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.)
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.
$ 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>
#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)