+=head2 guestfs_set_progress_callback
+
+ typedef void (*guestfs_progress_cb) (guestfs_h *g, void *opaque,
+ int proc_nr, int serial,
+ uint64_t position, uint64_t total);
+ void guestfs_set_progress_callback (guestfs_h *g,
+ guestfs_progress_cb cb,
+ void *opaque);
+
+Some long-running operations can generate progress messages. If
+this callback is registered, then it will be called each time a
+progress message is generated (usually two seconds after the
+operation started, and three times per second thereafter until
+it completes, although the frequency may change in future versions).
+
+The callback receives two numbers: C<position> and C<total>.
+The units of C<total> are not defined, although for some
+operations C<total> may relate in some way to the amount of
+data to be transferred (eg. in bytes or megabytes), and
+C<position> may be the portion which has been transferred.
+
+The only defined and stable parts of the API are:
+
+=over 4
+
+=item *
+
+The callback can display to the user some type of progress bar or
+indicator which shows the ratio of C<position>:C<total>.
+
+=item *
+
+0 E<lt>= C<position> E<lt>= C<total>
+
+=item *
+
+If any progress notification is sent during a call, then a final
+progress notification is always sent when C<position> = C<total>.
+
+This is to simplify caller code, so callers can easily set the
+progress indicator to "100%" at the end of the operation, without
+requiring special code to detect this case.
+
+=back
+
+The callback also receives the procedure number and serial number of
+the call. These are only useful for debugging protocol issues, and
+the callback can normally ignore them. The callback may want to
+print these numbers in error messages or debugging messages.
+
+=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.
+
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