The linkat() function shall be equivalent to the link() function except
       that symbolic links shall be handled as specified by the value of flag
       (see below) and except in the case where either path1 or path2 or both
       are relative paths. In this case a relative path path1 is interpreted
       relative to the directory associated with the file descriptor fd1
       instead of the current working directory and similarly for path2 and
       the file descriptor fd2.  If the access mode of the open file
       description associated with the file descriptor is not O_SEARCH, the
       function shall check whether directory searches are permitted using the
       current permissions of the directory underlying the file descriptor. If
       the access mode is O_SEARCH, the function shall not perform the check.

       Values for flag are constructed by a bitwise-inclusive OR of flags from
       the following list, defined in <fcntl.h>:

       AT_SYMLINK_FOLLOW
             If path1 names a symbolic link, a new link for the target of the
             symbolic link is created.

       If linkat() is passed the special value AT_FDCWD in the fd1 or fd2
       parameter, the current working directory shall be used for the
       respective path argument. If both fd1 and fd2 have value AT_FDCWD, the
       behavior shall be identical to a call to link(), except that symbolic
       links shall be handled as specified by the value of flag.

       If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the
       path1 argument names a symbolic link, a new link is created for the
       symbolic link path1 and not its target.

RETURN VALUE
       Upon successful completion, these functions shall return 0. Otherwise,
       these functions shall return -1 and set errno to indicate the error.

ERRORS
       These functions 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 directory entry 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 a component of a pathname 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.

       ENOENT or ENOTDIR
              The path1 argument names an existing non-directory file, and the
              path2 argument contains at least one non-<slash> character and
              ends with one or more trailing <slash> characters. If path2
              without the trailing <slash> characters would name an existing
              file, an [ENOENT] error shall not occur.

       ENOSPC The directory to contain the link cannot be extended.

       ENOTDIR
              A component of either path prefix names an existing file that is
              neither a directory nor a symbolic link to a directory, or the
              path1 argument contains at least one non-<slash> character and
              ends with one or more trailing <slash> characters and the last
              pathname component names an existing file that is neither a
              directory nor a symbolic link to a directory, or the path1
              argument names an existing non-directory file and the path2
              argument names a nonexistent file, contains at least one
              non-<slash> character, and ends with one or more trailing
              <slash> characters.

       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 linkat() function shall fail if:

       EACCES The access mode of the open file description associated with fd1
              or fd2 is not O_SEARCH and the permissions of the directory
              underlying fd1 or fd2, respectively, do not permit directory
              searches.

       EBADF  The path1 or path2 argument does not specify an absolute path
              and the fd1 or fd2 argument, respectively, is neither AT_FDCWD
              nor a valid file descriptor open for reading or searching.

       ENOTDIR
              The path1 or path2 argument is not an absolute path and fd1 or
              fd2, respectively, is a file descriptor associated with a non-
              directory file.

       These functions may fail if:

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

       ENAMETOOLONG
              The length of a pathname exceeds {PATH_MAX}, or pathname
              resolution of a symbolic link produced an intermediate result
              with a length that exceeds {PATH_MAX}.

       The linkat() function may fail if:

       EINVAL The value of the flag argument is not valid.

       The following sections are informative.

EXAMPLES
   Creating aa Link to aa 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 aa Link to aa File Within aa 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.

       If path1 refers to a symbolic link, application developers should use
       linkat() with appropriate flags to select whether or not the symbolic
       link should be resolved.

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
       POSIX.1-2017 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.

       The purpose of the linkat() function is to link files in directories
       other than the current working directory without exposure to race
       conditions. Any part of the path of a file could be changed in parallel
       to a call to link(), resulting in unspecified behavior. By opening a
       file descriptor for the directory of both the existing file and the
       target location and using the linkat() function it can be guaranteed
       that the both filenames are in the desired directories.

       The AT_SYMLINK_FOLLOW flag allows for implementing both common
       behaviors of the link() function. The POSIX specification requires that
       if path1 is a symbolic link, a new link for the target of the symbolic
       link is created. Many systems by default or as an alternative provide a
       mechanism to avoid the implicit symbolic link lookup and create a new
       link for the symbolic link itself.

       Earlier versions of this standard specified only the link() function,
       and required it to behave like linkat() with the AT_SYMLINK_FOLLOW
       flag. However, historical practice from SVR4 and Linux kernels had
       link() behaving like linkat() with no flags, and many systems that
       attempted to provide a conforming link() function did so in a way that
       was rarely used, and when it was used did not conform to the standard
       (e.g., by not being atomic, or by dereferencing the symbolic link
       incorrectly). Since applications could not rely on link() following
       links in practice, the linkat() function was added taking a flag to
       specify the desired behavior for the application.

FUTURE DIRECTIONS
       None.

SEE ALSO
       rename(), symlink(), unlink()

       The Base Definitions volume of POSIX.1-2017, <fcntl.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              LINK(3)
