extattr - virtual file system named extended attributes
Named extended attributes allow additional meta-data to be
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
namespaces are defined universally, although individual file
implement additional namespaces, or not implement these
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
the entire current "value" associated with a given name. As
there are a
plethora of file systems with differing extended attributes,
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
VOP_GETEXTATTR(9) and VOP_SETEXTATTR(9).
This man page was written by Robert Watson.
In addition, the interface does not provide a mechanism to
current set of available attributes; it has been suggested
a NULL attribute name should cause a list of defined attributes for the
passed file or directory, but this is not currently implemented.
OpenBSD 3.6 December 23, 1999
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