[lwip-users] Re: [lwip] Q: implementation for select()

[Duncan Palmer]

On Tuesday 06 August 2002 16:46, Sebastian Bauer wrote:
> Hi!
>
> > > Hello,
> > > Is there an implementation for select() in the socket library
> > > available? Where can I find it?
> > > That would be very useful for my application.
> >
> > I'd also like to have it. Which parts of the select functionality do you
>
> need?
>
> I need at least the readfds.

If you just have one fd in your list of readfd's, its pretty trivial - you 
can just do a non-blocking read on conn->recvmbox. For more than one, its a 
bit more work. Did you have any implementation ideas? I've just had a bit of 
a think about it, and come up with this. What do you think? 

Add 2 lists of semaphores to each struct netconn - one to be used by readfds 
and one by writefds. Add fields to the struct netconn's to enable FD_SET and 
friends to be implemented.

To implement readfd's: The select call will create a new semaphore and add it 
to the list of semaphores on all connections which correspond to its list of 
readfds. It will then wait on this semaphore. When new data comes in (i.e. in 
all the places where conn->recvmbox is posted to), signal all read semaphores 
on the connection. This will wake up the select call which can delete the 
semaphore from all connections it was waiting from and return. 

To implement writefds: If the connection is a UDP connection, it doesn't 
block, so that's easy. If its a TCP connection, the select call creates a new 
semaphore and adds it to the list of semaphores on all connections which 
correspond to its list of writefds. Writes on TCP sockets block if 
tcp_sndbuf(conn->pcb.tcp) == 0, and all writes result in a call to sent_tcp() 
or err_tcp(). So, in each of these functions, signal all write semaphores on 
the connection. This will wake up the select call which can delete the 
semaphore from all connections it was waiting from and return. 

If we're awoken as a result of an error (conn->err is set), select needs to 
return failure.

This actually sounds easier than i thought it would be - does it look correct 
to you? 

>
> Would be nice if somebody could implement this. Otherwise I will try it
> because it's quiet important for me.
> But then where do all the patches go which are sent to the list?
> Is there a cvs which I can access?
>
> Ciao,
> Sebastian
>
>
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