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     extattr — virtual file system named extended attributes


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


     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

     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




     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.