| .\" This manpage is Copyright (C) 1992 Drew Eckhardt; |
| .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson. |
| .\" and Copyright (C) 2006, 2014 Michael Kerrisk |
| .\" |
| .\" %%%LICENSE_START(VERBATIM) |
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| .\" manual under the conditions for verbatim copying, provided that the |
| .\" entire resulting derived work is distributed under the terms of a |
| .\" permission notice identical to this one. |
| .\" |
| .\" Since the Linux kernel and libraries are constantly changing, this |
| .\" manual page may be incorrect or out-of-date. The author(s) assume no |
| .\" responsibility for errors or omissions, or for damages resulting from |
| .\" the use of the information contained herein. The author(s) may not |
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| .\" which is licensed free of charge, as they might when working |
| .\" professionally. |
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| .\" Formatted or processed versions of this manual, if unaccompanied by |
| .\" the source, must acknowledge the copyright and authors of this work. |
| .\" %%%LICENSE_END |
| .\" |
| .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu> |
| .\" Modified 1994-08-21 by Michael Haardt |
| .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" Modified 2005-04-04, as per suggestion by Michael Hardt for rename.2 |
| .\" |
| .TH LINK 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| link, linkat \- make a new name for a file |
| .SH SYNOPSIS |
| .nf |
| .B #include <unistd.h> |
| .PP |
| .BI "int link(const char *" oldpath ", const char *" newpath ); |
| .PP |
| .BR "#include <fcntl.h> " "/* Definition of AT_* constants */" |
| .B #include <unistd.h> |
| .PP |
| .BI "int linkat(int " olddirfd ", const char *" oldpath , |
| .BI " int " newdirfd ", const char *" newpath ", int " flags ); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR linkat (): |
| .nf |
| Since glibc 2.10: |
| _POSIX_C_SOURCE >= 200809L |
| Before glibc 2.10: |
| _ATFILE_SOURCE |
| .fi |
| .SH DESCRIPTION |
| .BR link () |
| creates a new link (also known as a hard link) to an existing file. |
| .PP |
| If |
| .I newpath |
| exists, it will |
| .I not |
| be overwritten. |
| .PP |
| This new name may be used exactly as the old one for any operation; |
| both names refer to the same file (and so have the same permissions |
| and ownership) and it is impossible to tell which name was the |
| "original". |
| .SS linkat() |
| The |
| .BR linkat () |
| system call operates in exactly the same way as |
| .BR link (), |
| except for the differences described here. |
| .PP |
| If the pathname given in |
| .I oldpath |
| is relative, then it is interpreted relative to the directory |
| referred to by the file descriptor |
| .I olddirfd |
| (rather than relative to the current working directory of |
| the calling process, as is done by |
| .BR link () |
| for a relative pathname). |
| .PP |
| If |
| .I oldpath |
| is relative and |
| .I olddirfd |
| is the special value |
| .BR AT_FDCWD , |
| then |
| .I oldpath |
| is interpreted relative to the current working |
| directory of the calling process (like |
| .BR link ()). |
| .PP |
| If |
| .I oldpath |
| is absolute, then |
| .I olddirfd |
| is ignored. |
| .PP |
| The interpretation of |
| .I newpath |
| is as for |
| .IR oldpath , |
| except that a relative pathname is interpreted relative |
| to the directory referred to by the file descriptor |
| .IR newdirfd . |
| .PP |
| The following values can be bitwise ORed in |
| .IR flags : |
| .TP |
| .BR AT_EMPTY_PATH " (since Linux 2.6.39)" |
| .\" commit 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3 |
| If |
| .I oldpath |
| is an empty string, create a link to the file referenced by |
| .IR olddirfd |
| (which may have been obtained using the |
| .BR open (2) |
