Copyright (c) 1999, 2000, 2001 Robert N. M. Watson
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$FreeBSD: head/share/man/man9/extattr.9 96711 2002-05-16 05:21:58Z trhodes $
.Dd December 23, 1999 .Os .Dt EXTATTR 9 .Sh NAME .Nm extattr .Nd virtual filesystem named extended attributes .Sh SYNOPSIS n sys/param.h n sys/vnode.h n sys/extattr.h .Sh 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 filesystems may implement additional namespaces, or not implement these namespaces: .Dv EXTATTR_NAMESPACE_USER , .Dv 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.
p Reads of extended attribute data may return specific contiguous regions of the meta-data, in the style of .Xr VOP_READ 9 , but writes will replace the entire current "value" associated with a given name. As there are a plethora of filesystems 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 filesystem. 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.
p Extended attributes are named using a null-terminated character string. Depending on underlying filesystem semantics, this name may or may not be case-sensitive. Appropriate vnode extended attribute calls are: .Xr VOP_GETEXTATTR 9 and .Xr VOP_SETEXTATTR 9 . .Sh SEE ALSO .Xr VFS 9 , .Xr VFS_EXTATTRCTL 9 , .Xr VOP_GETEXTATTR 9 , .Xr VOP_SETEXTATTR 9 .Sh AUTHORS This man page was written by .An Robert Watson . .Sh BUGS The extended attribute access interface is currently unable to return the total size of the attribute, or indicate that the buffer space provided by the caller is insufficient to hold all available data. In addition, the interface does not provide a mechanism to retrieve the current set of available attributes; it has been suggested that providing a .Dv NULL attribute name should cause a list of defined attributes for the passed file or directory, but this is not currently implemented.