.\" Copyright (C) Andreas Gruenbacher, February 2001

.\" Copyright (C) Silicon Graphics Inc, September 2001

.\" Copyright (C) 2015 Heinrich Schuchardt <xypron.glpk@gmx.de>

.\"

.\" %%%LICENSE_START(GPLv2+_DOC_FULL)

.\" This is free documentation; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

.\" published by the Free Software Foundation; either version 2 of

.\" the License, or (at your option) any later version.

.\"

.\" The GNU General Public License's references to "object code"

.\" and "executables" are to be interpreted as the output of any

.\" document formatting or typesetting system, including

.\" intermediate and printed output.

.\"

.\" This manual is distributed in the hope that it will be useful,

.\" but WITHOUT ANY WARRANTY; without even the implied warranty of

.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

.\" GNU General Public License for more details.

.\"

.\" You should have received a copy of the GNU General Public

.\" License along with this manual; if not, see

.\" <http://www.gnu.org/licenses/>.

.\" %%%LICENSE_END

.\"

.TH LISTXATTR 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

listxattr, llistxattr, flistxattr \- list extended attribute names

.SH SYNOPSIS

.fam C

.nf

.B #include <sys/types.h>

.B #include <sys/xattr.h>

.PP

.BI "ssize_t listxattr(const char\ *" path ", char\ *" list \

", size_t " size );

.BI "ssize_t llistxattr(const char\ *" path ", char\ *" list \

", size_t " size );

.BI "ssize_t flistxattr(int " fd ", char\ *" list ", size_t " size );

.fi

.fam T

.SH DESCRIPTION

Extended attributes are

.IR 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

.BR stat (2)

data).

A complete overview of extended attributes concepts can be found in

.BR xattr (7).

.PP

.BR listxattr ()

retrieves the list

of extended attribute names associated with the given

.I path

in the filesystem.

The retrieved list is placed in

.IR list ,

a caller-allocated buffer whose size (in bytes) is specified in the argument

.IR 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

.I list

is returned.

.PP

.BR llistxattr ()

is identical to

.BR 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.

.PP

.BR flistxattr ()

is identical to

.BR listxattr (),

only the open file referred to by

.I fd

(as returned by

.BR open (2))

is interrogated in place of

.IR path .

.PP

A single extended attribute

.I name

is a null-terminated string.

The name includes a namespace prefix; there may be several, disjoint

namespaces associated with an individual inode.

.PP

If

.I size

is specified as zero, these calls return the current size of the

list of extended attribute names (and leave

.I 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.)

.SS Example

The

.I list

of names is returned as an unordered array of null-terminated character

strings (attribute names are separated by null bytes (\(aq\\0\(aq)), like this:

.PP

.in +4n

.EX

user.name1\\0system.name1\\0user.name2\\0

.EE

.in

.PP

Filesystems that implement POSIX ACLs using

extended attributes might return a

.I list

like this:

.PP

.in +4n

.EX

system.posix_acl_access\\0system.posix_acl_default\\0

.EE

.in

.SH RETURN VALUE

On success, a nonnegative number is returned indicating the size of the

extended attribute name list.

On failure, \-1 is returned and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B 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.

.TP

.B ENOTSUP

Extended attributes are not supported by the filesystem, or are disabled.

.TP

.B ERANGE

The

.I size

of the

.I list

buffer is too small to hold the result.

.PP

In addition, the errors documented in

.BR stat (2)

can also occur.

.SH VERSIONS

These system calls have been available on Linux since kernel 2.4;

glibc support is provided since version 2.3.

.SH CONFORMING TO

These system calls are Linux-specific.

.\" .SH AUTHORS

.\" Andreas Gruenbacher,

.\" .RI < a.gruenbacher@computer.org >

.\" and the SGI XFS development team,

.\" .RI < linux-xfs@oss.sgi.com >.

.\" Please send any bug reports or comments to these addresses.

.SH BUGS

.\" The xattr(7) page refers to this text:

As noted in

.BR xattr (7),

the VFS imposes a limit of 64\ kB on the size of the extended

attribute name list returned by

.BR 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.

.SH EXAMPLE

The following program demonstrates the usage of

.BR listxattr ()

and

.BR getxattr (2).

For the file whose pathname is provided as a command-line argument,

it lists all extended file attributes and their values.

.PP

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

.BR 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

.BR ERANGE .

Calls to

.BR getxattr (2)

could be handled similarly.

.PP

The following output was recorded by first creating a file, setting

some extended file attributes,

and then listing the attributes with the example program.

.SS Example output

.in +4n

.EX

$ \fBtouch /tmp/foo\fP

$ \fBsetfattr -n user.fred -v chocolate /tmp/foo\fP

$ \fBsetfattr -n user.frieda -v bar /tmp/foo\fP

$ \fBsetfattr -n user.empty /tmp/foo\fP

$ \fB./listxattr /tmp/foo\fP

user.fred: chocolate

user.frieda: bar

user.empty: <no value>

.EE

.in

.SS Program source (listxattr.c)

.EX

#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);

}

.EE

.SH SEE ALSO

.BR getfattr (1),

.BR setfattr (1),

.BR getxattr (2),

.BR open (2),

.BR removexattr (2),

.BR setxattr (2),

.BR stat (2),

.BR symlink (7),

.BR xattr (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.