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NAME

     extattr - virtual file system named extended attributes

SYNOPSIS

     #include <sys/param.h>
     #include <sys/vnode.h>
     #include <sys/extattr.h>

DESCRIPTION

     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).

SEE ALSO

     VFS(9), VFS_EXTATTRCTL(9), VOP_GETEXTATTR(9), VOP_LISTEXTATTR(9),
     VOP_SETEXTATTR(9)

AUTHORS

     This manual page was written by 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.