documentation for POSIX version of gettimeofday(2) -- the horizon leans forward offering you space for new steps of change

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+.\"	$OpenBSD: clock_gettime.2,v 1.1 1997/05/08 20:02:46 kstailey 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. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. 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 May 8, 1997
+.Dt CLOCK_GETTIME 2
+.Os BSD 4
+.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" "const 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 specifed by
+.Fa clock_id .
+The format of the value is
+.Pp
+.Fa clock_id
+can be one of three values: CLOCK_REALTIME for time that increments as
+a wall clock should, CLOCK_VIRTUAL for time that increments only when
+the CPU is running in user mode on behalf of the calling process, or
+CLOCK_PROF for time that increments when the CPU is running in user or
+kernel mode.
+.Pp
+The structure pointed to by
+.Fa tp
+is defined in 
+.Ao Pa sys/time.h Ac
+as:
+.Pp
+.Bd -literal
+struct timespec {
+	time_t	tv_sec;		/* seconds */
+	long	tv_nsec;	/* and nanoseconds */
+};
+.Ed
+.Pp
+Only the super-user 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 super-user
+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
+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 [EFAULT]
+.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 super-user attempted to set the time.
+.El
+.Sh SEE ALSO
+.Xr date 1 ,
+.Xr adjtime 2 ,
+.Xr ctime 3 ,
+.Xr timed 8
+.Sh HISTORY
+The
+.Fn gettimeofday
+function call appeared in
+.Bx 4.2 .