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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

SYNOPSIS         top

       #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         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 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         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.

       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         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.1‐2017 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         top

       None.

SEE ALSO         top

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

       The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
       unistd.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: unistd.h(0p)link(1p)ln(1p)fstatvfs(3p)open(3p)rename(3p)symlink(3p)unlink(3p)