select(sock) indicates not-ready, but sock.recv does not block

Roy Smith roy at
Mon Feb 17 14:04:13 CET 2014

In article <mailman.7083.1392618926.18130.python-list at>,
 Nikolaus Rath <Nikolaus at> wrote:

> Hello,
> I have a problem with using select. I can reliably reproduce a situation
> where,), (), (), 0) returns ((),(),())
> (i.e., no data ready for reading), but an immediately following
> sock.recv() returns data without blocking.
> I am pretty sure that this is not a race condition. The behavor is 100%
> reproducible, the program is single threaded, and even waiting for 10
> seconds before the select() call does not change the result.
> I'm running Python 3.3.3 under Linux 3.12.
> Has anyone an idea what might be going wrong here?
> Thanks,
> -Nikolaus

Can you post the code which demonstrates this?

Also, with any kind of networking problem, tcpdump is your fried.  When 
you run your code, use tcpdump to watch all the network traffic on 
whatever port your socket is bound to.  That might give you some clues 
what's going on.

Likewise, I would also strace the process and watch all the network 
system calls.  The problem you're describing might be unexpected 
behavior in Python, or it might be in the kernel.  Watching the actual 
system calls that are generated will narrow it down to which.

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