.\"	$OpenBSD: clock_gettime.2,v 1.19 2011/07/07 21:00:59 deraadt Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd $Mdocdate: July 7 2011 $
.Dt CLOCK_GETTIME 2
.Os
.Sh NAME
.Nm clock_gettime ,
.Nm clock_settime ,
.Nm clock_getres
.Nd get/set/calibrate date and time
.Sh SYNOPSIS
.Fd #include <sys/time.h>
.Ft int
.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
.Ft int
.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
.Ft int
.Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
.Sh DESCRIPTION
The
.Fn clock_gettime
and
.Fn clock_settime
allow the calling process to retrieve or set the value used by a clock
which is specified by
.Fa clock_id .
.Pp
.Fa clock_id
can be one of four values:
.Dv CLOCK_REALTIME
for time that increments as
a wall clock should,
.Dv CLOCK_VIRTUAL
for time that increments only when
the CPU is running in user mode on behalf of the calling process,
.Dv CLOCK_PROF
for time that increments when the CPU is running in user or kernel mode, or
.Dv CLOCK_MONOTONIC
for time that increments at a steady rate (monotonically).
.Pp
The structure pointed to by
.Fa tp
is defined in
.Aq Pa sys/time.h
as:
.Bd -literal -offset indent
struct timespec {
	time_t	tv_sec;		/* seconds */
	long	tv_nsec;	/* and nanoseconds */
};
.Ed
.Pp
Only the superuser may set the time of day.
If the system securelevel is greater than 1 (see
.Xr init 8 ) ,
the time may only be advanced.
This limitation is imposed to prevent a malicious superuser
from setting arbitrary time stamps on files.
The system time can still be adjusted backwards using the
.Xr adjtime 2
system call even when the system is secure.
.Pp
The resolution (granularity) of a clock is returned by the
.Fn clock_getres
call.
This value is placed in a (non-null)
.Fa *tp .
.Sh RETURN VALUES
A 0 return value indicates that the call succeeded.
A \-1 return value indicates an error occurred, and in this
case an error code is stored into the global variable
.Va errno .
.Sh ERRORS
The following error codes may be set in
.Va errno :
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa clock_id
was not a valid value.
.It Bq Er EFAULT
The
.Fa tp
argument address referenced invalid memory.
.It Bq Er EPERM
A user other than the superuser attempted to set the time.
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr adjtime 2 ,
.Xr ctime 3
.Sh STANDARDS
The
.Fn clock_gettime ,
.Fn clock_settime ,
and
.Fn clock_getres
functions conform to
.St -p1003.1b-93 .