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
13 link, linkat — link one file to another file relative to two directory
14 file descriptors
15
17 #include <unistd.h>
18 int link(const char *path1, const char *path2);
20 int linkat(int fd1, const char *path1, int fd2,
21 const char *path2, int flag);
22
24 The link() function shall create a new link (directory entry) for the
25 existing file, path1.
26 The path1 argument points to a pathname naming an existing file. The
28 path2 argument points to a pathname naming the new directory entry to
29 be created. The link() function shall atomically create a new link for
30 the existing file and the link count of the file shall be incremented
31 by one.
32 If path1 names a directory, link() shall fail unless the process has
34 appropriate privileges and the implementation supports using link() on
35 directories.
36 If path1 names a symbolic link, it is implementation-defined whether
38 link() follows the symbolic link, or creates a new link to the symbolic
39 link itself.
40 Upon successful completion, link() shall mark for update the last file
42 status change timestamp of the file. Also, the last data modification
43 and last file status change timestamps of the directory that contains
44 the new entry shall be marked for update.
45 If link() fails, no link shall be created and the link count of the
47 file shall remain unchanged.
48 The implementation may require that the calling process has permission
50 to access the existing file.
51 The linkat() function shall be equivalent to the link() function except
53 that symbolic links shall be handled as specified by the value of flag
54 (see below) and except in the case where either path1 or path2 or both
55 are relative paths. In this case a relative path path1 is interpreted
56 relative to the directory associated with the file descriptor fd1
57 instead of the current working directory and similarly for path2 and
58 the file descriptor fd2. If the file descriptor was opened without
59 O_SEARCH, the function shall check whether directory searches are per‐
60 mitted using the current permissions of the directory underlying the
61 file descriptor. If the file descriptor was opened with O_SEARCH, the
62 function shall not perform the check.
63 Values for flag are constructed by a bitwise-inclusive OR of flags from
65 the following list, defined in <fcntl.h>:
66 AT_SYMLINK_FOLLOW
68 If path1 names a symbolic link, a new link for the target of the
69 symbolic link is created.
70 If linkat() is passed the special value AT_FDCWD in the fd1 or fd2
72 parameter, the current working directory shall be used for the respec‐
73 tive path argument. If both fd1 and fd2 have value AT_FDCWD, the behav‐
74 ior shall be identical to a call to link(), except that symbolic links
75 shall be handled as specified by the value of flag.
76 If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the
78 path1 argument names a symbolic link, a new link is created for the
79 symbolic link path1 and not its target.
80
82 Upon successful completion, these functions shall return 0. Otherwise,
83 these functions shall return −1 and set errno to indicate the error.
84
86 These functions shall fail if:
87 EACCES A component of either path prefix denies search permission, or
89 the requested link requires writing in a directory that denies
90 write permission, or the calling process does not have permis‐
91 sion to access the existing file and this is required by the
92 implementation.
93 EEXIST The path2 argument resolves to an existing directory entry or
95 refers to a symbolic link.
96 ELOOP A loop exists in symbolic links encountered during resolution of
98 the path1 or path2 argument.
99 EMLINK The number of links to the file named by path1 would exceed
101 {LINK_MAX}.
102 ENAMETOOLONG
104 The length of a component of a pathname is longer than
105 {NAME_MAX}.
106 ENOENT A component of either path prefix does not exist; the file named
108 by path1 does not exist; or path1 or path2 points to an empty
109 string.
110 ENOSPC The directory to contain the link cannot be extended.
112 ENOTDIR
114 A component of either path prefix names an existing file that is
115 neither a directory nor a symbolic link to a directory, or the
116 path1 argument contains at least one non-<slash> character and
117 ends with one or more trailing <slash> characters and the last
118 pathname component names an existing file that is neither a
119 directory nor a symbolic link to a directory, or the path1 argu‐
120 ment names an existing non-directory file and the path2 argument
121 names a nonexistent file, contains at least one non-<slash>
122 character, and ends with one or more trailing <slash> charac‐
123 ters.
124 EPERM The file named by path1 is a directory and either the calling
126 process does not have appropriate privileges or the implementa‐
127 tion prohibits using link() on directories.
128 EROFS The requested link requires writing in a directory on a read-
130 only file system.
131 EXDEV The link named by path2 and the file named by path1 are on dif‐
133 ferent file systems and the implementation does not support
134 links between file systems.
135 EXDEV path1 refers to a named STREAM.
137 The linkat() function shall fail if:
139 EBADF The path1 or path2 argument does not specify an absolute path
141 and the fd1 or fd2 argument, respectively, is neither AT_FDCWD
142 nor a valid file descriptor open for reading or searching.
143 ENOTDIR
145 The path1 or path2 argument is not an absolute path and fd1 or
146 fd2, respectively, is a file descriptor associated with a non-
147 directory file.
148 These functions may fail if:
150 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
152 resolution of the path1 or path2 argument.
153 ENAMETOOLONG
155 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
156 tion of a symbolic link produced an intermediate result with a
157 length that exceeds {PATH_MAX}.
158 The linkat() function may fail if:
160 EINVAL The value of the flag argument is not valid.
162 The following sections are informative.
164
166 Creating a Link to a File
167 The following example shows how to create a link to a file named
168 /home/cnd/mod1 by creating a new directory entry named /modules/pass1.
169 #include <unistd.h>
171 char *path1 = "/home/cnd/mod1";
173 char *path2 = "/modules/pass1";
174 int status;
175 ...
176 status = link (path1, path2);
177 Creating a Link to a File Within a Program
179 In the following program example, the link() function links the
180 /etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd
181 (defined as SAVEFILE), which is used to save the current password file.
182 Then, after removing the current password file (defined as PASSWDFILE),
183 the new password file is saved as the current password file using the
184 link() function again.
185 #include <unistd.h>
187 #define LOCKFILE "/etc/ptmp"
189 #define PASSWDFILE "/etc/passwd"
190 #define SAVEFILE "/etc/opasswd"
191 ...
192 /* Save current password file */
193 link (PASSWDFILE, SAVEFILE);
194 /* Remove current password file. */
196 unlink (PASSWDFILE);
197 /* Save new password file as current password file. */
199 link (LOCKFILE,PASSWDFILE);
200
202 Some implementations do allow links between file systems.
203 If path1 refers to a symbolic link, application developers should use
205 linkat() with appropriate flags to select whether or not the symbolic
206 link should be resolved.
207
209 Linking to a directory is restricted to the superuser in most histori‐
210 cal implementations because this capability may produce loops in the
211 file hierarchy or otherwise corrupt the file system. This volume of
212 POSIX.1‐2008 continues that philosophy by prohibiting link() and
213 unlink() from doing this. Other functions could do it if the implemen‐
214 tor designed such an extension.
215 Some historical implementations allow linking of files on different
217 file systems. Wording was added to explicitly allow this optional
218 behavior.
219 The exception for cross-file system links is intended to apply only to
221 links that are programmatically indistinguishable from ``hard'' links.
222 The purpose of the linkat() function is to link files in directories
224 other than the current working directory without exposure to race con‐
225 ditions. Any part of the path of a file could be changed in parallel to
226 a call to link(), resulting in unspecified behavior. By opening a file
227 descriptor for the directory of both the existing file and the target
228 location and using the linkat() function it can be guaranteed that the
229 both filenames are in the desired directories.
230 The AT_SYMLINK_FOLLOW flag allows for implementing both common behav‐
232 iors of the link() function. The POSIX specification requires that if
233 path1 is a symbolic link, a new link for the target of the symbolic
234 link is created. Many systems by default or as an alternative provide a
235 mechanism to avoid the implicit symbolic link lookup and create a new
236 link for the symbolic link itself.
237 Earlier versions of this standard specified only the link() function,
239 and required it to behave like linkat() with the AT_SYMLINK_FOLLOW
240 flag. However, historical practice from SVR4 and Linux kernels had
241 link() behaving like linkat() with no flags, and many systems that
242 attempted to provide a conforming link() function did so in a way that
243 was rarely used, and when it was used did not conform to the standard
244 (e.g., by not being atomic, or by dereferencing the symbolic link
245 incorrectly). Since applications could not rely on link() following
246 links in practice, the linkat() function was added taking a flag to
247 specify the desired behavior for the application.
248
250 None.
251
253 rename(), symlink(), unlink()
254 The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <unistd.h>
256
258 Portions of this text are reprinted and reproduced in electronic form
259 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
260 -- Portable Operating System Interface (POSIX), The Open Group Base
261 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
262 cal and Electronics Engineers, Inc and The Open Group. (This is
263 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
264 event of any discrepancy between this version and the original IEEE and
265 The Open Group Standard, the original IEEE and The Open Group Standard
266 is the referee document. The original Standard can be obtained online
267 at http://www.unix.org/online.html .
268 Any typographical or formatting errors that appear in this page are
270 most likely to have been introduced during the conversion of the source
271 files to man page format. To report such errors, see https://www.ker‐
272 nel.org/doc/man-pages/reporting_bugs.html .
273IEEE/The Open Group 2013 LINK(3P)