1LINK(3P) POSIX Programmer's Manual LINK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 link, linkat — link one file to another file
13
15 #include <unistd.h>
16 int link(const char *path1, const char *path2);
18 #include <fcntl.h>
20 int linkat(int fd1, const char *path1, int fd2,
22 const char *path2, int flag);
23
25 The link() function shall create a new link (directory entry) for the
26 existing file, path1.
27 The path1 argument points to a pathname naming an existing file. The
29 path2 argument points to a pathname naming the new directory entry to
30 be created. The link() function shall atomically create a new link for
31 the existing file and the link count of the file shall be incremented
32 by one.
33 If path1 names a directory, link() shall fail unless the process has
35 appropriate privileges and the implementation supports using link() on
36 directories.
37 If path1 names a symbolic link, it is implementation-defined whether
39 link() follows the symbolic link, or creates a new link to the symbolic
40 link itself.
41 Upon successful completion, link() shall mark for update the last file
43 status change timestamp of the file. Also, the last data modification
44 and last file status change timestamps of the directory that contains
45 the new entry shall be marked for update.
46 If link() fails, no link shall be created and the link count of the
48 file shall remain unchanged.
49 The implementation may require that the calling process has permission
51 to access the existing file.
52 The linkat() function shall be equivalent to the link() function except
54 that symbolic links shall be handled as specified by the value of flag
55 (see below) and except in the case where either path1 or path2 or both
56 are relative paths. In this case a relative path path1 is interpreted
57 relative to the directory associated with the file descriptor fd1
58 instead of the current working directory and similarly for path2 and
59 the file descriptor fd2. If the access mode of the open file descrip‐
60 tion associated with the file descriptor is not O_SEARCH, the function
61 shall check whether directory searches are permitted using the current
62 permissions of the directory underlying the file descriptor. If the
63 access mode is O_SEARCH, the function shall not perform the check.
64 Values for flag are constructed by a bitwise-inclusive OR of flags from
66 the following list, defined in <fcntl.h>:
67 AT_SYMLINK_FOLLOW
69 If path1 names a symbolic link, a new link for the target of the
70 symbolic link is created.
71 If linkat() is passed the special value AT_FDCWD in the fd1 or fd2
73 parameter, the current working directory shall be used for the respec‐
74 tive path argument. If both fd1 and fd2 have value AT_FDCWD, the behav‐
75 ior shall be identical to a call to link(), except that symbolic links
76 shall be handled as specified by the value of flag.
77 If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the
79 path1 argument names a symbolic link, a new link is created for the
80 symbolic link path1 and not its target.
81
83 Upon successful completion, these functions shall return 0. Otherwise,
84 these functions shall return -1 and set errno to indicate the error.
85
87 These functions shall fail if:
88 EACCES A component of either path prefix denies search permission, or
90 the requested link requires writing in a directory that denies
91 write permission, or the calling process does not have permis‐
92 sion to access the existing file and this is required by the
93 implementation.
94 EEXIST The path2 argument resolves to an existing directory entry or
96 refers to a symbolic link.
97 ELOOP A loop exists in symbolic links encountered during resolution of
99 the path1 or path2 argument.
100 EMLINK The number of links to the file named by path1 would exceed
102 {LINK_MAX}.
103 ENAMETOOLONG
105 The length of a component of a pathname is longer than
106 {NAME_MAX}.
107 ENOENT A component of either path prefix does not exist; the file named
109 by path1 does not exist; or path1 or path2 points to an empty
110 string.
111 ENOENT or ENOTDIR
113 The path1 argument names an existing non-directory file, and the
114 path2 argument contains at least one non-<slash> character and
115 ends with one or more trailing <slash> characters. If path2
116 without the trailing <slash> characters would name an existing
117 file, an [ENOENT] error shall not occur.
118 ENOSPC The directory to contain the link cannot be extended.
120 ENOTDIR
122 A component of either path prefix names an existing file that is
123 neither a directory nor a symbolic link to a directory, or the
124 path1 argument contains at least one non-<slash> character and
125 ends with one or more trailing <slash> characters and the last
126 pathname component names an existing file that is neither a
127 directory nor a symbolic link to a directory, or the path1 argu‐
128 ment names an existing non-directory file and the path2 argument
129 names a nonexistent file, contains at least one non-<slash>
130 character, and ends with one or more trailing <slash> charac‐
131 ters.
132 EPERM The file named by path1 is a directory and either the calling
134 process does not have appropriate privileges or the implementa‐
135 tion prohibits using link() on directories.
136 EROFS The requested link requires writing in a directory on a read-
138 only file system.
139 EXDEV The link named by path2 and the file named by path1 are on dif‐
141 ferent file systems and the implementation does not support
142 links between file systems.
143 EXDEV path1 refers to a named STREAM.
145 The linkat() function shall fail if:
147 EACCES The access mode of the open file description associated with fd1
149 or fd2 is not O_SEARCH and the permissions of the directory
150 underlying fd1 or fd2, respectively, do not permit directory
151 searches.
152 EBADF The path1 or path2 argument does not specify an absolute path
154 and the fd1 or fd2 argument, respectively, is neither AT_FDCWD
155 nor a valid file descriptor open for reading or searching.
156 ENOTDIR
158 The path1 or path2 argument is not an absolute path and fd1 or
159 fd2, respectively, is a file descriptor associated with a non-
160 directory file.
161 These functions may fail if:
163 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
165 resolution of the path1 or path2 argument.
166 ENAMETOOLONG
168 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
169 tion of a symbolic link produced an intermediate result with a
170 length that exceeds {PATH_MAX}.
171 The linkat() function may fail if:
173 EINVAL The value of the flag argument is not valid.
175 The following sections are informative.
177
179 Creating a Link to a File
180 The following example shows how to create a link to a file named
181 /home/cnd/mod1 by creating a new directory entry named /modules/pass1.
182 #include <unistd.h>
185 char *path1 = "/home/cnd/mod1";
187 char *path2 = "/modules/pass1";
188 int status;
189 ...
190 status = link (path1, path2);
191 Creating a Link to a File Within a Program
193 In the following program example, the link() function links the
194 /etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd
195 (defined as SAVEFILE), which is used to save the current password file.
196 Then, after removing the current password file (defined as PASSWDFILE),
197 the new password file is saved as the current password file using the
198 link() function again.
199 #include <unistd.h>
202 #define LOCKFILE "/etc/ptmp"
204 #define PASSWDFILE "/etc/passwd"
205 #define SAVEFILE "/etc/opasswd"
206 ...
207 /* Save current password file */
208 link (PASSWDFILE, SAVEFILE);
209 /* Remove current password file. */
211 unlink (PASSWDFILE);
212 /* Save new password file as current password file. */
214 link (LOCKFILE,PASSWDFILE);
215
217 Some implementations do allow links between file systems.
218 If path1 refers to a symbolic link, application developers should use
220 linkat() with appropriate flags to select whether or not the symbolic
221 link should be resolved.
222
224 Linking to a directory is restricted to the superuser in most histori‐
225 cal implementations because this capability may produce loops in the
226 file hierarchy or otherwise corrupt the file system. This volume of
227 POSIX.1‐2017 continues that philosophy by prohibiting link() and
228 unlink() from doing this. Other functions could do it if the implemen‐
229 tor designed such an extension.
230 Some historical implementations allow linking of files on different
232 file systems. Wording was added to explicitly allow this optional
233 behavior.
234 The exception for cross-file system links is intended to apply only to
236 links that are programmatically indistinguishable from ``hard'' links.
237 The purpose of the linkat() function is to link files in directories
239 other than the current working directory without exposure to race con‐
240 ditions. Any part of the path of a file could be changed in parallel to
241 a call to link(), resulting in unspecified behavior. By opening a file
242 descriptor for the directory of both the existing file and the target
243 location and using the linkat() function it can be guaranteed that the
244 both filenames are in the desired directories.
245 The AT_SYMLINK_FOLLOW flag allows for implementing both common behav‐
247 iors of the link() function. The POSIX specification requires that if
248 path1 is a symbolic link, a new link for the target of the symbolic
249 link is created. Many systems by default or as an alternative provide a
250 mechanism to avoid the implicit symbolic link lookup and create a new
251 link for the symbolic link itself.
252 Earlier versions of this standard specified only the link() function,
254 and required it to behave like linkat() with the AT_SYMLINK_FOLLOW
255 flag. However, historical practice from SVR4 and Linux kernels had
256 link() behaving like linkat() with no flags, and many systems that
257 attempted to provide a conforming link() function did so in a way that
258 was rarely used, and when it was used did not conform to the standard
259 (e.g., by not being atomic, or by dereferencing the symbolic link
260 incorrectly). Since applications could not rely on link() following
261 links in practice, the linkat() function was added taking a flag to
262 specify the desired behavior for the application.
263
265 None.
266
268 rename(), symlink(), unlink()
269 The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <unistd.h>
271
273 Portions of this text are reprinted and reproduced in electronic form
274 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
275 table Operating System Interface (POSIX), The Open Group Base Specifi‐
276 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
277 Electrical and Electronics Engineers, Inc and The Open Group. In the
278 event of any discrepancy between this version and the original IEEE and
279 The Open Group Standard, the original IEEE and The Open Group Standard
280 is the referee document. The original Standard can be obtained online
281 at http://www.opengroup.org/unix/online.html .
282 Any typographical or formatting errors that appear in this page are
284 most likely to have been introduced during the conversion of the source
285 files to man page format. To report such errors, see https://www.ker‐
286 nel.org/doc/man-pages/reporting_bugs.html .
287IEEE/The Open Group 2017 LINK(3P)