listxattr(2) System Calls Manual listxattr(2)
NAME
listxattr, llistxattr, flistxattr - list extended attribute names
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/xattr.h>
ssize_t listxattr(const char *path, char *_Nullable list, size_t size);
ssize_t llistxattr(const char *path, char *_Nullable list, size_t size);
ssize_t flistxattr(int fd, char *_Nullable list, size_t size);
DESCRIPTION
Extended attributes are name:value pairs associated with inodes (files, directories, symbolic links, etc.). They are exten‐
sions 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 at‐
tributes 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
RETURN VALUE
On success, a nonnegative number is returned indicating the size of the extended attribute name list. On failure, -1 is re‐
turned and errno is set to indicate the error.
ERRORS
E2BIG The size of the list of extended attribute names is larger than the maximum size allowed; the list cannot be re‐
trieved. 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 since Linux 2.4; glibc support is provided since glibc 2.3.
STANDARDS
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 listx‐
attr(). 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.
EXAMPLES
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 pro‐
gram. 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 buf‐
fer 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 at‐
tributes 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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/xattr.h>
int
main(int argc, char *argv[])
{
char *buf, *key, *val;
ssize_t buflen, keylen, vallen;
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), setxattr(2), stat(2), symlink(7), xattr(7)
Linux man-pages 6.03 2023-02-05 listxattr(2)