[Libwebsockets] Example of custom event loop integration?

Felipe Gasper felipe at felipegasper.com
Sun May 30 13:25:35 CEST 2021


> On May 30, 2021, at 1:35 AM, Andy Green <andy at warmcat.com> wrote:
> 
> On 5/30/21 1:21 AM, Felipe Gasper wrote:
> 
> Hi Felipe -
> 
>> 	I’m considering writing a Perl binding to libwebsockets. For this to work I need to be able to integrate libwebsockets into whatever event loop the Perl application runs. (There are many loops, and three widely-used abstraction interfaces over them.)
>> 	I found the HTTP-server foreign event lib integration example: https://libwebsockets.org/git/libwebsockets/tree/minimal-examples/http-server/minimal-http-server-eventlib-foreign/minimal-http-server-eventlib-foreign.c
>> 	This example, though, appears only to use event libraries that libwebsockets natively supports. Does libwebsockets include an interface like libcurl’s multi_socket interface (cf. https://curl.se/libcurl/c/libcurl-multi.html)? That seems to be what `struct lws_event_loop_ops` and `struct lws_plugin_evlib` represent … are there any examples of giving these structs to libwebsockets at runtime?
> 
> For custom loops in the application, there is a gnarly hack from a decade ago it sounds like you found, the EXTERNAL_POLL support, I really don't recommend using this.  Modern lws offers hi res scheduling services (for, eg, UDP retry) that are used internally and encouraged for user code, since it radically improves portability, that are coming from modulating the wait via event lib timers or timeout computation. EXTERNAL_POLL's concept was much dumber, just bringing a zero-wait service call inside an existing custom event loop, it cannot make a satisfying result for both latency and cpu load.  It also has a dependency on protocols[0] which is user code business, and not available for the kind of generic integration you are looking at.

Ah, ok. So best to avoid EXTERNAL_POLL if possible.

Many Perl applications run their own select/poll loop. It’s not performant, but it is what it is. I’d like to support that rather than requiring users to use libev, libuv, etc. (which may not be available if they lack compiler access).

So, is there a supported/recommended mechanism that allows an application to do its own custom polling without recompiling libwebsockets to include a custom plugin? To start/stop libwebsockets as part of that custom loop? Is there an example somewhere?

Thank you!

cheers,
-Felipe


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