hires.pm

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In scalar context, the remaining time in the timer is returned.

In list context, both the remaining time and the interval are returned.

There are usually three or four interval timers (signals) available: the
C<$which> can be C<ITIMER_REAL>, C<ITIMER_VIRTUAL>, C<ITIMER_PROF>, or
C<ITIMER_REALPROF>.  Note that which ones are available depends: true
UNIX platforms usually have the first three, but (for example) Win32
and Cygwin have only C<ITIMER_REAL>, and only Solaris seems to have
C<ITIMER_REALPROF> (which is used to profile multithreaded programs).

C<ITIMER_REAL> results in C<alarm()>-like behaviour.  Time is counted in
I<real time>; that is, wallclock time.  C<SIGALRM> is delivered when
the timer expires.

C<ITIMER_VIRTUAL> counts time in (process) I<virtual time>; that is,
only when the process is running.  In multiprocessor/user/CPU systems
this may be more or less than real or wallclock time.  (This time is
also known as the I<user time>.)  C<SIGVTALRM> is delivered when the
timer expires.

C<ITIMER_PROF> counts time when either the process virtual time or when
the operating system is running on behalf of the process (such as I/O).
(This time is also known as the I<system time>.)  (The sum of user
time and system time is known as the I<CPU time>.)  C<SIGPROF> is
delivered when the timer expires.  C<SIGPROF> can interrupt system calls.

The semantics of interval timers for multithreaded programs are
system-specific, and some systems may support additional interval
timers.  For example, it is unspecified which thread gets the signals.
See your C<setitimer()> documentation.

=item getitimer ( $which )

Return the remaining time in the interval timer specified by C<$which>.

In scalar context, the remaining time is returned.

In list context, both the remaining time and the interval are returned.
The interval is always what you put in using C<setitimer()>.

=item clock_gettime ( $which )

Return as seconds the current value of the POSIX high resolution timer
specified by C<$which>.  All implementations that support POSIX high
resolution timers are supposed to support at least the C<$which> value
of C<CLOCK_REALTIME>, which is supposed to return results close to the
results of C<gettimeofday>, or the number of seconds since 00:00:00:00
January 1, 1970 Greenwich Mean Time (GMT).  Do not assume that
CLOCK_REALTIME is zero, it might be one, or something else.
Another potentially useful (but not available everywhere) value is
C<CLOCK_MONOTONIC>, which guarantees a monotonically increasing time
value (unlike time(), which can be adjusted).  See your system
documentation for other possibly supported values.

=item clock_getres ( $which )

Return as seconds the resolution of the POSIX high resolution timer
specified by C<$which>.  All implementations that support POSIX high
resolution timers are supposed to support at least the C<$which> value
of C<CLOCK_REALTIME>, see L</clock_gettime>.

=item clock_nanosleep ( $which, $nanoseconds, $flags = 0)

Sleeps for the number of nanoseconds (1e9ths of a second) specified.
Returns the number of nanoseconds actually slept.  The $which is the
"clock id", as with clock_gettime() and clock_getres().  The flags
default to zero but C<TIMER_ABSTIME> can specified (must be exported
explicitly) which means that C<$nanoseconds> is not a time interval
(as is the default) but instead an absolute time.  Can sleep for more
than one second.  Can also sleep for zero seconds, which often works
like a I<thread yield>.  See also C<Time::HiRes::sleep()>,
C<Time::HiRes::usleep()>, and C<Time::HiRes::nanosleep()>.

Do not expect clock_nanosleep() to be exact down to one nanosecond.
Getting even accuracy of one thousand nanoseconds is good.

=item clock()

Return as seconds the I<process time> (user + system time) spent by
the process since the first call to clock() (the definition is B<not>
"since the start of the process", though if you are lucky these times
may be quite close to each other, depending on the system).  What this
means is that you probably need to store the result of your first call
to clock(), and subtract that value from the following results of clock().

The time returned also includes the process times of the terminated
child processes for which wait() has been executed.  This value is
somewhat like the second value returned by the times() of core Perl,
but not necessarily identical.  Note that due to backward
compatibility limitations the returned value may wrap around at about
2147 seconds or at about 36 minutes.

=item stat

=item stat FH

=item stat EXPR

As L<perlfunc/stat> but with the access/modify/change file timestamps
in subsecond resolution, if the operating system and the filesystem
both support such timestamps.  To override the standard stat():

    use Time::HiRes qw(stat);

Test for the value of &Time::HiRes::d_hires_stat to find out whether
the operating system supports subsecond file timestamps: a value
larger than zero means yes. There are unfortunately no easy
ways to find out whether the filesystem supports such timestamps.
UNIX filesystems often do; NTFS does; FAT doesn't (FAT timestamp
granularity is B<two> seconds).

A zero return value of &Time::HiRes::d_hires_stat means that
Time::HiRes::stat is a no-op passthrough for CORE::stat(),
and therefore the timestamps will stay integers.  The same
thing will happen if the filesystem does not do subsecond timestamps,
even if the &Time::HiRes::d_hires_stat is non-zero.

In any case do not expect nanosecond resolution, or even a microsecond
resolution.  Also note that the modify/access timestamps might have
different resolutions, and that they need not be synchronized, e.g.
if the operations are

    write
    stat # t1
    read
    stat # t2

the access time stamp from t2 need not be greater-than the modify
time stamp from t1: it may be equal or I<less>.

