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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: CLOCKS_PER_SEC
Date: Fri, 14 Oct 2016 11:14:01 -0700
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Anton Shepelev <anton.txt@g{oogle}mail.com> writes:
> Keith Thompson:
[...]
>>That  would  be  difficult  to  do accurately.  The
>>clock() function measures CPU time, not real time.
>
> Is it not proportional to real time?

No.  For example, a process waiting on I/O, or swapped out,
consumes little or no CPU time, and on a multicore system a
process can consume more than one CPU second per real-time second.
(Obviously this can be system-specific.)

>>You'd have to execute a loop that consumes a  known
>>amount  of  CPU time and compare that to the values
>>returned by clock() before and after the loop.
>
> Well, I should go the other way round, i.e.  wait  a
> known  length  of  time  and  compare  it  with what
> clock() has counted:
>
>    const Seconds = 10;
>    t1 := clock();
>    native_delay( Seconds );
>    t2 := clock();
>    CLOCKS_PER_SECOND := (t2 - t1) / Seconds;
>
> where native_delay() is whatsoever delay routine the
> host language or OS provides.

The value returned by clock() will probably not advance while
native_delay() is running.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"