NAME
extattr —
file system extended
attributes
SYNOPSIS
#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/extattr.h>
DESCRIPTION
Extended attributes allow additional meta-data to be associated with vnodes
representing files and directories. The semantics 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 required to provide the namespace to
which the operation refers. If the same name is present in multiple
namespaces, the extended attributes associated 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 semantics of these
attributes are intended to be as follows: user attribute data is protected
according the normal discretionary and mandatory protections associated with
the data in the file or directory; system attribute data is protected such
that appropriate privilege is required to directly access or manipulate these
attributes.
Reads of extended attribute data may return specific contiguous 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 supporting
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 nul-terminated character 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_SETEXTATTR(9).
SEE ALSO
vfsops(9),
vnodeops(9)