[Python-Dev] Signals, threads, blocking C functions

Michael Hudson mwh at python.net
Wed Sep 6 12:34:23 CEST 2006

"Gustavo Carneiro" <gjcarneiro at gmail.com> writes:

> On 9/4/06, Nick Maclaren <nmm1 at cus.cam.ac.uk> wrote:
>> "Gustavo Carneiro" <gjcarneiro at gmail.com> wrote:
>> >   I am now thinking of something along these lines:
>> > typedef void (*PyPendingCallNotify)(void *user_data);
>> > PyAPI_FUNC(void) Py_AddPendingCallNotify(PyPendingCallNotify callback,
>> >     void *user_data);
>> > PyAPI_FUNC(void) Py_RemovePendingCallNotify(PyPendingCallNotify
>> >     callback, void *user_data);
>> Why would that help?  The problems are semantic, not syntactic.
>> Anthony Baxter isn't exaggerating the problem, despite what you may
>> think from his posting.
>   You guys are tough customers to please. 


> I am just trying to solve a problem here, not create a new one; you
> have to believe me.

We believe you, but you are stirring the ashes of old problems.

>      1. In PyGTK we have a gobject.MainLoop.run() method, which blocks
> essentially forever in a poll() system call, and only wakes if/when it
> has to process timeout or IO event;
>      2. When we only have one thread, we can guarantee that e.g.
> SIGINT will always be caught by the thread running the
> g_main_loop_run(), so we know poll() will be interrupted and a EINTR
> will be generated, giving us control temporarily back to check for
> python signals;
>      3. When we have multiple thread, we cannot make this assumption,
> so instead we install a timeout to periodically check for signals.
>   We want to get rid of timeouts.  Now my idea: add a Python API to say:
>      "dear Python, please call me when you start having pending calls,
> even if from a signal handler context, ok?"

This seems a reasonable proposal.  But it's totally a Python 2.6
thing, so how about taking a deep breath, working on a patch and
submitting it when it's ready?

Having to wake a process up a few times a second is ugly and annoying,
sure, but it is not a release delaying problem.


  It is never worth a first class man's time to express a majority
  opinion.  By definition, there are plenty of others to do that.
                                                        -- G. H. Hardy

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