[Lazarus] Epiktimer elapsed = negative value

[Giuliano Colla]

Il 07/07/2014 11:28, Michael Schnell ha scritto:
> Moreover:
>
> Even if the CPU's hardware tick counter is used, in a multi-CPU 
> system, each CPU might have it's own counter and those are not 
> guaranteed to be in sync. as the OS might move the program from one 
> CPU to the other, this method does not seem to be safe, either.
>

I don't know Epiktimer implementation, but I have some experience about 
Linux timers, because I've implemented several "real time" schedulers on 
top of Linux kernel.
The POSIX clock_gettime() function provides a parameter to specify which 
timer should be used. From clock_gettime man page:
>        clock_gettime returns the current timespec value of tp for the 
> specific
>        clock,  which_clock.   The values that clockid_t currently 
> supports for
>        POSIX.1b timers, as defined in include/linux/time.h, are:
>
>        CLOCK_REALTIME
>               Systemwide realtime clock.
>
>        CLOCK_MONOTONIC
>               Represents monotonic time. Cannot be set.
>
>        CLOCK_PROCESS_CPUTIME_ID
>               High resolution per-process timer.
>
>        CLOCK_THREAD_CPUTIME_ID
>               Thread-specific timer.
>
>        CLOCK_REALTIME_HR
>               High resolution version of CLOCK_REALTIME.
>
>        CLOCK_MONOTONIC_HR
>               High resolution version of CLOCK_MONOTONIC.
>

In my C code a simple function like this:
> struct timespec curr_time () {
>     struct timespec ts;
>     if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
>         handle_error("clock_gettime");
>     return (ts);
>     }
always return correct time. Pascal code should bind to the proper libc 
function. Maybe CLOCK_MONOTONIC or CLOCK_MONOTONIC_HR is even safer.

Giuliano