[Libwebsockets] Using libwebsockets as a codec

Andy Green andy at warmcat.com
Tue Jun 9 16:17:12 CEST 2020

On 6/9/20 3:02 PM, Andy Green wrote:
> On 6/9/20 2:54 PM, Ganesh Murthy wrote:
>> Our application has its own IO layer and is capable of providing 
>> libwebsockets with raw byte buffers. Does lws have a pure codec mode 
>> which takes in these buffers and produces http events that I can then 
>> go on and use? This question was already asked 4 years ago here  - 
>> https://libwebsockets.org/pipermail/libwebsockets/2016-November/002888.html 
>> The closest I could find to an api that accepts buffers is 
>> https://github.com/warmcat/libwebsockets/blob/master/include/libwebsockets/lws-adopt.h#L173 
>> but I don't have access to file descriptors.
> "Codec" means [en]coder/decoder it might mean something to you but it 
> doesn't give me any picture of what you are talking about when you say 
> that phrase wrt lws.
> Do you mean perhaps lws "raw" protocol?  This just passes payload around 
> without applying or removing any framing, eg
> https://libwebsockets.org/git/libwebsockets/tree/minimal-examples/raw/minimal-raw-netcat 

Never mind I read the thread you linked...

What I was saying in there about flow control is even more relevant now 
lws supports, eg, ws-over-h2.

The general feeling was that the focus was on your backend and the 
network stuff should just be a black box that plays well with what you 
had... due to things like tx credit management and flow control 
generally, the network part needs to have its own event loop that it is 
in charge of.  Eg the OP mentioned for flow control

 > If the write buffer fills up, the read-buffer is disabled till there 
is some write space.

that seems to make a lot of assumptions about where the data is going 
and relationship between tx situation and rx situation that might be so 
in, eg, a proxy, but not be true for anything else.

You can fill and drain these buffers from another thread and just let 
lws do whatever it's doing with them asynchronously... I am pretty sure 
that can be made to work.  Anything else, maybe I am just too dumb to 
see the light but it seemed a lot of grinding and clanking noises that 
could be expected were getting glossed over.


> -Andy

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