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

Andy Green andy at warmcat.com
Wed Nov 23 16:19:50 CET 2016



On November 23, 2016 10:53:57 PM GMT+08:00, Alan Conway <aconway at redhat.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.

>should have only one context but: a context per connection avoids all
>thread-safety issues around concurrent use of a context, and gives me a
>place to store the connection-lifetime info I need. Obviously there'll
>be extra overhead but would it work?

No.

-Andy

>Cheers,
>Alan.




More information about the Libwebsockets mailing list