+=head1 PRIVATE DATA AREA
+
+You can attach named pieces of private data to the libguestfs handle,
+and fetch them by name for the lifetime of the handle. This is called
+the private data area and is only available from the C API.
+
+To attach a named piece of data, use the following call:
+
+ void guestfs_set_private (guestfs_h *g, const char *key, void *data);
+
+C<key> is the name to associate with this data, and C<data> is an
+arbitrary pointer (which can be C<NULL>). Any previous item with the
+same name is overwritten.
+
+You can use any C<key> you want, but names beginning with an
+underscore character are reserved for internal libguestfs purposes
+(for implementing language bindings). It is recommended to prefix the
+name with some unique string to avoid collisions with other users.
+
+To retrieve the pointer, use:
+
+ void *guestfs_get_private (guestfs_h *g, const char *key);
+
+This function returns C<NULL> if either no data is found associated
+with C<key>, or if the user previously set the C<key>'s C<data>
+pointer to C<NULL>.
+
+Libguestfs does not try to look at or interpret the C<data> pointer in
+any way. As far as libguestfs is concerned, it need not be a valid
+pointer at all. In particular, libguestfs does I<not> try to free the
+data when the handle is closed. If the data must be freed, then the
+caller must either free it before calling L</guestfs_close> or must
+set up a close callback to do it (see L</guestfs_set_close_callback>,
+and note that only one callback can be registered for a handle).
+
+The private data area is implemented using a hash table, and should be
+reasonably efficient for moderate numbers of keys.
+