Re: Observing Time
Re: Observing Time
- Subject: Re: Observing Time
- From: email@hidden
- Date: Fri, 26 Jul 2013 23:16:44 +0900
On Jul 26, 2013, at 10:06 PM, Ken Thomases <email@hidden> wrote:
> On Jul 25, 2013, at 11:37 PM, email@hidden wrote:
>
>> On Jul 26, 2013, at 12:09 PM, Ken Thomases <email@hidden> wrote:
>>
>>> Also, the above code doesn't adjust the timer to fire on the second as Rick suggested. You're asking it to fire every so many seconds (delayInSeconds) but you aren't specifying when during the second to fire. Rather than passing 0 as the second parameter of dispatch_walltime(), you should compute an adjustment to try to get close to a whole second. Since dispatch_walltime() uses gettimeofday() when you pass NULL for the first parameter, I'd use that same call to fill in a timeval structure and then pass NSEC_PER_SEC - (tp.tv_usec * NSEC_PER_USEC) as the second parameter.
>>>
>>> Regards,
>>> Ken
>>>
>> Thanks Ken, no I hadn't yet bothered to do this, as dispatch_walltime() was initially close enough to work on the other bits of the app.
>> It would make a difference to reduce some lag.
>> What is the part in the second parameter your are doing there…
>>> NSEC_PER_SEC - (tp.tv_usec * NSEC_PER_USEC)
>> Looks like something to adust it but what is tp.tv_usec ?
>
> I hypothesized a struct timeval variable named "tp" that you passed to gettimeofday(). Then you would use the sub-second part (the tv_usec field) to figure out roughly how far off of a whole second dispatch_walltime() would be when passed NULL for the first parameter, since it will also use gettimeofday().
>
> Regards,
> Ken
>
Ok yes, that much kind of made sense. But I'm not clear how I can know the delta from the whole second before it begins firing. (Honestly I've gotten my head a bit turned around by mach_time)
I'm going to have to spend some time to understand this I think.
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