1link(2) System Calls Manual link(2)
2
6 link, linkat - make a new name for a file
7
9 Standard C library (libc, -lc)
10
12 #include <unistd.h>
13 int link(const char *oldpath, const char *newpath);
15 #include <fcntl.h> /* Definition of AT_* constants */
17 #include <unistd.h>
18 int linkat(int olddirfd, const char *oldpath,
20 int newdirfd, const char *newpath, int flags);
21 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23 linkat():
25 Since glibc 2.10:
26 _POSIX_C_SOURCE >= 200809L
27 Before glibc 2.10:
28 _ATFILE_SOURCE
29
31 link() creates a new link (also known as a hard link) to an existing
32 file.
33 If newpath exists, it will not be overwritten.
35 This new name may be used exactly as the old one for any operation;
37 both names refer to the same file (and so have the same permissions and
38 ownership) and it is impossible to tell which name was the "original".
39 linkat()
41 The linkat() system call operates in exactly the same way as link(),
42 except for the differences described here.
43 If the pathname given in oldpath is relative, then it is interpreted
45 relative to the directory referred to by the file descriptor olddirfd
46 (rather than relative to the current working directory of the calling
47 process, as is done by link() for a relative pathname).
48 If oldpath is relative and olddirfd is the special value AT_FDCWD, then
50 oldpath is interpreted relative to the current working directory of the
51 calling process (like link()).
52 If oldpath is absolute, then olddirfd is ignored.
54 The interpretation of newpath is as for oldpath, except that a relative
56 pathname is interpreted relative to the directory referred to by the
57 file descriptor newdirfd.
58 The following values can be bitwise ORed in flags:
60 AT_EMPTY_PATH (since Linux 2.6.39)
62 If oldpath is an empty string, create a link to the file refer‐
63 enced by olddirfd (which may have been obtained using the
64 open(2) O_PATH flag). In this case, olddirfd can refer to any
65 type of file except a directory. This will generally not work
66 if the file has a link count of zero (files created with O_TMP‐
67 FILE and without O_EXCL are an exception). The caller must have
68 the CAP_DAC_READ_SEARCH capability in order to use this flag.
69 This flag is Linux-specific; define _GNU_SOURCE to obtain its
70 definition.
71 AT_SYMLINK_FOLLOW (since Linux 2.6.18)
73 By default, linkat(), does not dereference oldpath if it is a
74 symbolic link (like link()). The flag AT_SYMLINK_FOLLOW can be
75 specified in flags to cause oldpath to be dereferenced if it is
76 a symbolic link. If procfs is mounted, this can be used as an
77 alternative to AT_EMPTY_PATH, like this:
78 linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
80 newname, AT_SYMLINK_FOLLOW);
81 Before Linux 2.6.18, the flags argument was unused, and had to be spec‐
83 ified as 0.
84 See openat(2) for an explanation of the need for linkat().
86
88 On success, zero is returned. On error, -1 is returned, and errno is
89 set to indicate the error.
90
92 EACCES Write access to the directory containing newpath is denied, or
93 search permission is denied for one of the directories in the
94 path prefix of oldpath or newpath. (See also path_resolu‐
95 tion(7).)
96 EDQUOT The user's quota of disk blocks on the filesystem has been ex‐
98 hausted.
99 EEXIST newpath already exists.
101 EFAULT oldpath or newpath points outside your accessible address space.
103 EIO An I/O error occurred.
105 ELOOP Too many symbolic links were encountered in resolving oldpath or
107 newpath.
108 EMLINK The file referred to by oldpath already has the maximum number
110 of links to it. For example, on an ext4(5) filesystem that does
111 not employ the dir_index feature, the limit on the number of
112 hard links to a file is 65,000; on btrfs(5), the limit is 65,535
113 links.
114 ENAMETOOLONG
116 oldpath or newpath was too long.
117 ENOENT A directory component in oldpath or newpath does not exist or is
119 a dangling symbolic link.
120 ENOMEM Insufficient kernel memory was available.
122 ENOSPC The device containing the file has no room for the new directory
124 entry.
125 ENOTDIR
127 A component used as a directory in oldpath or newpath is not, in
128 fact, a directory.
129 EPERM oldpath is a directory.
131 EPERM The filesystem containing oldpath and newpath does not support
133 the creation of hard links.
134 EPERM (since Linux 3.6)
136 The caller does not have permission to create a hard link to
137 this file (see the description of /proc/sys/fs/pro‐
138 tected_hardlinks in proc(5)).
139 EPERM oldpath is marked immutable or append-only. (See
141 ioctl_iflags(2).)
142 EROFS The file is on a read-only filesystem.
144 EXDEV oldpath and newpath are not on the same mounted filesystem.
146 (Linux permits a filesystem to be mounted at multiple points,
147 but link() does not work across different mounts, even if the
148 same filesystem is mounted on both.)
149 The following additional errors can occur for linkat():
151 EBADF oldpath (newpath) is relative but olddirfd (newdirfd) is neither
153 AT_FDCWD nor a valid file descriptor.
154 EINVAL An invalid flag value was specified in flags.
156 ENOENT AT_EMPTY_PATH was specified in flags, but the caller did not
158 have the CAP_DAC_READ_SEARCH capability.
159 ENOENT An attempt was made to link to the /proc/self/fd/NN file corre‐
161 sponding to a file descriptor created with
162 open(path, O_TMPFILE | O_EXCL, mode);
164 See open(2).
166 ENOENT An attempt was made to link to a /proc/self/fd/NN file corre‐
168 sponding to a file that has been deleted.
169 ENOENT oldpath is a relative pathname and olddirfd refers to a direc‐
171 tory that has been deleted, or newpath is a relative pathname
172 and newdirfd refers to a directory that has been deleted.
173 ENOTDIR
175 oldpath is relative and olddirfd is a file descriptor referring
176 to a file other than a directory; or similar for newpath and
177 newdirfd
178 EPERM AT_EMPTY_PATH was specified in flags, oldpath is an empty
180 string, and olddirfd refers to a directory.
181
183 POSIX.1-2001 says that link() should dereference oldpath if it is a
184 symbolic link. However, since Linux 2.0, Linux does not do so: if old‐
185 path is a symbolic link, then newpath is created as a (hard) link to
186 the same symbolic link file (i.e., newpath becomes a symbolic link to
187 the same file that oldpath refers to). Some other implementations be‐
188 have in the same manner as Linux. POSIX.1-2008 changes the specifica‐
189 tion of link(), making it implementation-dependent whether or not old‐
190 path is dereferenced if it is a symbolic link. For precise control
191 over the treatment of symbolic links when creating a link, use
192 linkat().
193 glibc
195 On older kernels where linkat() is unavailable, the glibc wrapper func‐
196 tion falls back to the use of link(), unless the AT_SYMLINK_FOLLOW is
197 specified. When oldpath and newpath are relative pathnames, glibc con‐
198 structs pathnames based on the symbolic links in /proc/self/fd that
199 correspond to the olddirfd and newdirfd arguments.
200
202 link() POSIX.1-2008.
203
205 link() SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).
206 linkat()
208 POSIX.1-2008. Linux 2.6.16, glibc 2.4.
209
211 Hard links, as created by link(), cannot span filesystems. Use sym‐
212 link(2) if this is required.
213
215 On NFS filesystems, the return code may be wrong in case the NFS server
216 performs the link creation and dies before it can say so. Use stat(2)
217 to find out if the link got created.
218
220 ln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolu‐
221 tion(7), symlink(7)
222Linux man-pages 6.04 2023-03-30 link(2)