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Time::HiRes - High resolution alarm, sleep, gettimeofday, interval timers


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

  usleep ($microseconds);

  ualarm ($microseconds);
  ualarm ($microseconds, $interval_microseconds);

  $t0 = [gettimeofday];
  ($seconds, $microseconds) = gettimeofday;

  $elapsed = tv_interval ( $t0, [$seconds, $microseconds]);
  $elapsed = tv_interval ( $t0, [gettimeofday]);
  $elapsed = tv_interval ( $t0 );

  use Time::HiRes qw ( time alarm sleep );

  $now_fractions = time;
  sleep ($floating_seconds);
  alarm ($floating_seconds);
  alarm ($floating_seconds, $floating_interval);

  use Time::HiRes qw( setitimer getitimer

  setitimer ($which, $floating_seconds, $floating_interval );
  getitimer ($which);


The Time::HiRes module implements a Perl interface to the usleep, ualarm, gettimeofday, and setitimer/getitimer system calls, in other words, high resolution time and timers. See the "EXAMPLES" section below and the test scripts for usage; see your system documentation for the description of the underlying nanosleep or usleep, ualarm, gettimeofday, and setitimer/getitimer calls.

If your system lacks gettimeofday() or an emulation of it you don't get gettimeofday() or the one-argument form of tv_interval(). If your system lacks all of nanosleep(), usleep(), and select(), you don't get Time::HiRes::usleep() or Time::HiRes::sleep(). If your system lacks both ualarm() and setitimer() you don't get Time::HiRes::ualarm() or Time::HiRes::alarm().

If you try to import an unimplemented function in the use statement it will fail at compile time.

If your subsecond sleeping is implemented with nanosleep() instead of usleep(), you can mix subsecond sleeping with signals since nanosleep() does not use signals. This, however, is not portable, and you should first check for the truth value of &Time::HiRes::d_nanosleep to see whether you have nanosleep, and then carefully read your nanosleep() C API documentation for any peculiarities. (There is no separate interface to call nanosleep(); just use Time::HiRes::sleep() or Time::HiRes::usleep() with small enough values.)

Unless using nanosleep for mixing sleeping with signals, give some thought to whether Perl is the tool you should be using for work requiring nanosecond accuracies.

The following functions can be imported from this module. No functions are exported by default.

gettimeofday ()

In array context returns a two-element array with the seconds and microseconds since the epoch. In scalar context returns floating seconds like Time::HiRes::time() (see below).

usleep ( $useconds )

Sleeps for the number of microseconds specified. Returns the number of microseconds actually slept. Can sleep for more than one second, unlike the usleep system call. See also Time::HiRes::sleep() below.

ualarm ( $useconds [, $interval_useconds ] )

Issues a ualarm call; the $interval_useconds is optional and will be zero if unspecified, resulting in alarm-like behaviour.

Note that the interaction between alarms and sleeps are unspecified.


tv_interval ( $ref_to_gettimeofday [, $ref_to_later_gettimeofday] )

Returns the floating seconds between the two times, which should have been returned by gettimeofday(). If the second argument is omitted, then the current time is used.

time ()

Returns a floating seconds since the epoch. This function can be imported, resulting in a nice drop-in replacement for the time provided with core Perl; see the "EXAMPLES" below.

NOTE 1: This higher resolution timer can return values either less or more than the core time(), depending on whether your platform rounds the higher resolution timer values up, down, or to the nearest second to get the core time(), but naturally the difference should be never more than half a second.

NOTE 2: Since Sunday, September 9th, 2001 at 01:46:40 AM GMT, when the time() seconds since epoch rolled over to 1_000_000_000, the default floating point format of Perl and the seconds since epoch have conspired to produce an apparent bug: if you print the value of Time::HiRes::time() you seem to be getting only five decimals, not six as promised (microseconds). Not to worry, the microseconds are there (assuming your platform supports such granularity in the first place). What is going on is that the default floating point format of Perl only outputs 15 digits. In this case that means ten digits before the decimal separator and five after. To see the microseconds you can use either printf/sprintf with "%.6f", or the gettimeofday() function in list context, which will give you the seconds and microseconds as two separate values.

sleep ( $floating_seconds )

Sleeps for the specified amount of seconds. Returns the number of seconds actually slept (a floating point value). This function can be imported, resulting in a nice drop-in replacement for the sleep provided with perl, see the "EXAMPLES" below.

Note that the interaction between alarms and sleeps are unspecified.

alarm ( $floating_seconds [, $interval_floating_seconds ] )

The SIGALRM signal is sent after the specified number of seconds. Implemented using ualarm(). The $interval_floating_seconds argument is optional and will be zero if unspecified, resulting in alarm()-like behaviour. This function can be imported, resulting in a nice drop-in replacement for the alarm provided with perl, see the "EXAMPLES" below.

NOTE 1: With some combinations of operating systems and Perl releases SIGALRM restarts select(), instead of interrupting it. This means that an alarm() followed by a select() may together take the sum of the times specified for the the alarm() and the select(), not just the time of the alarm().

Note that the interaction between alarms and sleeps are unspecified.

setitimer ( $which, $floating_seconds [, $interval_floating_seconds ] )

Start up an interval timer: after a certain time, a signal arrives, and more signals may keep arriving at certain intervals. To disable an "itimer", use $floating_seconds of zero. If the $interval_floating_seconds is set to zero (or unspecified), the timer is disabled after the next delivered signal.

Use of interval timers may interfere with alarm(), sleep(), and usleep(). In standard-speak the "interaction is unspecified", which means that anything may happen: it may work, it may not.

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 available: the $which can be ITIMER_REAL, ITIMER_VIRTUAL, ITIMER_PROF, or 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 ITIMER_REAL, and only Solaris seems to have ITIMER_REALPROF (which is used to profile multithreaded programs).

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

ITIMER_VIRTUAL counts time in (process) 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 user time.) SIGVTALRM is delivered when the timer expires.

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 system time.) (The sum of user time and system time is known as the CPU time.) SIGPROF is delivered when the timer expires. SIGPROF can interrupt system calls.

The semantics of interval timers for multithreaded programs are system-specific, and some systems may support additional interval timers. See your setitimer() documentation.

getitimer ( $which )

Return the remaining time in the interval timer specified by $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 setitimer().


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;	

# 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);


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 (*)(UV ret[2])

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

Here is an example of using NVtime from C:

double (*myNVtime)();
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)());


negative time not invented yet

You tried to use a negative time argument.

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?


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


D. Wegscheid <> R. Schertler <> J. Hietaniemi <> G. Aas <>


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

Copyright (c) 2002,2003,2004 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.