[Libwebsockets] Please explain what is rx_flow_control , and what is fx flow , and how it is related to binary data

Andy Green andy at warmcat.com
Mon Jun 20 10:39:19 CEST 2016

On 06/20/2016 04:28 PM, Meir Yanovich wrote:
> Hello
> i want to add support to my server to able handling binary data from
> client .

There is no difference between text and binary ws messages for lws, 
except you use LWS_WRITE_BINARY.

> i have only 1 example which is test-echo.c .
> i can see the handling of the incoming binary data using
> LWS_WRITE_BINARY protocol.
> i can see there is some kind of "rx" handling in the code :

This is unrelated to binary or not.  There is no requirement to do rx 
flow control just because you want to send a binary ws mesage.

> do_rx:
> pss->final = lws_is_final_fragment(wsi);
> pss->binary = lws_frame_is_binary(wsi);
> lwsl_info("+++ test-echo: RX len %d final %d, pss->len=%d\n",
>  len, pss->final, (int)pss->len);
> memcpy(&pss->buf[LWS_PRE], in, len);
> assert((int)pss->len == -1);
> pss->len = (unsigned int)len;
> pss->rx += len;
> lws_rx_flow_control(wsi, 0);
> lws_callback_on_writable(wsi);
> break;
> how does it all connects ?

Binary ws messages and rx flow control are unrelated.

RX flow control allows the user code to halt processing of incoming RX 
until later.

Often in network programming, the rate you can accept new input is not 
decided by the guy sending it, it's decided by the guy you are sending to.

In the case your onward destination can't accept any more right now, you 
have to create "backpressure" that stops more new input being processed.

This -->

 > lws_rx_flow_control(wsi, 0);
 > lws_callback_on_writable(wsi);

is part of stopping new rx coming until the onward recipient accepted 
some data.

Take a look at what the mirror protocol example does about it as well.

> also as i side note ,
> in test-echo.c i can see there is also the client code included i can't
> understand how i start the client from the same file of the server , (
> the server is starting great )

Just do something similar to what it does in test-client.c does.  If 
he's not connected, he tries to connect asynchronously every couple of 

It's OK in lws >2.0 to mix server and client connections for ws or http 
in the same context.


> Thanks
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