flistxattr
LISTXATTR(2) Linux Programmer's Manual LISTXATTR(2)
NAME
listxattr, llistxattr, flistxattr - list extended attribute names
SYNOPSIS
#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);
DESCRIPTION
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 sym-
bolic 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 sec-
ond 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
RETURN VALUE
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.
ERRORS
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.
VERSIONS
These system calls have been available on Linux since kernel 2.4; glibc
support is provided since version 2.3.
CONFORMING TO
These system calls are Linux-specific.
BUGS
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 to-
tal size of attribute names attached to a file exceeds this limit, it
is no longer possible to retrieve the list of attribute names.
EXAMPLE
The following program demonstrates the usage of listxattr() and getx-
attr(2). For the file whose pathname is provided as a command-line ar-
gument, 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 pro-
gram. 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 (per-
haps 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);
}
SEE ALSO
getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2), setx-
attr(2), stat(2), symlink(7), xattr(7)
COLOPHON
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