1FDOPENDIR(3P) POSIX Programmer's Manual FDOPENDIR(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 fdopendir, opendir — open directory associated with file descriptor
13
15 #include <dirent.h>
16 DIR *fdopendir(int fd);
18 DIR *opendir(const char *dirname);
19
21 The fdopendir() function shall be equivalent to the opendir() function
22 except that the directory is specified by a file descriptor rather than
23 by a name. The file offset associated with the file descriptor at the
24 time of the call determines which entries are returned.
25 Upon successful return from fdopendir(), the file descriptor is under
27 the control of the system, and if any attempt is made to close the file
28 descriptor, or to modify the state of the associated description, other
29 than by means of closedir(), readdir(), readdir_r(), rewinddir(), or
30 seekdir(), the behavior is undefined. Upon calling closedir() the file
31 descriptor shall be closed.
32 It is unspecified whether the FD_CLOEXEC flag will be set on the file
34 descriptor by a successful call to fdopendir().
35 The opendir() function shall open a directory stream corresponding to
37 the directory named by the dirname argument. The directory stream is
38 positioned at the first entry. If the type DIR is implemented using a
39 file descriptor, applications shall only be able to open up to a total
40 of {OPEN_MAX} files and directories.
41 If the type DIR is implemented using a file descriptor, the descriptor
43 shall be obtained as if the O_DIRECTORY flag was passed to open().
44
46 Upon successful completion, these functions shall return a pointer to
47 an object of type DIR. Otherwise, these functions shall return a null
48 pointer and set errno to indicate the error.
49
51 The fdopendir() function shall fail if:
52 EBADF The fd argument is not a valid file descriptor open for reading.
54 ENOTDIR
56 The descriptor fd is not associated with a directory.
57 The opendir() function shall fail if:
59 EACCES Search permission is denied for the component of the path prefix
61 of dirname or read permission is denied for dirname.
62 ELOOP A loop exists in symbolic links encountered during resolution of
64 the dirname argument.
65 ENAMETOOLONG
67 The length of a component of a pathname is longer than
68 {NAME_MAX}.
69 ENOENT A component of dirname does not name an existing directory or
71 dirname is an empty string.
72 ENOTDIR
74 A component of dirname names an existing file that is neither a
75 directory nor a symbolic link to a directory.
76 The opendir() function may fail if:
78 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
80 resolution of the dirname argument.
81 EMFILE All file descriptors available to the process are currently
83 open.
84 ENAMETOOLONG
86 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
87 tion of a symbolic link produced an intermediate result with a
88 length that exceeds {PATH_MAX}.
89 ENFILE Too many files are currently open in the system.
91 The following sections are informative.
93
95 Open a Directory Stream
96 The following program fragment demonstrates how the opendir() function
97 is used.
98 #include <dirent.h>
101 ...
102 DIR *dir;
103 struct dirent *dp;
104 ...
105 if ((dir = opendir (".")) == NULL) {
106 perror ("Cannot open .");
107 exit (1);
108 }
109 while ((dp = readdir (dir)) != NULL) {
111 ...
112 Find And Open a File
114 The following program searches through a given directory looking for
115 files whose name does not begin with a dot and whose size is larger
116 than 1 MiB.
117 #include <stdio.h>
120 #include <dirent.h>
121 #include <fcntl.h>
122 #include <sys/stat.h>
123 #include <stdint.h>
124 #include <stdlib.h>
125 #include <unistd.h>
126 int
128 main(int argc, char *argv[])
129 {
130 struct stat statbuf;
131 DIR *d;
132 struct dirent *dp;
133 int dfd, ffd;
134 if ((d = fdopendir((dfd = open("./tmp", O_RDONLY)))) == NULL) {
136 fprintf(stderr, "Cannot open ./tmp directory\n");
137 exit(1);
138 }
139 while ((dp = readdir(d)) != NULL) {
140 if (dp->d_name[0] == '.')
141 continue;
142 /* there is a possible race condition here as the file
143 * could be renamed between the readdir and the open */
144 if ((ffd = openat(dfd, dp->d_name, O_RDONLY)) == -1) {
145 perror(dp->d_name);
146 continue;
147 }
148 if (fstat(ffd, &statbuf) == 0 && statbuf.st_size > (1024*1024)) {
149 /* found it ... */
150 printf("%s: %jdK\n", dp->d_name,
151 (intmax_t)(statbuf.st_size / 1024));
152 }
153 close(ffd);
154 }
155 closedir(d); // note this implicitly closes dfd
156 return 0;
157 }
158
160 The opendir() function should be used in conjunction with readdir(),
161 closedir(), and rewinddir() to examine the contents of the directory
162 (see the EXAMPLES section in readdir()). This method is recommended
163 for portability.
164
166 The purpose of the fdopendir() function is to enable opening files in
167 directories other than the current working directory without exposure
168 to race conditions. Any part of the path of a file could be changed in
169 parallel to a call to opendir(), resulting in unspecified behavior.
170 Based on historical implementations, the rules about file descriptors
172 apply to directory streams as well. However, this volume of
173 POSIX.1‐2017 does not mandate that the directory stream be implemented
174 using file descriptors. The description of closedir() clarifies that if
175 a file descriptor is used for the directory stream, it is mandatory
176 that closedir() deallocate the file descriptor. When a file descriptor
177 is used to implement the directory stream, it behaves as if the
178 FD_CLOEXEC had been set for the file descriptor.
179 The directory entries for dot and dot-dot are optional. This volume of
181 POSIX.1‐2017 does not provide a way to test a priori for their exis‐
182 tence because an application that is portable must be written to look
183 for (and usually ignore) those entries. Writing code that presumes that
184 they are the first two entries does not always work, as many implemen‐
185 tations permit them to be other than the first two entries, with a
186 ``normal'' entry preceding them. There is negligible value in providing
187 a way to determine what the implementation does because the code to
188 deal with dot and dot-dot must be written in any case and because such
189 a flag would add to the list of those flags (which has proven in itself
190 to be objectionable) and might be abused.
191 Since the structure and buffer allocation, if any, for directory opera‐
193 tions are defined by the implementation, this volume of POSIX.1‐2017
194 imposes no portability requirements for erroneous program constructs,
195 erroneous data, or the use of unspecified values such as the use or
196 referencing of a dirp value or a dirent structure value after a direc‐
197 tory stream has been closed or after a fork() or one of the exec func‐
198 tion calls.
199
201 None.
202
204 closedir(), dirfd(), fstatat(), open(), readdir(), rewinddir(), sym‐
205 link()
206 The Base Definitions volume of POSIX.1‐2017, <dirent.h>, <sys_types.h>
208
210 Portions of this text are reprinted and reproduced in electronic form
211 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
212 table Operating System Interface (POSIX), The Open Group Base Specifi‐
213 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
214 Electrical and Electronics Engineers, Inc and The Open Group. In the
215 event of any discrepancy between this version and the original IEEE and
216 The Open Group Standard, the original IEEE and The Open Group Standard
217 is the referee document. The original Standard can be obtained online
218 at http://www.opengroup.org/unix/online.html .
219 Any typographical or formatting errors that appear in this page are
221 most likely to have been introduced during the conversion of the source
222 files to man page format. To report such errors, see https://www.ker‐
223 nel.org/doc/man-pages/reporting_bugs.html .
224IEEE/The Open Group 2017 FDOPENDIR(3P)