[alsa-devel] snd_seq_query_port_subscribers not updating subs index

[Charles Flèche]

On my systems it does increment the index number, and I think I took this
code first from code examples of the Alsa doc... I'll  make sure to always
increment the index by hand.

Thanks again, Takashi

On Thu, Apr 12, 2018 at 4:54 PM, Takashi Iwai <tiwai at suse.de> wrote:

> On Thu, 12 Apr 2018 16:49:27 +0200,
> Charles Flèche wrote:
> >
> > > Sort of, yes
> >
> > >From the snd_seq_query_port_subscribers doc: "To list up all the
> > subscriptions, call this function with the index numbers from 0 until
> this
> > returns a negative value."
> >
> > So does it mean than in certain cases snd_seq_query_port_subscribers
> > increments the index itself, and sometime it does not ?
>
> It shouldn't increment the index number by itself; at least the kernel
> interface doesn't do that.  But the API doesn't check whether the
> backend (plugin) didn't touch the returned index value or not, so the
> possibility is still open :)
>
>
> Takashi
>
> >
> > Thanks, Takashi
> >
> > On Thu, Apr 12, 2018 at 4:44 PM, Takashi Iwai <tiwai at suse.de> wrote:
> >
> > > On Sun, 08 Apr 2018 18:37:58 +0200,
> > > Charles Flèche wrote:
> > > >
> > > > Hi all,
> > > >
> > > > I'm iterating through subscribers with
> snd_seq_query_port_subscribers to
> > > > disconnect them. It works most of the time, but when a2j (an alsa ->
> jack
> > > > MIDI bridge) runs, snd_seq_query_port_subscribers does not update the
> > > subs
> > > > index, and the next call returns 0, leading to an infinite loop. I
> have
> > > to
> > > > manually increase the index when the disconnection fails (meaning
> that I
> > > > hit an a2j connection).
> > > >
> > > > while (snd_seq_query_port_subscribers(seq, subs) >= 0) {
> > > >   addr = *snd_seq_query_subscribe_get_addr(subs);
> > > >   if (snd_seq_disconnect_to(seq, port, addr.client, addr.port) < 0) {
> > > >     snd_seq_query_subscribe_set_index(subs,
> > > > snd_seq_query_subscribe_get_index(subs) + 1);
> > > >   }
> > > > }
> > > >
> > > > 1. Is the non-updated subs index an expected behavior ?
> > >
> > > Sort of, yes.  It's no iterator operation like query_next_client.
> > > It just queries the given index and fills out the result.  The user
> > > needs to iterate with incrementing the index manually.
> > >
> > > > 2. Is my fix a correct one or could it leads to other issues ?
> > >
> > > It's the correct implementation.
> > >
> > >
> > > HTH,
> > >
> > > Takashi
> > >
> > [2  <text/html; UTF-8 (quoted-printable)>]
> >
>