1UNLINK(P) POSIX Programmer's Manual UNLINK(P)
2
6 unlink - remove a directory entry
7
9 #include <unistd.h>
10 int unlink(const char *path);
12
15 The unlink() function shall remove a link to a file. If path names a
16 symbolic link, unlink() shall remove the symbolic link named by path
17 and shall not affect any file or directory named by the contents of the
18 symbolic link. Otherwise, unlink() shall remove the link named by the
19 pathname pointed to by path and shall decrement the link count of the
20 file referenced by the link.
21 When the file's link count becomes 0 and no process has the file open,
23 the space occupied by the file shall be freed and the file shall no
24 longer be accessible. If one or more processes have the file open when
25 the last link is removed, the link shall be removed before unlink()
26 returns, but the removal of the file contents shall be postponed until
27 all references to the file are closed.
28 The path argument shall not name a directory unless the process has
30 appropriate privileges and the implementation supports using unlink()
31 on directories.
32 Upon successful completion, unlink() shall mark for update the st_ctime
34 and st_mtime fields of the parent directory. Also, if the file's link
35 count is not 0, the st_ctime field of the file shall be marked for
36 update.
37
39 Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
40 returned and errno set to indicate the error. If -1 is returned, the
41 named file shall not be changed.
42
44 The unlink() function shall fail and shall not unlink the file if:
45 EACCES Search permission is denied for a component of the path prefix,
47 or write permission is denied on the directory containing the
48 directory entry to be removed.
49 EBUSY The file named by the path argument cannot be unlinked because
51 it is being used by the system or another process and the imple‐
52 mentation considers this an error.
53 ELOOP A loop exists in symbolic links encountered during resolution of
55 the path argument.
56 ENAMETOOLONG
58 The length of the path argument exceeds {PATH_MAX} or a pathname
59 component is longer than {NAME_MAX}.
60 ENOENT A component of path does not name an existing file or path is an
62 empty string.
63 ENOTDIR
65 A component of the path prefix is not a directory.
66 EPERM The file named by path is a directory, and either the calling
68 process does not have appropriate privileges, or the implementa‐
69 tion prohibits using unlink() on directories.
70 EPERM or EACCES
72 The S_ISVTX flag is set on the directory containing the file
74 referred to by the path argument and the caller is not the file
75 owner, nor is the caller the directory owner, nor does the call‐
76 er have appropriate privileges.
77 EROFS The directory entry to be unlinked is part of a read-only file
79 system.
80 The unlink() function may fail and not unlink the file if:
83 EBUSY The file named by path is a named STREAM.
85 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
87 resolution of the path argument.
88 ENAMETOOLONG
90 As a result of encountering a symbolic link in resolution of the
91 path argument, the length of the substituted pathname string
92 exceeded {PATH_MAX}.
93 ETXTBSY
95 The entry to be unlinked is the last directory entry to a pure
96 procedure (shared text) file that is being executed.
97 The following sections are informative.
100
102 Removing a Link to a File
103 The following example shows how to remove a link to a file named
104 /home/cnd/mod1 by removing the entry named /modules/pass1.
105 #include <unistd.h>
108 char *path = "/modules/pass1";
111 int status;
112 ...
113 status = unlink(path);
114 Checking for an Error
116 The following example fragment creates a temporary password lock file
117 named LOCKFILE, which is defined as /etc/ptmp, and gets a file descrip‐
118 tor for it. If the file cannot be opened for writing, unlink() is used
119 to remove the link between the file descriptor and LOCKFILE.
120 #include <sys/types.h>
123 #include <stdio.h>
124 #include <fcntl.h>
125 #include <errno.h>
126 #include <unistd.h>
127 #include <sys/stat.h>
128 #define LOCKFILE "/etc/ptmp"
131 int pfd; /* Integer for file descriptor returned by open call. */
134 FILE *fpfd; /* File pointer for use in putpwent(). */
135 ...
136 /* Open password Lock file. If it exists, this is an error. */
137 if ((pfd = open(LOCKFILE, O_WRONLY| O_CREAT | O_EXCL, S_IRUSR
138 | S_IWUSR | S_IRGRP | S_IROTH)) == -1) {
139 fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
140 exit(1);
141 }
142 /* Lock file created; proceed with fdopen of lock file so that
145 putpwent() can be used.
146 */
147 if ((fpfd = fdopen(pfd, "w")) == NULL) {
148 close(pfd);
149 unlink(LOCKFILE);
150 exit(1);
151 }
152 Replacing Files
154 The following example fragment uses unlink() to discard links to files,
155 so that they can be replaced with new versions of the files. The first
156 call removes the link to LOCKFILE if an error occurs. Successive calls
157 remove the links to SAVEFILE and PASSWDFILE so that new links can be
158 created, then removes the link to LOCKFILE when it is no longer needed.
159 #include <sys/types.h>
162 #include <stdio.h>
163 #include <fcntl.h>
164 #include <errno.h>
165 #include <unistd.h>
166 #include <sys/stat.h>
167 #define LOCKFILE "/etc/ptmp"
170 #define PASSWDFILE "/etc/passwd"
171 #define SAVEFILE "/etc/opasswd"
172 ...
173 /* If no change was made, assume error and leave passwd unchanged. */
174 if (!valid_change) {
175 fprintf(stderr, "Could not change password for user %s\n", user);
176 unlink(LOCKFILE);
177 exit(1);
178 }
179 /* Change permissions on new password file. */
182 chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);
183 /* Remove saved password file. */
186 unlink(SAVEFILE);
187 /* Save current password file. */
190 link(PASSWDFILE, SAVEFILE);
191 /* Remove current password file. */
194 unlink(PASSWDFILE);
195 /* Save new password file as current password file. */
198 link(LOCKFILE,PASSWDFILE);
199 /* Remove lock file. */
202 unlink(LOCKFILE);
203 exit(0);
206
208 Applications should use rmdir() to remove a directory.
209
211 Unlinking a directory is restricted to the superuser in many historical
212 implementations for reasons given in link() (see also rename()).
213 The meaning of [EBUSY] in historical implementations is "mount point
215 busy". Since this volume of IEEE Std 1003.1-2001 does not cover the
216 system administration concepts of mounting and unmounting, the descrip‐
217 tion of the error was changed to "resource busy". (This meaning is used
218 by some device drivers when a second process tries to open an exclusive
219 use device.) The wording is also intended to allow implementations to
220 refuse to remove a directory if it is the root or current working
221 directory of any process.
222
224 None.
225
227 close() , link() , remove() , rmdir() , the Base Definitions volume of
228 IEEE Std 1003.1-2001, <unistd.h>
229
231 Portions of this text are reprinted and reproduced in electronic form
232 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
233 -- Portable Operating System Interface (POSIX), The Open Group Base
234 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
235 Electrical and Electronics Engineers, Inc and The Open Group. In the
236 event of any discrepancy between this version and the original IEEE and
237 The Open Group Standard, the original IEEE and The Open Group Standard
238 is the referee document. The original Standard can be obtained online
239 at http://www.opengroup.org/unix/online.html .
240IEEE/The Open Group 2003 UNLINK(P)