freebsd-dev/lib/libc/sys/clock_gettime.2

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.Dd December 29, 2009
.Dt CLOCK_GETTIME 2
.Os
.Sh NAME
.Nm clock_gettime ,
.Nm clock_settime ,
.Nm clock_getres
.Nd get/set/calibrate date and time
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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
system calls allow the calling process to retrieve or set the value
used by a clock which is specified by
.Fa clock_id .
.Pp
The
.Fa clock_id
argument
can be one of the following values:
.Dv CLOCK_REALTIME ,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_REALTIME_FAST
for time that increments as
a wall clock should;
.Dv CLOCK_MONOTONIC ,
.Dv CLOCK_MONOTONIC_PRECISE ,
.Dv CLOCK_MONOTONIC_FAST
which increments in SI seconds;
.Dv CLOCK_UPTIME ,
.Dv CLOCK_UPTIME_PRECISE ,
.Dv CLOCK_UPTIME_FAST
which starts at zero when the kernel boots and increments
monotonically in SI seconds while the machine is running;
.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_SECOND
which returns the current second without performing a full time counter
query, using in-kernel cached value of current second.
.Pp
The clock IDs
.Fa CLOCK_REALTIME_FAST ,
.Fa CLOCK_MONOTONIC_FAST ,
.Fa CLOCK_UPTIME_FAST
are analogs of corresponding IDs without _FAST suffix but do not perform
a full time counter query, so their accuracy is one timer tick.
Similarly,
.Fa CLOCK_REALTIME_PRECISE ,
.Fa CLOCK_MONOTONIC_PRECISE ,
.Fa CLOCK_UPTIME_PRECISE
are used to get the most exact value as possible, at the expense of
execution time.
.Pp
The structure pointed to by
.Fa tp
is defined in
.In sys/timespec.h
as:
.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, using only
.Fa CLOCK_REALTIME .
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
system call.
This value is placed in a (non-NULL)
.Fa *tp .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The following error codes may be set in
.Va errno :
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa clock_id
argument
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 STANDARDS
The
.Fn clock_gettime ,
.Fn clock_settime ,
and
.Fn clock_getres
system calls conform to
.St -p1003.1b-93 .
The clock IDs
.Fa CLOCK_REALTIME_FAST ,
.Fa CLOCK_REALTIME_PRECISE ,
.Fa CLOCK_MONOTONIC_FAST ,
.Fa CLOCK_MONOTONIC_PRECISE ,
.Fa CLOCK_UPTIME ,
.Fa CLOCK_UPTIME_FAST ,
.Fa CLOCK_UPTIME_PRECISE ,
.Fa CLOCK_SECOND
are FreeBSD extensions to the POSIX interface.