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
12 unlink, unlinkat — remove a directory entry
13
15 #include <unistd.h>
16 int unlink(const char *path);
18 #include <fcntl.h>
20 int unlinkat(int fd, const char *path, int flag);
22
24 The unlink() function shall remove a link to a file. If path names a
25 symbolic link, unlink() shall remove the symbolic link named by path
26 and shall not affect any file or directory named by the contents of the
27 symbolic link. Otherwise, unlink() shall remove the link named by the
28 pathname pointed to by path and shall decrement the link count of the
29 file referenced by the link.
30 When the file's link count becomes 0 and no process has the file open,
32 the space occupied by the file shall be freed and the file shall no
33 longer be accessible. If one or more processes have the file open when
34 the last link is removed, the link shall be removed before unlink()
35 returns, but the removal of the file contents shall be postponed until
36 all references to the file are closed.
37 The path argument shall not name a directory unless the process has
39 appropriate privileges and the implementation supports using unlink()
40 on directories.
41 Upon successful completion, unlink() shall mark for update the last
43 data modification and last file status change timestamps of the parent
44 directory. Also, if the file's link count is not 0, the last file sta‐
45 tus change timestamp of the file shall be marked for update.
46 The unlinkat() function shall be equivalent to the unlink() or rmdir()
48 function except in the case where path specifies a relative path. In
49 this case the directory entry to be removed is determined relative to
50 the directory associated with the file descriptor fd instead of the
51 current working directory. If the access mode of the open file descrip‐
52 tion associated with the file descriptor is not O_SEARCH, the function
53 shall check whether directory searches are permitted using the current
54 permissions of the directory underlying the file descriptor. If the
55 access mode is O_SEARCH, the function shall not perform the check.
56 Values for flag are constructed by a bitwise-inclusive OR of flags from
58 the following list, defined in <fcntl.h>:
59 AT_REMOVEDIR
61 Remove the directory entry specified by fd and path as a direc‐
62 tory, not a normal file.
63 If unlinkat() is passed the special value AT_FDCWD in the fd parameter,
65 the current working directory shall be used and the behavior shall be
66 identical to a call to unlink() or rmdir() respectively, depending on
67 whether or not the AT_REMOVEDIR bit is set in flag.
68
70 Upon successful completion, these functions shall return 0. Otherwise,
71 these functions shall return -1 and set errno to indicate the error. If
72 -1 is returned, the named file shall not be changed.
73
75 These functions shall fail and shall not unlink the file if:
76 EACCES Search permission is denied for a component of the path prefix,
78 or write permission is denied on the directory containing the
79 directory entry to be removed.
80 EBUSY The file named by the path argument cannot be unlinked because
82 it is being used by the system or another process and the imple‐
83 mentation considers this an error.
84 ELOOP A loop exists in symbolic links encountered during resolution of
86 the path argument.
87 ENAMETOOLONG
89 The length of a component of a pathname is longer than
90 {NAME_MAX}.
91 ENOENT A component of path does not name an existing file or path is an
93 empty string.
94 ENOTDIR
96 A component of the path prefix names an existing file that is
97 neither a directory nor a symbolic link to a directory, or the
98 path argument contains at least one non-<slash> character and
99 ends with one or more trailing <slash> characters and the last
100 pathname component names an existing file that is neither a
101 directory nor a symbolic link to a directory.
102 EPERM The file named by path is a directory, and either the calling
104 process does not have appropriate privileges, or the implementa‐
105 tion prohibits using unlink() on directories.
106 EPERM or EACCES
108 The S_ISVTX flag is set on the directory containing the file
109 referred to by the path argument and the process does not sat‐
110 isfy the criteria specified in the Base Definitions volume of
111 POSIX.1‐2017, Section 4.3, Directory Protection.
112 EROFS The directory entry to be unlinked is part of a read-only file
114 system.
115 The unlinkat() function shall fail if:
117 EACCES The access mode of the open file description associated with fd
119 is not O_SEARCH and the permissions of the directory underlying
120 fd do not permit directory searches.
121 EBADF The path argument does not specify an absolute path and the fd
123 argument is neither AT_FDCWD nor a valid file descriptor open
124 for reading or searching.
125 ENOTDIR
127 The path argument is not an absolute path and fd is a file
128 descriptor associated with a non-directory file.
129 EEXIST or ENOTEMPTY
131 The flag parameter has the AT_REMOVEDIR bit set and the path
132 argument names a directory that is not an empty directory, or
133 there are hard links to the directory other than dot or a single
134 entry in dot-dot.
135 ENOTDIR
137 The flag parameter has the AT_REMOVEDIR bit set and path does
138 not name a directory.
139 These functions may fail and not unlink the file if:
141 EBUSY The file named by path is a named STREAM.
143 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
145 resolution of the path argument.
146 ENAMETOOLONG
148 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
149 tion of a symbolic link produced an intermediate result with a
150 length that exceeds {PATH_MAX}.
151 ETXTBSY
153 The entry to be unlinked is the last directory entry to a pure
154 procedure (shared text) file that is being executed.
155 The unlinkat() function may fail if:
157 EINVAL The value of the flag argument is not valid.
159 The following sections are informative.
161
163 Removing a Link to a File
164 The following example shows how to remove a link to a file named
165 /home/cnd/mod1 by removing the entry named /modules/pass1.
