[Python-Dev] Signals, threads, blocking C functions (original) (raw)

Gustavo Carneiro gjcarneiro at gmail.com
Sat Sep 2 14:10:04 CEST 2006

We have to resort to timeouts in pygtk in order to catch unix signals in threaded mode. The reason is this. We call gtk_main() (mainloop function) which blocks forever. Suppose there are threads in the program; then any thread can receive a signal (e.g. SIGINT). Python catches the signal, but doesn't do anything; it simply sets a flag in a global structure and calls Py_AddPendingCall(), and I guess it expects someone to call Py_MakePendingCalls(). However, the main thread is blocked calling a C function and has no way of being notified it needs to give control back to python to handle the signal. Hence, we use a 100ms timeout for polling. Unfortunately, timeouts needlessly consume CPU time and drain laptop batteries.

According to [1], all python needs to do to avoid this problem is block all signals in all but the main thread; then we can guarantee signal handlers are always called from the main thread, and pygtk doesn't need a timeout.

Another alternative would be to add a new API like Py_AddPendingCallNotification, which would let python notify extensions that new pending calls exist and need to be processed.

But I would really prefer the first alternative, as it could be fixed within python 2.5; no need to wait for 2.6.

Please, let's make Python ready for the enterprise! [2]

[1] https://bugzilla.redhat.com/bugzilla/process_bug.cgi#c3 [2] http://perkypants.org/blog/2006/09/02/rfte-python/

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