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extattr - virtual file system named extended attributes
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_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 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_GETEXTATTR(9), VOP_LISTEXTATTR(9), and VOP_SETEXTATTR(9).
VFS(9), VFS_EXTATTRCTL(9), VOP_GETEXTATTR(9), VOP_LISTEXTATTR(9),
This manual page was written by Robert Watson.
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.