select.poll and ppoll

[Marko Rauhamaa]

Steven D'Aprano <steve+comp.lang.python at pearwood.info>:

> I have select.poll, but I'm looking for something like ppoll instead. From 
> the Linux man page:
>
>    ppoll()
>        The relationship between poll() and ppoll() is analogous to the
>        relationship  between  select(2)  and  pselect(2):  like  pselect(2), 
>        ppoll()  allows  an  application to safely wait until either a file
>        descriptor becomes ready or until a signal is caught.
>
> Technically, *I* don't want this, it's one of my work-colleagues. He
> says:
>
> "My high-level goal is to run a callback function whenever the alsa
> mixer level changes. The C alsa API provides
> snd_mixer_elem_set_callback, but the Python API (import alsaaudio)
> seems to need me to get poll(2) descriptors"

I've never used ppoll or pselect. They are used to fix naive signal
handling in a main loop:

    while not signaled:
        select.poll(...)
        ...

The loop suffers from a race condition: the "signaled" flag, which is
set by a signal handler, might change between "while" and "select.poll".

The standard, classic way to solve the race condition is to replace the
"signaled" flag with an internal pipe. The signal handler writes a byte
into the pipe and select.poll() wakes up when the pipe becomes readable.
IOW, ppoll() and pselect() are not needed.


Marko