Re: [lwip-users] Socket API thread safe?

[Peter Graf]

Hi Leon,

> > Leon Woestenberg wrote:
> > > This is a recurring topic (and rightly so).
> > > lwIP socket API is *not* multi-threaded safe in the *generic* sense, 
> > > i.e. for a full-pre-emptive environment (this is as far as I know).
> > So how do you know (given a proper implementation of sysarch and 
> > lightweight prot. of course)? Can you provide an example?
> I do not know for certain, but lets take an example where I think it
> goes wrong:
>
> 1 - network interface hardware receives packet, raises interrupt
> 2 - the lwIP netif device driver's foobar_input() funtion is called
>     from a task that is made runnable, triggered by the interrupt.
> 3 - the packet happens to update the ARP table
> 4 - During 3, a higher priority task becomes runnable (and thus running)
> and sends out a packet on its own private UDP socket.
>
> We have a concurrent reader and writer in the ARP module and have
> incorrect behaviour.

Thanks. I think you are right. Now it becomes clearer that I never saw an 
effect like this, because in my case:

a. The task that is triggered by the interrupt has highest possible priority
b. I have little movement in the ARP table. There are few local hosts, most 
traffic goes to the gateway.
c. I use TCP rather than UDP

Can we say that the essential thread-safety problems are restricted to ARP? 
To me it looks like (almost) the only case where the sequential API is 
circumvented.

> I have explicitly not mentioned a API here. I am just curious if this
> case is covered. I am SURE the raw API is non-thread-safe, I just wish
> to hear the netconn_ API is.

I can not say much about the netconn API, since I use the sockets layer on 
top, which provides further mutual exclusion. My view is just practical 
experience anyway - I can not prove correctness. I have, however, used lwIP 
quite a lot, and at found it stable enought to be _useful_ in a 
full-pre-emptive environment.

All the best
Peter