1LISTXATTR(2) Linux Programmer's Manual LISTXATTR(2)
2
6 listxattr, llistxattr, flistxattr - list extended attribute names
7
9 #include <sys/xattr.h>
10 ssize_t listxattr(const char *path, char *list, size_t size);
12 ssize_t llistxattr(const char *path, char *list, size_t size);
13 ssize_t flistxattr(int fd, char *list, size_t size);
14
16 Extended attributes are name:value pairs associated with inodes (files,
17 directories, symbolic links, etc.). They are extensions to the normal
18 attributes which are associated with all inodes in the system (i.e.,
19 the stat(2) data). A complete overview of extended attributes concepts
20 can be found in xattr(7).
21 listxattr() retrieves the list of extended attribute names associated
23 with the given path in the filesystem. The retrieved list is placed in
24 list, a caller-allocated buffer whose size (in bytes) is specified in
25 the argument size. The list is the set of (null-terminated) names, one
26 after the other. Names of extended attributes to which the calling
27 process does not have access may be omitted from the list. The length
28 of the attribute name list is returned.
29 llistxattr() is identical to listxattr(), except in the case of a sym‐
31 bolic link, where the list of names of extended attributes associated
32 with the link itself is retrieved, not the file that it refers to.
33 flistxattr() is identical to listxattr(), only the open file referred
35 to by fd (as returned by open(2)) is interrogated in place of path.
36 A single extended attribute name is a null-terminated string. The name
38 includes a namespace prefix; there may be several, disjoint namespaces
39 associated with an individual inode.
40 If size is specified as zero, these calls return the current size of
42 the list of extended attribute names (and leave list unchanged). This
43 can be used to determine the size of the buffer that should be supplied
44 in a subsequent call. (But, bear in mind that there is a possibility
45 that the set of extended attributes may change between the two calls,
46 so that it is still necessary to check the return status from the sec‐
47 ond call.)
48 Example
50 The list of names is returned as an unordered array of null-terminated
51 character strings (attribute names are separated by null bytes ('\0')),
52 like this:
53 user.name1\0system.name1\0user.name2\0
55 Filesystems that implement POSIX ACLs using extended attributes might
57 return a list like this:
58 system.posix_acl_access\0system.posix_acl_default\0
60
62 On success, a nonnegative number is returned indicating the size of the
63 extended attribute name list. On failure, -1 is returned and errno is
64 set to indicate the error.
65
67 E2BIG The size of the list of extended attribute names is larger than
68 the maximum size allowed; the list cannot be retrieved. This
69 can happen on filesystems that support an unlimited number of
70 extended attributes per file such as XFS, for example. See
71 BUGS.
72 ENOTSUP
74 Extended attributes are not supported by the filesystem, or are
75 disabled.
76 ERANGE The size of the list buffer is too small to hold the result.
78 In addition, the errors documented in stat(2) can also occur.
80
82 These system calls have been available on Linux since kernel 2.4; glibc
83 support is provided since version 2.3.
84
86 These system calls are Linux-specific.
87
89 As noted in xattr(7), the VFS imposes a limit of 64 kB on the size of
90 the extended attribute name list returned by listxattr(7). If the to‐
91 tal size of attribute names attached to a file exceeds this limit, it
92 is no longer possible to retrieve the list of attribute names.
93
95 The following program demonstrates the usage of listxattr() and getx‐
96 attr(2). For the file whose pathname is provided as a command-line ar‐
97 gument, it lists all extended file attributes and their values.
98 To keep the code simple, the program assumes that attribute keys and
100 values are constant during the execution of the program. A production
101 program should expect and handle changes during execution of the pro‐
102 gram. For example, the number of bytes required for attribute keys
103 might increase between the two calls to listxattr(). An application
104 could handle this possibility using a loop that retries the call (per‐
105 haps up to a predetermined maximum number of attempts) with a larger
106 buffer each time it fails with the error ERANGE. Calls to getxattr(2)
107 could be handled similarly.
108 The following output was recorded by first creating a file, setting
110 some extended file attributes, and then listing the attributes with the
111 example program.
112 Example output
114 $ touch /tmp/foo
115 $ setfattr -n user.fred -v chocolate /tmp/foo
116 $ setfattr -n user.frieda -v bar /tmp/foo
117 $ setfattr -n user.empty /tmp/foo
118 $ ./listxattr /tmp/foo
119 user.fred: chocolate
120 user.frieda: bar
121 user.empty: <no value>
122 Program source (listxattr.c)
124 #include <malloc.h>
125 #include <stdio.h>
126 #include <stdlib.h>
127 #include <string.h>
128 #include <sys/types.h>
129 #include <sys/xattr.h>
130 int
132 main(int argc, char *argv[])
133 {
134 ssize_t buflen, keylen, vallen;
135 char *buf, *key, *val;
136 if (argc != 2) {
138 fprintf(stderr, "Usage: %s path\n", argv[0]);
139 exit(EXIT_FAILURE);
140 }
141 /*
143 * Determine the length of the buffer needed.
144 */
145 buflen = listxattr(argv[1], NULL, 0);
146 if (buflen == -1) {
147 perror("listxattr");
148 exit(EXIT_FAILURE);
149 }
150 if (buflen == 0) {
151 printf("%s has no attributes.\n", argv[1]);
152 exit(EXIT_SUCCESS);
153 }
154 /*
156 * Allocate the buffer.
157 */
158 buf = malloc(buflen);
159 if (buf == NULL) {
160 perror("malloc");
161 exit(EXIT_FAILURE);
162 }
163 /*
165 * Copy the list of attribute keys to the buffer.
166 */
167 buflen = listxattr(argv[1], buf, buflen);
168 if (buflen == -1) {
169 perror("listxattr");
170 exit(EXIT_FAILURE);
171 }
172 /*
174 * Loop over the list of zero terminated strings with the
175 * attribute keys. Use the remaining buffer length to determine
176 * the end of the list.
177 */
178 key = buf;
179 while (buflen > 0) {
180 /*
182 * Output attribute key.
183 */
184 printf("%s: ", key);
185 /*
187 * Determine length of the value.
188 */
189 vallen = getxattr(argv[1], key, NULL, 0);
190 if (vallen == -1)
191 perror("getxattr");
192 if (vallen > 0) {
194 /*
196 * Allocate value buffer.
197 * One extra byte is needed to append 0x00.
198 */
199 val = malloc(vallen + 1);
200 if (val == NULL) {
201 perror("malloc");
202 exit(EXIT_FAILURE);
203 }
204 /*
206 * Copy value to buffer.
207 */
208 vallen = getxattr(argv[1], key, val, vallen);
209 if (vallen == -1)
210 perror("getxattr");
211 else {
212 /*
213 * Output attribute value.
214 */
215 val[vallen] = 0;
216 printf("%s", val);
217 }
218 free(val);
220 } else if (vallen == 0)
221 printf("<no value>");
222 printf("\n");
224 /*
226 * Forward to next attribute key.
227 */
228 keylen = strlen(key) + 1;
229 buflen -= keylen;
230 key += keylen;
231 }
232 free(buf);
234 exit(EXIT_SUCCESS);
235 }
236
238 getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2), setx‐
239 attr(2), stat(2), symlink(7), xattr(7)
240
242 This page is part of release 5.12 of the Linux man-pages project. A
243 description of the project, information about reporting bugs, and the
244 latest version of this page, can be found at
245 https://www.kernel.org/doc/man-pages/.
246Linux 2021-03-22 LISTXATTR(2)