166 #include <unistd.h>
169 char *path = "/modules/pass1";
171 int status;
172 ...
173 status = unlink(path);
174 Checking for an Error
176 The following example fragment creates a temporary password lock file
177 named LOCKFILE, which is defined as /etc/ptmp, and gets a file descrip‐
178 tor for it. If the file cannot be opened for writing, unlink() is used
179 to remove the link between the file descriptor and LOCKFILE.
180 #include <sys/types.h>
183 #include <stdio.h>
184 #include <fcntl.h>
185 #include <errno.h>
186 #include <unistd.h>
187 #include <sys/stat.h>
188 #define LOCKFILE "/etc/ptmp"
190 int pfd; /* Integer for file descriptor returned by open call. */
192 FILE *fpfd; /* File pointer for use in putpwent(). */
193 ...
194 /* Open password Lock file. If it exists, this is an error. */
195 if ((pfd = open(LOCKFILE, O_WRONLY| O_CREAT | O_EXCL, S_IRUSR
196 | S_IWUSR | S_IRGRP | S_IROTH)) == -1) {
197 fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
198 exit(1);
199 }
200 /* Lock file created; proceed with fdopen of lock file so that
202 putpwent() can be used.
203 */
204 if ((fpfd = fdopen(pfd, "w")) == NULL) {
205 close(pfd);
206 unlink(LOCKFILE);
207 exit(1);
208 }
209 Replacing Files
211 The following example fragment uses unlink() to discard links to files,
212 so that they can be replaced with new versions of the files. The first
213 call removes the link to LOCKFILE if an error occurs. Successive calls
214 remove the links to SAVEFILE and PASSWDFILE so that new links can be
215 created, then removes the link to LOCKFILE when it is no longer needed.
216 #include <sys/types.h>
219 #include <stdio.h>
220 #include <fcntl.h>
221 #include <errno.h>
222 #include <unistd.h>
223 #include <sys/stat.h>
224 #define LOCKFILE "/etc/ptmp"
226 #define PASSWDFILE "/etc/passwd"
227 #define SAVEFILE "/etc/opasswd"
228 ...
229 /* If no change was made, assume error and leave passwd unchanged. */
230 if (!valid_change) {
231 fprintf(stderr, "Could not change password for user %s\n", user);
232 unlink(LOCKFILE);
233 exit(1);
234 }
235 /* Change permissions on new password file. */
237 chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);
238 /* Remove saved password file. */
240 unlink(SAVEFILE);
241 /* Save current password file. */
243 link(PASSWDFILE, SAVEFILE);
244 /* Remove current password file. */
246 unlink(PASSWDFILE);
247 /* Save new password file as current password file. */
249 link(LOCKFILE,PASSWDFILE);
250 /* Remove lock file. */
252 unlink(LOCKFILE);
253 exit(0);
255
257 Applications should use rmdir() to remove a directory.
258
260 Unlinking a directory is restricted to the superuser in many historical
261 implementations for reasons given in link() (see also rename()).
262 The meaning of [EBUSY] in historical implementations is ``mount point
264 busy''. Since this volume of POSIX.1‐2017 does not cover the system
265 administration concepts of mounting and unmounting, the description of
266 the error was changed to ``resource busy''. (This meaning is used by
267 some device drivers when a second process tries to open an exclusive
268 use device.) The wording is also intended to allow implementations to
269 refuse to remove a directory if it is the root or current working
270 directory of any process.
271 The standard developers reviewed TR 24715‐2006 and noted that LSB-con‐
273 forming implementations may return [EISDIR] instead of [EPERM] when
274 unlinking a directory. A change to permit this behavior by changing the
275 requirement for [EPERM] to [EPERM] or [EISDIR] was considered, but
276 decided against since it would break existing strictly conforming and
277 conforming applications. Applications written for portability to both
278 POSIX.1‐2008 and the LSB should be prepared to handle either error
279 code.
280 The purpose of the unlinkat() function is to remove directory entries
282 in directories other than the current working directory without expo‐
283 sure to race conditions. Any part of the path of a file could be
284 changed in parallel to a call to unlink(), resulting in unspecified
285 behavior. By opening a file descriptor for the target directory and
286 using the unlinkat() function it can be guaranteed that the removed
287 directory entry is located relative to the desired directory.
288
290 None.
291
293 close(), link(), remove(), rename(), rmdir(), symlink()
294 The Base Definitions volume of POSIX.1‐2017, Section 4.3, Directory
296 Protection, <fcntl.h>, <unistd.h>
297
299 Portions of this text are reprinted and reproduced in electronic form
300 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
301 table Operating System Interface (POSIX), The Open Group Base Specifi‐
302 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
303 Electrical and Electronics Engineers, Inc and The Open Group. In the
304 event of any discrepancy between this version and the original IEEE and
305 The Open Group Standard, the original IEEE and The Open Group Standard
306 is the referee document. The original Standard can be obtained online
307 at http://www.opengroup.org/unix/online.html .
308 Any typographical or formatting errors that appear in this page are
310 most likely to have been introduced during the conversion of the source
311 files to man page format. To report such errors, see https://www.ker‐
312 nel.org/doc/man-pages/reporting_bugs.html .
313IEEE/The Open Group 2017 UNLINK(3P)