[Libwebsockets] How to set the "user" callback parameter for adopted sockets?

Alan Conway aconway at redhat.com
Thu Nov 24 15:42:02 CET 2016


On Wed, 2016-11-23 at 23:19 +0800, Andy Green wrote:
> 
> On November 23, 2016 10:53:57 PM GMT+08:00, Alan Conway <aconway at redh
> at.com> wrote:
> > 
> > On Wed, 2016-11-23 at 09:46 -0500, Alan Conway wrote:
> > > 
> > > On Wed, 2016-11-23 at 08:50 +0800, Andy Green wrote:
> > > > 
> > > > 
> > > > On Tue, 2016-11-22 at 19:02 -0500, Alan Conway wrote:
> > > > > 
> > > > > 
> > > > > 
> > > > > I'm making progress! I can crash my router from a web brower
> > > > > :)
> > > > > 
> > > > > I can't figure out how to set the "user" parameter for lws
> > > > > callbacks
> > > > > for a socket adopted with lws_adopt_socket(). I can access
> > > > > the
> > > > 
> > > > That's not what *user_space is for.
> > > > 
> > > > It's for protocol-specific local data, per-connection.  When
> > > > the
> > > > protocol changes, which it may often do with a http/1.1
> > > > keepalive
> > > > connection visiting urls that map on to different plugins, the
> > > > user
> > > > data allocation is destroyed by lws and a new one sized
> > > > appropriately
> > > > for the new protocol allocated.
> > > 
> > > For me, the protocol starts as "http" and may change to "binary"
> > > or
> > > "amqp" (which I treat as both "amqp"). Does that mean that if I
> > > store
> > > context for the http protocol, it will be wiped out in the
> > > upgrade?
> > > Is
> > > there any way to preserve some per-connection data between the
> > > two or
> > > is each protocol session treated as entirely separate from
> > > previous
> > > ones?
> > > 
> > > > 
> > > > 
> > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > allocated space *after* the adopt call with lws_wsi_user(),
> > > > > but
> > > > > lws_adopt_socket() itself is firing my callbacks so that's
> > > > > too
> > > > > late.
> > > > > 
> > > > > The docs mention setting up your session data on the
> > > > > LWS_CALLBACK_ESTABLISHED event, but at that point I have
> > > > > nothing
> > > > > I
> > > > > can
> > > > > use to find data from my application.
> > > > 
> > > > LWS doesn't have this concept of external shadowed data per-
> > > > connection
> > > > built-in.
> > > > 
> > > > You have two ways to do it
> > > > 
> > > >  - add a new opaque void * in struct lws and set it at adopt-
> > > > time,
> > > > provide an accessor to get it from the wsi.  This is a bit of a
> > > > burden
> > > > for everyone who doesn't care about this then.
> > > 
> > > For current purposes I need to use a released, packaged version
> > > of
> > > lws
> > > so modifying it isn't an option - I do hope to make contributions
> > > later...
> > > 
> > > > 
> > > > 
> > > >  - Set the context user pointer at context creation time.  You
> > > > can
> > > > get
> > > > this from a wsi in the callback.  You can then use this
> > > > context-
> > > > wide
> > > > pointer to dereference the wsi pointer to your private
> > > > connection
> > > > pointer on your side, entirely outside of lws.
> > > 
> > > I could use either the fd or the wsi pointer to look up my
> > > application
> > > data, but it's an expensive lookup that requires a map of all the
> > > known
> > > fds/wsi-pointers. I could use thread-local storage as a hack to
> > > get
> > > my
> > > context into the initial "http" established callback, but that
> > > doesn't
> > > help if it will get wiped out in the upgrade to "amqp"... Hmmm.
> > > 
> > 
> > What if I use a separate context per connection? I know you said I
> 
> No, that is a very bad idea.

Why? What will go wrong?







More information about the Libwebsockets mailing list