Named 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 to
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_USEREXTATTR_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_READ9,
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 null-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_GETEXTATTR9,
VOP_LISTEXTATTR9,
and
VOP_SETEXTATTR9.
This manual page was written by
An 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 attributes for the passed file
or directory, but this is not currently implemented.
