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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
LISTXATTR(2) Linux Programmer's Manual LISTXATTR(2)
listxattr, llistxattr, flistxattr - list extended attribute names
#include <sys/types.h>
#include <sys/xattr.h>
ssize_t listxattr(const char *path, char *list, size_t size);
ssize_t llistxattr(const char *path, char *list, size_t size);
ssize_t flistxattr(int fd, char *list, size_t size);
Extended attributes are name:value pairs associated with inodes
(files, directories, symbolic links, etc.). They are extensions to
the normal attributes which are associated with all inodes in the
system (i.e., the stat(2) data). A complete overview of extended
attributes concepts can be found in xattr(7).
listxattr() retrieves the list of extended attribute names associated
with the given path in the filesystem. The retrieved list is placed
in list, a caller-allocated buffer whose size (in bytes) is specified
in the argument size. The list is the set of (null-terminated)
names, one after the other. Names of extended attributes to which
the calling process does not have access may be omitted from the
list. The length of the attribute name list is returned.
llistxattr() is identical to listxattr(), except in the case of a
symbolic link, where the list of names of extended attributes
associated with the link itself is retrieved, not the file that it
refers to.
flistxattr() is identical to listxattr(), only the open file referred
to by fd (as returned by open(2)) is interrogated in place of path.
A single extended attribute name is a null-terminated string. The
name includes a namespace prefix; there may be several, disjoint
namespaces associated with an individual inode.
If size is specified as zero, these calls return the current size of
the list of extended attribute names (and leave list unchanged).
This can be used to determine the size of the buffer that should be
supplied in a subsequent call. (But, bear in mind that there is a
possibility that the set of extended attributes may change between
the two calls, so that it is still necessary to check the return
status from the second call.)
Example
The list of names is returned as an unordered array of null-
terminated character strings (attribute names are separated by null
bytes ('\0')), like this:
user.name1\0system.name1\0user.name2\0
Filesystems that implement POSIX ACLs using extended attributes might
return a list like this:
system.posix_acl_access\0system.posix_acl_default\0
On success, a nonnegative number is returned indicating the size of
the extended attribute name list. On failure, -1 is returned and
errno is set appropriately.
E2BIG The size of the list of extended attribute names is larger
than the maximum size allowed; the list cannot be retrieved.
This can happen on filesystems that support an unlimited
number of extended attributes per file such as XFS, for
example. See BUGS.
ENOTSUP
Extended attributes are not supported by the filesystem, or
are disabled.
ERANGE The size of the list buffer is too small to hold the result.
In addition, the errors documented in stat(2) can also occur.
These system calls have been available on Linux since kernel 2.4;
glibc support is provided since version 2.3.
These system calls are Linux-specific.
As noted in xattr(7), the VFS imposes a limit of 64 kB on the size of
the extended attribute name list returned by listxattr(7). If the
total size of attribute names attached to a file exceeds this limit,
it is no longer possible to retrieve the list of attribute names.
The following program demonstrates the usage of listxattr() and
getxattr(2). For the file whose pathname is provided as a command-
line argument, it lists all extended file attributes and their
values.
To keep the code simple, the program assumes that attribute keys and
values are constant during the execution of the program. A
production program should expect and handle changes during execution
of the program. For example, the number of bytes required for
attribute keys might increase between the two calls to listxattr().
An application could handle this possibility using a loop that
retries the call (perhaps up to a predetermined maximum number of
attempts) with a larger buffer each time it fails with the error
ERANGE. Calls to getxattr(2) could be handled similarly.
The following output was recorded by first creating a file, setting
some extended file attributes, and then listing the attributes with
the example program.
Example output
$ touch /tmp/foo
$ setfattr -n user.fred -v chocolate /tmp/foo
$ setfattr -n user.frieda -v bar /tmp/foo
$ setfattr -n user.empty /tmp/foo
$ ./listxattr /tmp/foo
user.fred: chocolate
user.frieda: bar
user.empty: <no value>
Program source (listxattr.c)
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/xattr.h>
int
main(int argc, char *argv[])
{
ssize_t buflen, keylen, vallen;
char *buf, *key, *val;
if (argc != 2) {
fprintf(stderr, "Usage: %s path\n", argv[0]);
exit(EXIT_FAILURE);
}
/*
* Determine the length of the buffer needed.
*/
buflen = listxattr(argv[1], NULL, 0);
if (buflen == -1) {
perror("listxattr");
exit(EXIT_FAILURE);
}
if (buflen == 0) {
printf("%s has no attributes.\n", argv[1]);
exit(EXIT_SUCCESS);
}
/*
* Allocate the buffer.
*/
buf = malloc(buflen);
if (buf == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
/*
* Copy the list of attribute keys to the buffer.
*/
buflen = listxattr(argv[1], buf, buflen);
if (buflen == -1) {
perror("listxattr");
exit(EXIT_FAILURE);
}
/*
* Loop over the list of zero terminated strings with the
* attribute keys. Use the remaining buffer length to determine
* the end of the list.
*/
key = buf;
while (buflen > 0) {
/*
* Output attribute key.
*/
printf("%s: ", key);
/*
* Determine length of the value.
*/
vallen = getxattr(argv[1], key, NULL, 0);
if (vallen == -1)
perror("getxattr");
if (vallen > 0) {
/*
* Allocate value buffer.
* One extra byte is needed to append 0x00.
*/
val = malloc(vallen + 1);
if (val == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
/*
* Copy value to buffer.
*/
vallen = getxattr(argv[1], key, val, vallen);
if (vallen == -1)
perror("getxattr");
else {
/*
* Output attribute value.
*/
val[vallen] = 0;
printf("%s", val);
}
free(val);
} else if (vallen == 0)
printf("<no value>");
printf("\n");
/*
* Forward to next attribute key.
*/
keylen = strlen(key) + 1;
buflen -= keylen;
key += keylen;
}
free(buf);
exit(EXIT_SUCCESS);
}
getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2),
setxattr(2), stat(2), symlink(7), xattr(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-03-13 LISTXATTR(2)
Pages that refer to this page: getxattr(2), removexattr(2), setxattr(2), syscalls(2), symlink(7), xattr(7)