[lwip-users] Sending with thread safety

[Mike He]

Hello,

I am looking for some help understanding the proper way to send packets 
with lwIP while maintaining thread safety. As I understand from the 
documentation, using raw API, I have to call lwIP functions from inside 
lwIP's execution context.

For operating in server mode, this is easy to understand and is 
well-documented in the examples given. When a request is received, the 
response is sent back in the tcp_recv callback, so all the code is in 
lwIP's context.

However, for sending data with lwIP that is not the response of a 
request - for example as a client, or as part of a full-duplex protocol 
such as Websocket - the trigger to send a packet in these cases is 
initiated by something outside the lwIP context, and I am not sure the 
proper way to bring the packet into the lwIP context. Assuming the 
connection is set up already, then to send a packet, the documentation 
says to call tcp_write to enqueue and tcp_output to send.

1. Where is the proper place to call these functions in the lwIP 
context? Is it in the lwIPHostTimerHandler?
2. Do I need to call both tcp_write and tcp_output in the lwIP context? 
Or can I enqueue with tcp_write outside of the context, and just call 
tcp_output from inside the lwIP context?
3. Is the basic procedure then to set a flag from outside the context 
(passing any data through global or file scope), then call the lwIP 
functions from inside its execution context?

Thanks for any help,
Mike