=back

=head1 EXAMPLES

  use Time::HiRes qw(usleep ualarm gettimeofday tv_interval);

  $microseconds = 750_000;
  usleep($microseconds);

  # signal alarm in 2.5s & every .1s thereafter
  ualarm(2_500_000, 100_000);
  # cancel that ualarm
  ualarm(0);

  # get seconds and microseconds since the epoch
  ($s, $usec) = gettimeofday();

  # measure elapsed time 
  # (could also do by subtracting 2 gettimeofday return values)
  $t0 = [gettimeofday];
  # do bunch of stuff here
  $t1 = [gettimeofday];
  # do more stuff here
  $t0_t1 = tv_interval $t0, $t1;

  $elapsed = tv_interval ($t0, [gettimeofday]);
  $elapsed = tv_interval ($t0);	# equivalent code

  #
  # replacements for time, alarm and sleep that know about
  # floating seconds
  #
  use Time::HiRes;
  $now_fractions = Time::HiRes::time;
  Time::HiRes::sleep (2.5);
  Time::HiRes::alarm (10.6666666);

  use Time::HiRes qw ( time alarm sleep );
  $now_fractions = time;
  sleep (2.5);
  alarm (10.6666666);

  # Arm an interval timer to go off first at 10 seconds and
  # after that every 2.5 seconds, in process virtual time

  use Time::HiRes qw ( setitimer ITIMER_VIRTUAL time );

  $SIG{VTALRM} = sub { print time, "\n" };
  setitimer(ITIMER_VIRTUAL, 10, 2.5);

  use Time::HiRes qw( clock_gettime clock_getres CLOCK_REALTIME );
  # Read the POSIX high resolution timer.
  my $high = clock_getres(CLOCK_REALTIME);
  # But how accurate we can be, really?
  my $reso = clock_getres(CLOCK_REALTIME);

  use Time::HiRes qw( clock_nanosleep TIMER_ABSTIME );
  clock_nanosleep(CLOCK_REALTIME, 1e6);
  clock_nanosleep(CLOCK_REALTIME, 2e9, TIMER_ABSTIME);

  use Time::HiRes qw( clock );
  my $clock0 = clock();
  ... # Do something.
  my $clock1 = clock();
  my $clockd = $clock1 - $clock0;

  use Time::HiRes qw( stat );
  my ($atime, $mtime, $ctime) = (stat("istics"))[8, 9, 10];

=head1 C API

In addition to the perl API described above, a C API is available for
extension writers.  The following C functions are available in the
modglobal hash:

  name             C prototype
  ---------------  ----------------------
  Time::NVtime     double (*)()
  Time::U2time     void (*)(pTHX_ UV ret[2])

Both functions return equivalent information (like C<gettimeofday>)
but with different representations.  The names C<NVtime> and C<U2time>
were selected mainly because they are operating system independent.
(C<gettimeofday> is Unix-centric, though some platforms like Win32 and
VMS have emulations for it.)

Here is an example of using C<NVtime> from C:

  double (*myNVtime)(); /* Returns -1 on failure. */
  SV **svp = hv_fetch(PL_modglobal, "Time::NVtime", 12, 0);
  if (!svp)         croak("Time::HiRes is required");
  if (!SvIOK(*svp)) croak("Time::NVtime isn't a function pointer");
  myNVtime = INT2PTR(double(*)(), SvIV(*svp));
  printf("The current time is: %f\n", (*myNVtime)());

=head1 DIAGNOSTICS

=head2 useconds or interval more than ...

In ualarm() you tried to use number of microseconds or interval (also
in microseconds) more than 1_000_000 and setitimer() is not available
in your system to emulate that case.

=head2 negative time not invented yet

You tried to use a negative time argument.

=head2 internal error: useconds < 0 (unsigned ... signed ...)

Something went horribly wrong-- the number of microseconds that cannot
become negative just became negative.  Maybe your compiler is broken?

=head1 CAVEATS

Notice that the core C<time()> maybe rounding rather than truncating.
What this means is that the core C<time()> may be reporting the time
as one second later than C<gettimeofday()> and C<Time::HiRes::time()>.

Adjusting the system clock (either manually or by services like ntp)
may cause problems, especially for long running programs that assume
a monotonously increasing time (note that all platforms do not adjust
time as gracefully as UNIX ntp does).  For example in Win32 (and derived
platforms like Cygwin and MinGW) the Time::HiRes::time() may temporarily
drift off from the system clock (and the original time())  by up to 0.5
seconds. Time::HiRes will notice this eventually and recalibrate.
Note that since Time::HiRes 1.77 the clock_gettime(CLOCK_MONOTONIC)
might help in this (in case your system supports CLOCK_MONOTONIC).

=head1 SEE ALSO

Perl modules L<BSD::Resource>, L<Time::TAI64>.

Your system documentation for C<clock>, C<clock_gettime>,
C<clock_getres>, C<clock_nanosleep>, C<clock_settime>, C<getitimer>,
C<gettimeofday>, C<setitimer>, C<sleep>, C<stat>, C<ualarm>.

=head1 AUTHORS

D. Wegscheid <[e-mail]>
R. Schertler <[e-mail]>
J. Hietaniemi <[e-mail]>
G. Aas <[e-mail]>

=head1 COPYRIGHT AND LICENSE

Copyright (c) 1996-2002 Douglas E. Wegscheid.  All rights reserved.

Copyright (c) 2002, 2003, 2004, 2005, 2006, 2007 Jarkko Hietaniemi.  All rights reserved.

This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut