1UNLINK(3P) POSIX Programmer's Manual UNLINK(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 unlink, unlinkat — remove a directory entry relative to directory file
14 descriptor
15
17 #include <unistd.h>
18 int unlink(const char *path);
20 int unlinkat(int fd, const char *path, int flag);
21
23 The unlink() function shall remove a link to a file. If path names a
24 symbolic link, unlink() shall remove the symbolic link named by path
25 and shall not affect any file or directory named by the contents of the
26 symbolic link. Otherwise, unlink() shall remove the link named by the
27 pathname pointed to by path and shall decrement the link count of the
28 file referenced by the link.
29 When the file's link count becomes 0 and no process has the file open,
31 the space occupied by the file shall be freed and the file shall no
32 longer be accessible. If one or more processes have the file open when
33 the last link is removed, the link shall be removed before unlink()
34 returns, but the removal of the file contents shall be postponed until
35 all references to the file are closed.
36 The path argument shall not name a directory unless the process has
38 appropriate privileges and the implementation supports using unlink()
39 on directories.
40 Upon successful completion, unlink() shall mark for update the last
42 data modification and last file status change timestamps of the parent
43 directory. Also, if the file's link count is not 0, the last file sta‐
44 tus change timestamp of the file shall be marked for update.
45 The unlinkat() function shall be equivalent to the unlink() or rmdir()
47 function except in the case where path specifies a relative path. In
48 this case the directory entry to be removed is determined relative to
49 the directory associated with the file descriptor fd instead of the
50 current working directory. If the file descriptor was opened without
51 O_SEARCH, the function shall check whether directory searches are per‐
52 mitted using the current permissions of the directory underlying the
53 file descriptor. If the file descriptor was opened with O_SEARCH, the
54 function shall not perform the check.
55 Values for flag are constructed by a bitwise-inclusive OR of flags from
57 the following list, defined in <fcntl.h>:
58 AT_REMOVEDIR
60 Remove the directory entry specified by fd and path as a direc‐
61 tory, not a normal file.
62 If unlinkat() is passed the special value AT_FDCWD in the fd parameter,
64 the current working directory shall be used and the behavior shall be
65 identical to a call to unlink() or rmdir() respectively, depending on
66 whether or not the AT_REMOVEDIR bit is set in flag.
67
69 Upon successful completion, these functions shall return 0. Otherwise,
70 these functions shall return −1 and set errno to indicate the error. If
71 −1 is returned, the named file shall not be changed.
72
74 These functions shall fail and shall not unlink the file if:
75 EACCES Search permission is denied for a component of the path prefix,
77 or write permission is denied on the directory containing the
78 directory entry to be removed.
79 EBUSY The file named by the path argument cannot be unlinked because
81 it is being used by the system or another process and the imple‐
82 mentation considers this an error.
83 ELOOP A loop exists in symbolic links encountered during resolution of
85 the path argument.
86 ENAMETOOLONG
88 The length of a component of a pathname is longer than
89 {NAME_MAX}.
90 ENOENT A component of path does not name an existing file or path is an
92 empty string.
93 ENOTDIR
95 A component of the path prefix names an existing file that is
96 neither a directory nor a symbolic link to a directory, or the
97 path argument contains at least one non-<slash> character and
98 ends with one or more trailing <slash> characters and the last
99 pathname component names an existing file that is neither a
100 directory nor a symbolic link to a directory.
101 EPERM The file named by path is a directory, and either the calling
103 process does not have appropriate privileges, or the implementa‐
104 tion prohibits using unlink() on directories.
105 EPERM or EACCES
107 The S_ISVTX flag is set on the directory containing the file
108 referred to by the path argument and the process does not sat‐
109 isfy the criteria specified in the Base Definitions volume of
110 POSIX.1‐2008, Section 4.2, Directory Protection.
111 EROFS The directory entry to be unlinked is part of a read-only file
113 system.
114 The unlinkat() function shall fail if:
116 EACCES fd was not opened with O_SEARCH and the permissions of the
118 directory underlying fd do not permit directory searches.
119 EBADF The path argument does not specify an absolute path and the fd
121 argument is neither AT_FDCWD nor a valid file descriptor open
122 for reading or searching.
123 ENOTDIR
125 The path argument is not an absolute path and fd is a file
126 descriptor associated with a non-directory file.
127 EEXIST or ENOTEMPTY
129 The flag parameter has the AT_REMOVEDIR bit set and the path
130 argument names a directory that is not an empty directory, or
131 there are hard links to the directory other than dot or a single
132 entry in dot-dot.
133 ENOTDIR
135 The flag parameter has the AT_REMOVEDIR bit set and path does
136 not name a directory.
137 These functions may fail and not unlink the file if:
139 EBUSY The file named by path is a named STREAM.
141 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
143 resolution of the path argument.
144 ENAMETOOLONG
146 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
147 tion of a symbolic link produced an intermediate result with a
148 length that exceeds {PATH_MAX}.
149 ETXTBSY
151 The entry to be unlinked is the last directory entry to a pure
152 procedure (shared text) file that is being executed.
153 The unlinkat() function may fail if:
155 EINVAL The value of the flag argument is not valid.
157 The following sections are informative.
159
161 Removing a Link to a File
162 The following example shows how to remove a link to a file named
163 /home/cnd/mod1 by removing the entry named /modules/pass1.
