1LISTXATTR(2) Linux Programmer's Manual LISTXATTR(2)
2
6 listxattr, llistxattr, flistxattr - list extended attribute names
7
9 #include <sys/types.h>
10 #include <sys/xattr.h>
11 ssize_t listxattr(const char *path, char *list, size_t size);
13 ssize_t llistxattr(const char *path, char *list, size_t size);
14 ssize_t flistxattr(int fd, char *list, size_t size);
15
17 Extended attributes are name:value pairs associated with inodes (files,
18 directories, symbolic links, etc.). They are extensions to the normal
19 attributes which are associated with all inodes in the system (i.e.,
20 the stat(2) data). A complete overview of extended attributes concepts
21 can be found in xattr(7).
22 listxattr() retrieves the list of extended attribute names associated
24 with the given path in the filesystem. The retrieved list is placed in
25 list, a caller-allocated buffer whose size (in bytes) is specified in
26 the argument size. The list is the set of (null-terminated) names, one
27 after the other. Names of extended attributes to which the calling
28 process does not have access may be omitted from the list. The length
29 of the attribute name list is returned.
30 llistxattr() is identical to listxattr(), except in the case of a sym‐
32 bolic link, where the list of names of extended attributes associated
33 with the link itself is retrieved, not the file that it refers to.
34 flistxattr() is identical to listxattr(), only the open file referred
36 to by fd (as returned by open(2)) is interrogated in place of path.
37 A single extended attribute name is a null-terminated string. The name
39 includes a namespace prefix; there may be several, disjoint namespaces
40 associated with an individual inode.
41 If size is specified as zero, these calls return the current size of
43 the list of extended attribute names (and leave list unchanged). This
44 can be used to determine the size of the buffer that should be supplied
45 in a subsequent call. (But, bear in mind that there is a possibility
46 that the set of extended attributes may change between the two calls,
47 so that it is still necessary to check the return status from the sec‐
48 ond call.)
49 Example
51 The list of names is returned as an unordered array of null-terminated
52 character strings (attribute names are separated by null bytes ('\0')),
53 like this:
54 user.name1\0system.name1\0user.name2\0
56 Filesystems that implement POSIX ACLs using extended attributes might
58 return a list like this:
59 system.posix_acl_access\0system.posix_acl_default\0
61
63 On success, a nonnegative number is returned indicating the size of the
64 extended attribute name list. On failure, -1 is returned and errno is
65 set appropriately.
66
68 E2BIG The size of the list of extended attribute names is larger than
69 the maximum size allowed; the list cannot be retrieved. This
70 can happen on filesystems that support an unlimited number of
71 extended attributes per file such as XFS, for example. See
72 BUGS.
73 ENOTSUP
75 Extended attributes are not supported by the filesystem, or are
76 disabled.
77 ERANGE The size of the list buffer is too small to hold the result.
79 In addition, the errors documented in stat(2) can also occur.
81
83 These system calls have been available on Linux since kernel 2.4; glibc
84 support is provided since version 2.3.
85
87 These system calls are Linux-specific.
88
90 As noted in xattr(7), the VFS imposes a limit of 64 kB on the size of
91 the extended attribute name list returned by listxattr(7). If the
92 total size of attribute names attached to a file exceeds this limit, it
93 is no longer possible to retrieve the list of attribute names.
94
96 The following program demonstrates the usage of listxattr() and getx‐
97 attr(2). For the file whose pathname is provided as a command-line
98 argument, it lists all extended file attributes and their values.
99 To keep the code simple, the program assumes that attribute keys and
101 values are constant during the execution of the program. A production
102 program should expect and handle changes during execution of the pro‐
103 gram. For example, the number of bytes required for attribute keys
104 might increase between the two calls to listxattr(). An application
105 could handle this possibility using a loop that retries the call (per‐
106 haps up to a predetermined maximum number of attempts) with a larger
107 buffer each time it fails with the error ERANGE. Calls to getxattr(2)
108 could be handled similarly.
109 The following output was recorded by first creating a file, setting
111 some extended file attributes, and then listing the attributes with the
112 example program.
113 Example output
115 $ touch /tmp/foo
116 $ setfattr -n user.fred -v chocolate /tmp/foo
117 $ setfattr -n user.frieda -v bar /tmp/foo
118 $ setfattr -n user.empty /tmp/foo
119 $ ./listxattr /tmp/foo
120 user.fred: chocolate
121 user.frieda: bar
122 user.empty: <no value>
123 Program source (listxattr.c)
125 #include <malloc.h>
126 #include <stdio.h>
127 #include <stdlib.h>
128 #include <string.h>
129 #include <sys/types.h>
130 #include <sys/xattr.h>
131 int
133 main(int argc, char *argv[])
134 {
135 ssize_t buflen, keylen, vallen;
136 char *buf, *key, *val;
137 if (argc != 2) {
139 fprintf(stderr, "Usage: %s path\n", argv[0]);
140 exit(EXIT_FAILURE);
141 }
142 /*
144 * Determine the length of the buffer needed.
145 */
146 buflen = listxattr(argv[1], NULL, 0);
147 if (buflen == -1) {
148 perror("listxattr");
149 exit(EXIT_FAILURE);
150 }
151 if (buflen == 0) {
152 printf("%s has no attributes.\n", argv[1]);
153 exit(EXIT_SUCCESS);
154 }
155 /*
157 * Allocate the buffer.
158 */
159 buf = malloc(buflen);
160 if (buf == NULL) {
161 perror("malloc");
162 exit(EXIT_FAILURE);
163 }
164 /*
166 * Copy the list of attribute keys to the buffer.
167 */
168 buflen = listxattr(argv[1], buf, buflen);
169 if (buflen == -1) {
170 perror("listxattr");
171 exit(EXIT_FAILURE);
172 }
173 /*
175 * Loop over the list of zero terminated strings with the
176 * attribute keys. Use the remaining buffer length to determine
177 * the end of the list.
178 */
179 key = buf;
180 while (buflen > 0) {
181 /*
183 * Output attribute key.
184 */
185 printf("%s: ", key);
186 /*
188 * Determine length of the value.
189 */
190 vallen = getxattr(argv[1], key, NULL, 0);
191 if (vallen == -1)
192 perror("getxattr");
193 if (vallen > 0) {
195 /*
197 * Allocate value buffer.
198 * One extra byte is needed to append 0x00.
199 */
200 val = malloc(vallen + 1);
201 if (val == NULL) {
202 perror("malloc");
203 exit(EXIT_FAILURE);
204 }
205 /*
207 * Copy value to buffer.
208 */
209 vallen = getxattr(argv[1], key, val, vallen);
210 if (vallen == -1)
211 perror("getxattr");
212 else {
213 /*
214 * Output attribute value.
215 */
216 val[vallen] = 0;
217 printf("%s", val);
218 }
219 free(val);
221 } else if (vallen == 0)
222 printf("<no value>");
223 printf("\n");
225 /*
227 * Forward to next attribute key.
228 */
229 keylen = strlen(key) + 1;
230 buflen -= keylen;
231 key += keylen;
232 }
233 free(buf);
235 exit(EXIT_SUCCESS);
236 }
237
239 getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2), setx‐
240 attr(2), stat(2), symlink(7), xattr(7)
241
243 This page is part of release 4.16 of the Linux man-pages project. A
244 description of the project, information about reporting bugs, and the
245 latest version of this page, can be found at
246 https://www.kernel.org/doc/man-pages/.
247Linux 2017-09-15 LISTXATTR(2)