Re: [lwip-devel] Netconn callback order

[Jonathan Larmour]

Bill Florac wrote:
> Is there a reason why the callback from netconn_new_with_callback() must
> occur before the data is put into the mailbox rather than after
> (recv_udp() function in api_msg.c)?  I would like to make the receiving
> of packets (UDP) event based rather than have to poll and wait for them.
> If the callback occurs after the posting then I could get the data from
> the callback. Or, better yet, pass a pointer to the netbuf in the
> callback and return a flag if the data has been eaten skipping the
> mailbox all together.

The primary user of callback stuff is select() in the sockets API. I think
it's this way round to avoid a race condition where a socket gets data read
from it before the select. The relevant bit being that there's a RCVMINUS
event on a successful read, and a RCVPLUS when data arrives. In
event_callback() in sockets.c, these are used to set sock->rcvevent which
is essentially the number of pbufs waiting to be read.

If what you proposed were implemented then depending on when threads run, I
believe sock->rcvevent could wrap around, breaking event_callback().

Jifl
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