Re: [PATCH] Let RTEMS provide clock()

[Craig Howland <howland at LGSInnovations dot com>]

On 09/06/2017 09:56 AM, Joel Sherrill wrote:
> On 9/6/2017 3:53 AM, Sebastian Huber wrote:
> > On 06/09/17 10:41, Corinna Vinschen wrote:
> >
> > > I just don't quite understand.  Is sysconf(_SC_CLK_TCK) different
> > > from CLOCKS_PER_SEC on RTEMS?
> >
> > Yes, the CLOCKS_PER_SEC is a POSIX defined constant and the
> > sysconf(_SC_CLK_TCK) returns the actual system tick frequency. Which is
> > usually 100 or 1000 ticks per second.
>
> Making it more complicated, the system tick frequency is
> something that the end user can set for each application.
> We just need to make sure that the constant CLOCKS_PER_SECOND
> is appropriate for use with clock() as defined here:
>
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock.html
>
> Sebastian.. I assume that all that matches up per POSIX, right?
>
> --joel
CLOCKS_PER_SEC is not necessarily constant on all systems.  It is only POSIX 
which requires it to be fixed (at 1 million).  The C standard defines it as "an 
expression with type clock_t that is the number per second of the value returned 
by the clock function."  (In addition, POSIX does not require microsecond 
accuracy, so the _times_r() return value could simply be scaled to 
CLOCKS_PER_SEC if so desired as an alternative way of doing it.)