Re: (*prepare) and (*check) need user_data

[Owen Taylor <otaylor redhat com>]

Tim Janik <timj@gtk.org> writes:

> On 23 Mar 1999, Owen Taylor wrote:
> 
> > 
> > Tim Janik <timj@gtk.org> writes:
> > 
> > > hi owen,
> > > 
> > > i discovered that for most applications of GSource in conjunction
> > > with a poll fd, you'd want to add the pollfd right away with the source
> > > and remove it again, like:
> > > 
> > > static void
> > > destroy_poll_fd (gpointer data)
> > > {
> > >   GPollFD *pfd = data;
> > > 
> > >   g_main_remove_poll (pfd);
> > >   g_free (pfd);
> > > }
> > > 
> > > 
> > >   pfd = g_new0 (GPollFD, 1);
> > >   pfd->fd = some_fd;
> > >   pfd->events |= G_IO_IN | G_IO_OUT | G_IO_PRI;
> > >   g_main_add_poll (pfd, X_PRIORITY);
> > >   g_source_add (X_PRIORITY, TRUE, &x_funcs, x, pfd, destroy_poll_fd);
> > > 
> > > to properly make use of the pollfd in prepare and check, the user_data pointer
> > > needs to be provided though, this can be achived while keeping binary
> > > compatibility with 1.2.0.
> > 
> > OK, I don't understand this at all. The user data  pointer
> > is only used to dispatch the call back to the user..
> > 
> > This should only be done in the dispatch functtion, or
> > you're breaking the model...
> 
> nope, i'm not breaking the model. i just use sources differently from the
> way e.g. timeouts or idles work. that is, i'm not using source_data for a function
> pointer and user_data as an argument to dispatch that function, rather i'm passing
> an object in as source_data and use user_data for a GPollFd*.
> 
> in (*prepare), i'm adjusting the GPollFD.events field acccording to the object's
> state.
> 
> in (*check), i'm evaluating the object state again *and* GPollFD.revents, and from
> that, i figure whether dispatching is required.
> 
> finally, (*dispatch) amounts to something as simple as object->dispatch().
> 
> what the above example meant to show is that user_data is extremely convenient
> to hold GPollFDs for fds that have the same life time as their corresponding
> GSource (which is pretty common when dealing with GPollFDs), given that you use
> sources differently than in the timout/idle way.
> 
> but to actually make use of the GPollFD, i.e. to evaluate .revents, you need a
> pointer to it, and that's what the user_data in (*check) and (*prepare) is for.

This is slightly bizarre, to me. 

Bite the bullet, create a structure, put your GPollFD 
pointer into it, and pass it as the gsource and get it over
with.

The reason why the user_data pointer is separate from the source
is basically to allow the source to get the appropriate 
GDestroyNotify behavior without duplicating code.

Nothing else in G* passes in _two_ gpointer of data to
each function. What if you need three? or four?
 
Regards,
                                        Owen