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

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

PROLOG         top

       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         top

       link, linkat — link one file to another file relative to two
       directory file descriptors

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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 file descriptor was
       opened without O_SEARCH, the function shall check whether directory
       searches are permitted using the current permissions of the directory
       underlying the file descriptor. If the file descriptor was opened
       with 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         top

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

ERRORS         top

       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.

       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:

       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         top

   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         top

       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         top

       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.1‐2008 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         top

       None.

SEE ALSO         top

       rename(3p), symlink(3p), unlink(3p)

       The Base Definitions volume of POSIX.1‐2008, fcntl.h(0p),
       unistd.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            LINK(3P)

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