Re: RFC: MI output during program execution

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 12 Aug 2005 07:49:29 -0400
> From: Bob Rossi <bob@brasko.net>
> Cc: gdb-patches@sources.redhat.com
> 
> On Fri, Aug 12, 2005 at 12:43:19PM +0300, Eli Zaretskii wrote:
> > > Date: Thu, 11 Aug 2005 21:28:10 -0400
> > > From: Bob Rossi <bob@brasko.net>
> > > Cc: Eli Zaretskii <eliz@gnu.org>, gdb-patches@sources.redhat.com
> > > 
> > > Thanks for all the guidence so far. Even though you have not attempted
> > > the observer approach, how do you feel about it? Is this something that
> > > you think could be accomplished with the current FSF GDB? Nick, Daniel
> > > and Eli, do you like this approach?
> > 
> > The record will show that I dislike using observers in core GDB
> > sources (i.e. in the sources released with official GDB
> > distributions).  But my opinions on this are usually voted down.
> 
> I would like to become more educated on the pros/cons of the two possible
> approaches in solving this problem. What do you not like about the
> observer approach that the hook approach solves better?

I'm not sure what you mean by ``the hook approach''.  From my point of
view, observers are simply one implementation of the hook approach.

I dislike using observers because, in a C program, that looks
suspiciously like the coder doesn't know what the program is doing.  C
is not a polymorphic language, so in a C program events happen and
data of certain type arrives via a 100% predictable and deterministic
path.  If you know where the event happens which you want to hook,
simply arrange for your function to be called directly by whatever
generates or receives that event.

In addition, using observers raises an issue with multiple observers
watching the same events and stepping on each other's feet.  In other
words, the observer facility doesn't promise anything about the order
in which the observers are invoked, and doesn't give us control on the
order, so a potential arises for conflicts, if several observers futz
with the same data structures or try to produce opposite effects.

I'm sure you will find more detail about this in the archives, just
search for threads that mentioned observers.