[zeromq-dev] zloop_timer() does not fire if added after zloop_start()...

Andy Ballingall TF ballingall at thefoundry.co.uk
Thu Dec 27 17:01:35 CET 2012

Hi Pieter,

On 27 December 2012 13:55, Pieter Hintjens <ph at imatix.com> wrote:
> On Thu, Dec 27, 2012 at 2:53 PM, Andy Ballingall TF
> <ballingall at thefoundry.co.uk> wrote:
>> Note how my timer registration succeeds, but the polling seems to
>> ignore it?
> Then it's because the tickless timer is being miscalculated; it should
> be re-calculated each time you add or remove a timer handler. If not,
> there's a bug.

I agree with Apostolis below that the zloop code seems correct, with
respect to the calculation of the next timer expiry. Replicating my
scenario with test code would be quite a bit of work, so I took
another look at the zloop implementation again, and I think I can see
what's happening.

Firstly, my timer mutations were occurring within a message handler,
and the mutation consisted of:
a) Remove existing timer
b) Create a new one.

The old removed timer and the new timer both have the same 'arg' (in
my case this is an instance of my communications class).

The reactor loop (in function zloop_start() ) first processes any poll
handlers, then at the end, checks if the zombies list has any timers
to clear.

By the time the zloop_start() function gets to line 420 (where it
checks for zombies), then the state is:

Zombies: 1 (arg='this')

Timers: 2
1 = old timer (arg='this', expiry=<old time>)
2 = new timer (arg = 'this', expiry = <new time>)

The loop at line 423 will blow away all the timers - including the one
I just added - as it matches the zombie arg.

That explains why my newly added timer wasn't firing, and why the poll
timeout was set to 1 hour: It's because my newly added timer had been
removed almost immediately!

If the timer deletion used a reference to the timer object rather than
'arg' to reference it, then it would ensure that I could call
zloop_timer_end() and subsequently zloop_timer() from within a
handler, without the zloop_start() function removing that newly added

In the end, this isn't a problem for me because I've changed my
implementation to use a heartbeat timer which never gets removed and
replaced, but I can see scenarios in the future where it would be good
to reset an existing timer, or delete and add a number of timers
inside a handler...

Anyway, thanks for your help.


> -Pieter
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Andy Ballingall
Senior Software Engineer

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