extattr(9)

NAME

extattr - virtual file system named extended attributes

SYNOPSIS

#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/extattr.h>

DESCRIPTION

Named extended attributes allow additional meta-data to be

associated
with vnodes representing files and directories. The seman

tics of this
additional data is that of a "name=value" pair, where a name

may be
defined or undefined, and if defined, associated with zero

or more bytes
of arbitrary binary data. Extended attribute names exist

within a set of
namespaces; each operation on an extended attribute is re

quired to provide the namespace to which to operation refers. If the

same name is
present in multiple namespaces, the extended attributes as

sociated with
the names are stored and manipulated independently. The

following two
namespaces are defined universally, although individual file

systems may
implement additional namespaces, or not implement these

namespaces:
EXTATTR_NAMESPACE_USER, EXTATTR_NAMESPACE_SYSTEM. The se

mantics of these
attributes are intended to be as follows: user attribute da

ta is protected according the normal discretionary and mandatory pro

tections associated with the data in the file or directory; system at

tribute data is
protected such that appropriate privilege is required to di

rectly access
or manipulate these attributes.

Reads of extended attribute data may return specific con

tiguous regions
of the meta-data, in the style of VOP_READ(9), but writes

will replace
the entire current "value" associated with a given name. As

there are a
plethora of file systems with differing extended attributes,

availability
and functionality of these functions may be limited, and

they should be
used with awareness of the underlying semantics of the sup

porting file
system. Authorization schemes for extended attribute data

may also vary
by file system, as well as maximum attribute size, and

whether or not any
or specific new attributes may be defined.

Extended attributes are named using a null-terminated char

acter string.
Depending on underlying file system semantics, this name may

or may not
be case-sensitive. Appropriate vnode extended attribute

calls are:
VOP_GETEXTATTR(9), VOP_LISTEXTATTR(9), and VOP_SETEXTAT

TR(9).

SEE ALSO

VFS(9), VFS_EXTATTRCTL(9), VOP_GETEXTATTR(9), VOP_LISTEXTAT

TR(9),
VOP_SETEXTATTR(9)

AUTHORS

This manual page was written by Robert Watson.

BUGS

In addition, the interface does not provide a mechanism to

retrieve the
current set of available attributes; it has been suggested

that providing
a NULL attribute name should cause a list of defined at

tributes for the
passed file or directory, but this is not currently imple

mented.

BSD December 23, 1999

BSD