man-pages/man2/listxattr.2

.\" Copyright (C) Andreas Gruenbacher, February 2001
.\" Copyright (C) Silicon Graphics Inc, September 2001
.\" Copyright (C) 2015 Heinrich Schuchardt <xypron.glpk@gmx.de>
.\"
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.TH LISTXATTR 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
listxattr, llistxattr, flistxattr \- list extended attribute names
.SH SYNOPSIS
.fam C
.nf
.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\e0\(aq)), like this:
.PP
.in +4n
.EX
user.name1\e0system.name1\e0user.name2\e0
.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\e0system.posix_acl_default\e0
.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 to indicate the error.
.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 EXAMPLES
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\en", 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.\en", 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("\en");

        /*
         * 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)