libc lwip recv MSG_PEEK doesn't behave as specified

[Christian Helmuth]

Hello,

On Tue, Jun 19, 2018 at 08:14:07PM +0200, Johannes Kliemann wrote:
> So having at least MSG_DONTWAIT would be quite helpful. But I think the
> general problem is the API. Having a native asynchronous TCP or IP
> session with a native TCP/IP stack would be the best case but I assume
> this won't happen soon. But also an asynchronous (or at least more
> compatible) approach using libc would already be useful (and potentially
> resolve the need for any recv flags ;) ).
> 
> So currently I need some flags but I would happily switch to a generally
> better approach if there is one.

I see two prominent directions we could explore. First on the libc
level, you may use select() with a zero timeout for polling file
descriptors and only read() if data is available. This option leaves
us with the question, when to poll. The second option may be a deeper
look into Emery's current lwip port which removes the socket API level
and provides the low-level functions, which should be non-blocking.

There's also a third option in libc/select.h, which provides a
mechanism to block in the entrypoint waiting for signals, RPC, and
also for libc file descriptors to become readable. The semantics are a
bit tricky, but I know of applications out there using this mechanism
successfully.

Greets
-- 
Christian Helmuth
Genode Labs

https://www.genode-labs.com/ · https://genode.org/
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