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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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listxattr(2) System Calls Manual listxattr(2)
listxattr, llistxattr, flistxattr - list extended attribute names
Standard C library (libc, -lc)
#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);
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 to indicate the error.
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.
Linux.
Linux 2.4, glibc 2.3.
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().
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 <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);
}
getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2),
setxattr(2), stat(2), symlink(7), xattr(7)
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Linux man-pages 6.9.1 2024-06-15 listxattr(2)
Pages that refer to this page: getxattr(2), removexattr(2), setxattr(2), syscalls(2), symlink(7), xattr(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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