| .B O_PATH |
| flag). |
| In this case, |
| .I olddirfd |
| can refer to any type of file except a directory. |
| This will generally not work if the file has a link count of zero (files |
| created with |
| .BR O_TMPFILE |
| and without |
| .BR O_EXCL |
| are an exception). |
| The caller must have the |
| .BR CAP_DAC_READ_SEARCH |
| capability in order to use this flag. |
| This flag is Linux-specific; define |
| .B _GNU_SOURCE |
| .\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed |
| to obtain its definition. |
| .TP |
| .BR AT_SYMLINK_FOLLOW " (since Linux 2.6.18)" |
| By default, |
| .BR linkat (), |
| does not dereference |
| .I oldpath |
| if it is a symbolic link (like |
| .BR link ()). |
| The flag |
| .B AT_SYMLINK_FOLLOW |
| can be specified in |
| .I flags |
| to cause |
| .I oldpath |
| to be dereferenced if it is a symbolic link. |
| If procfs is mounted, |
| this can be used as an alternative to |
| .BR AT_EMPTY_PATH , |
| like this: |
| .IP |
| .in +4n |
| .EX |
| linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd, |
| newname, AT_SYMLINK_FOLLOW); |
| .EE |
| .in |
| .PP |
| Before kernel 2.6.18, the |
| .I flags |
| argument was unused, and had to be specified as 0. |
| .PP |
| See |
| .BR openat (2) |
| for an explanation of the need for |
| .BR linkat (). |
| .SH RETURN VALUE |
| On success, zero is returned. |
| On error, \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EACCES |
| Write access to the directory containing |
| .I newpath |
| is denied, or search permission is denied for one of the directories |
| in the path prefix of |
| .I oldpath |
| or |
| .IR newpath . |
| (See also |
| .BR path_resolution (7).) |
| .TP |
| .B EDQUOT |
| The user's quota of disk blocks on the filesystem has been exhausted. |
| .TP |
| .B EEXIST |
| .I newpath |
| already exists. |
| .TP |
| .B EFAULT |
| .IR oldpath " or " newpath " points outside your accessible address space." |
| .TP |
| .B EIO |
| An I/O error occurred. |
| .TP |
| .B ELOOP |
| Too many symbolic links were encountered in resolving |
| .IR oldpath " or " newpath . |
| .TP |
| .B EMLINK |
| The file referred to by |
| .I oldpath |
| already has the maximum number of links to it. |
| For example, on an |
| .BR ext4 (5) |
| filesystem that does not employ the |
| .I dir_index |
| feature, the limit on the number of hard links to a file is 65,000; on |
| .BR btrfs (5), |
| the limit is 65,535 links. |
| .TP |
| .B ENAMETOOLONG |
| .IR oldpath " or " newpath " was too long." |
| .TP |
| .B ENOENT |
| A directory component in |
| .IR oldpath " or " newpath |
| does not exist or is a dangling symbolic link. |
| .TP |
| .B ENOMEM |
| Insufficient kernel memory was available. |
| .TP |
| .B ENOSPC |
| The device containing the file has no room for the new directory |
| entry. |
| .TP |
| .B ENOTDIR |
| A component used as a directory in |
| .IR oldpath " or " newpath |
| is not, in fact, a directory. |
| .TP |
| .B EPERM |
| .I oldpath |
| is a directory. |
| .TP |
| .B EPERM |
| The filesystem containing |
| .IR oldpath " and " newpath |
| does not support the creation of hard links. |
| .TP |
| .BR EPERM " (since Linux 3.6)" |
| The caller does not have permission to create a hard link to this file |
| (see the description of |
| .IR /proc/sys/fs/protected_hardlinks |
| in |
| .BR proc (5)). |
| .TP |
| .B EPERM |
| .I oldpath |
| is marked immutable or append-only. |
| (See |
| .BR ioctl_iflags (2).) |
| .TP |
| .B EROFS |
| The file is on a read-only filesystem. |
| .TP |
| .B EXDEV |
| .IR oldpath " and " newpath |
| are not on the same mounted filesystem. |
| (Linux permits a filesystem to be mounted at multiple points, but |
| .BR link () |
| does not work across different mount points, |
| even if the same filesystem is mounted on both.) |
| .PP |
| The following additional errors can occur for |
| .BR linkat (): |
| .TP |
| .B EBADF |
| .I olddirfd |
| or |
| .I newdirfd |
| is not a valid file descriptor. |
| .TP |
| .B EINVAL |
| An invalid flag value was specified in |
| .IR flags . |
| .TP |
| .B ENOENT |
| .B AT_EMPTY_PATH |
| was specified in |
| .IR flags , |
| but the caller did not have the |
| .B CAP_DAC_READ_SEARCH |
| capability. |
| .TP |
| .B ENOENT |
| An attempt was made to link to the |
| .I /proc/self/fd/NN |
| file corresponding to a file descriptor created with |
| .IP |
| .in +4n |
| .EX |
| open(path, O_TMPFILE | O_EXCL, mode); |
| .EE |
| .in |
| .IP |
| See |
| .BR open (2). |
| .TP |
| .B ENOENT |
| An attempt was made to link to a |
| .I /proc/self/fd/NN |
| file corresponding to a file that has been deleted. |
| .TP |
| .B ENOENT |
| .I oldpath |
| is a relative pathname and |
| .I olddirfd |
| refers to a directory that has been deleted, |
| or |
| .I newpath |
| is a relative pathname and |
| .I newdirfd |
| refers to a directory that has been deleted. |
| .TP |
| .B ENOTDIR |
| .I oldpath |
| is relative and |
| .I olddirfd |
| is a file descriptor referring to a file other than a directory; |
| or similar for |
| .I newpath |
| and |
| .I newdirfd |
| .TP |
| .B EPERM |
| .BR AT_EMPTY_PATH |
| was specified in |
| .IR flags , |
| .I oldpath |
| is an empty string, and |
| .IR olddirfd |
| refers to a directory. |
| .SH VERSIONS |
| .BR linkat () |
| was added to Linux in kernel 2.6.16; |
| library support was added to glibc in version 2.4. |
| .SH CONFORMING TO |
| .BR link (): |
| SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008. |
| .\" SVr4 documents additional ENOLINK and |
| .\" EMULTIHOP error conditions; POSIX.1 does not document ELOOP. |
| .\" X/OPEN does not document EFAULT, ENOMEM or EIO. |
| .PP |
| .BR linkat (): |
| POSIX.1-2008. |
| .SH NOTES |
| Hard links, as created by |
| .BR link (), |
| cannot span filesystems. |
| Use |
| .BR symlink (2) |
| if this is required. |
| .PP |
| POSIX.1-2001 says that |
| .BR link () |
| should dereference |
| .I oldpath |
| if it is a symbolic link. |
| However, since kernel 2.0, |
| .\" more precisely: since kernel 1.3.56 |
| Linux does not do so: if |
| .I oldpath |
| is a symbolic link, then |
| .I newpath |
| is created as a (hard) link to the same symbolic link file |
| (i.e., |
| .I newpath |
| becomes a symbolic link to the same file that |
| .I oldpath |
| refers to). |
| Some other implementations behave in the same manner as Linux. |
| .\" For example, the default Solaris compilation environment |
| .\" behaves like Linux, and contributors to a March 2005 |
| .\" thread in the Austin mailing list reported that some |
| .\" other (System V) implementations did/do the same -- MTK, Apr 05 |
| POSIX.1-2008 changes the specification of |
| .BR link (), |
| making it implementation-dependent whether or not |
| .I oldpath |
| is dereferenced if it is a symbolic link. |
| For precise control over the treatment of symbolic links when |
| creating a link, use |
| .BR linkat (). |
| .SS Glibc notes |
| On older kernels where |
| .BR linkat () |
| is unavailable, the glibc wrapper function falls back to the use of |
| .BR link (), |
| unless the |
| .B AT_SYMLINK_FOLLOW |
| is specified. |
| When |
| .I oldpath |
| and |
| .I newpath |
| are relative pathnames, |
| glibc constructs pathnames based on the symbolic links in |
| .IR /proc/self/fd |
| that correspond to the |
| .I olddirfd |
| and |
| .IR newdirfd |
| arguments. |
| .SH BUGS |
| On NFS filesystems, the return code may be wrong in case the NFS server |
| performs the link creation and dies before it can say so. |
| Use |
| .BR stat (2) |
| to find out if the link got created. |
| .SH SEE ALSO |
| .BR ln (1), |
| .BR open (2), |
| .BR rename (2), |
| .BR stat (2), |
| .BR symlink (2), |
| .BR unlink (2), |
| .BR path_resolution (7), |
| .BR symlink (7) |