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
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
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
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
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
Appropriate vnode extended attribute calls are: