[Lazarus] Better implementation of GetTickcount using clock_gettime

zeljko zeljko at holobit.net
Wed Apr 2 14:01:26 CEST 2014

On 04/02/2014 01:10 PM, Mattias Gaertner wrote:
> On Wed, 02 Apr 2014 13:45:08 +0300
> Anton Kavalenka <anton.k at tut.by> wrote:
>> [...]
>> Yes - CLOCK_MONOTONICC_RAW is prefreable, because general GetTickCount()
>> is number of millisecond ticks since system startup. It cannot be
>> adjusted by time services, it just grows up every 1 msec.
>> Ant typical GetTickCount() usage - find a difference between subsequent
>> values.
>> It would be amazing when difference gets negative.
> The NTP changes in CLOCK_MONOTONIC only accelerates or slows it down. It
> tries to fix the ticks for e.g. busy systems or VMs. It is still
> monotonic.
> CLOCK_MONOTONIC_RAW would better emulate the WinAPI
> function GetTickCount.
> I guess most programs need CLOCK_MONOTONIC.

My guess is that there no need to speculate what most programs need but 
what is more precise or better to say what is correct.
IMO CLOCK_MONOTONIC is affected by eg. ntp/adjtime() so result of 
GetTickCount() is not correct in all cases.
Imagine daemon service which relies on GetTickCount and then clock 
change due to the daylight via ntp (eg. ATicks := GetTickCount , and 
after function "if GetTickCount - ATicks > XXXXX THEN 
Maybe I'm wrong, but we should set CLOCK_MONOTONIC_RAW instead of 
CLOCK_MONOTONIC for linux targets if kernel version matches minimum for 
CLOCK_MONOTONIC_RAW (same thing should be done in fpc - Michael already 
commited CLOCK_MONOTONIC afair).


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