164 #include <unistd.h>
166 char *path = "/modules/pass1";
168 int status;
169 ...
170 status = unlink(path);
171 Checking for an Error
173 The following example fragment creates a temporary password lock file
174 named LOCKFILE, which is defined as /etc/ptmp, and gets a file descrip‐
175 tor for it. If the file cannot be opened for writing, unlink() is used
176 to remove the link between the file descriptor and LOCKFILE.
177 #include <sys/types.h>
179 #include <stdio.h>
180 #include <fcntl.h>
181 #include <errno.h>
182 #include <unistd.h>
183 #include <sys/stat.h>
184 #define LOCKFILE "/etc/ptmp"
186 int pfd; /* Integer for file descriptor returned by open call. */
188 FILE *fpfd; /* File pointer for use in putpwent(). */
189 ...
190 /* Open password Lock file. If it exists, this is an error. */
191 if ((pfd = open(LOCKFILE, O_WRONLY| O_CREAT | O_EXCL, S_IRUSR
192 | S_IWUSR | S_IRGRP | S_IROTH)) == -1) {
193 fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
194 exit(1);
195 }
196 /* Lock file created; proceed with fdopen of lock file so that
198 putpwent() can be used.
199 */
200 if ((fpfd = fdopen(pfd, "w")) == NULL) {
201 close(pfd);
202 unlink(LOCKFILE);
203 exit(1);
204 }
205 Replacing Files
207 The following example fragment uses unlink() to discard links to files,
208 so that they can be replaced with new versions of the files. The first
209 call removes the link to LOCKFILE if an error occurs. Successive calls
210 remove the links to SAVEFILE and PASSWDFILE so that new links can be
211 created, then removes the link to LOCKFILE when it is no longer needed.
212 #include <sys/types.h>
214 #include <stdio.h>
215 #include <fcntl.h>
216 #include <errno.h>
217 #include <unistd.h>
218 #include <sys/stat.h>
219 #define LOCKFILE "/etc/ptmp"
221 #define PASSWDFILE "/etc/passwd"
222 #define SAVEFILE "/etc/opasswd"
223 ...
224 /* If no change was made, assume error and leave passwd unchanged. */
225 if (!valid_change) {
226 fprintf(stderr, "Could not change password for user %s\n", user);
227 unlink(LOCKFILE);
228 exit(1);
229 }
230 /* Change permissions on new password file. */
232 chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);
233 /* Remove saved password file. */
235 unlink(SAVEFILE);
236 /* Save current password file. */
238 link(PASSWDFILE, SAVEFILE);
239 /* Remove current password file. */
241 unlink(PASSWDFILE);
242 /* Save new password file as current password file. */
244 link(LOCKFILE,PASSWDFILE);
245 /* Remove lock file. */
247 unlink(LOCKFILE);
248 exit(0);
250
252 Applications should use rmdir() to remove a directory.
253
255 Unlinking a directory is restricted to the superuser in many historical
256 implementations for reasons given in link() (see also rename()).
257 The meaning of [EBUSY] in historical implementations is ``mount point
259 busy''. Since this volume of POSIX.1‐2008 does not cover the system
260 administration concepts of mounting and unmounting, the description of
261 the error was changed to ``resource busy''. (This meaning is used by
262 some device drivers when a second process tries to open an exclusive
263 use device.) The wording is also intended to allow implementations to
264 refuse to remove a directory if it is the root or current working
265 directory of any process.
266 The standard developers reviewed TR 24715‐2006 and noted that LSB-con‐
268 forming implementations may return [EISDIR] instead of [EPERM] when
269 unlinking a directory. A change to permit this behavior by changing the
270 requirement for [EPERM] to [EPERM] or [EISDIR] was considered, but
271 decided against since it would break existing strictly conforming and
272 conforming applications. Applications written for portability to both
273 POSIX.1‐2008 and the LSB should be prepared to handle either error
274 code.
275 The purpose of the unlinkat() function is to remove directory entries
277 in directories other than the current working directory without expo‐
278 sure to race conditions. Any part of the path of a file could be
279 changed in parallel to a call to unlink(), resulting in unspecified
280 behavior. By opening a file descriptor for the target directory and
281 using the unlinkat() function it can be guaranteed that the removed
282 directory entry is located relative to the desired directory.
283
285 None.
286
288 close(), link(), remove(), rename(), rmdir(), symlink()
289 The Base Definitions volume of POSIX.1‐2008, Section 4.2, Directory
291 Protection, <fcntl.h>, <unistd.h>
292
294 Portions of this text are reprinted and reproduced in electronic form
295 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
296 -- Portable Operating System Interface (POSIX), The Open Group Base
297 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
298 cal and Electronics Engineers, Inc and The Open Group. (This is
299 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
300 event of any discrepancy between this version and the original IEEE and
301 The Open Group Standard, the original IEEE and The Open Group Standard
302 is the referee document. The original Standard can be obtained online
303 at http://www.unix.org/online.html .
304 Any typographical or formatting errors that appear in this page are
306 most likely to have been introduced during the conversion of the source
307 files to man page format. To report such errors, see https://www.ker‐
308 nel.org/doc/man-pages/reporting_bugs.html .
309IEEE/The Open Group 2013 UNLINK(3P)