adjtime.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)adjtime.2	8.2	4/3/91
.TH adjtime 2
.SH Name
adjtime \- correct the time to allow synchronization of the system clock
.SH Syntax
.B #include <sys/time.h>
.PP
.B adjtime(delta, olddelta)
.br
.B struct timeval *delta; 
.br
.B struct timeval *olddelta;
.SH Description
.NXR "adjtime system call"
.NXA "gettimeofday system call" "adjtime system call"
.NXR "clock" "synchronizing"
The
.PN adjtime
system call
changes the system time, as returned by
.PN gettimeofday ,
moving it backward or forward 
by the number of microseconds corresponding to the timeval
\fIdelta\fP.
.PP
The time is maintained by incrementing it with a machine-dependent tick
every clock interrupt.
If \fIdelta\fP is negative, the clock is
slowed down by incrementing it in smaller ticks until
the correction is made.
If \fIdelta\fP is positive, a larger tick
is used.
Thus, the time is always
a monotonically increasing function.
A time correction from an earlier call to 
.PN adjtime
may not be finished when 
.PN adjtime
is called again.
If \fIolddelta\fP is nonzero,
then the structure pointed to will contain, upon return, the
number of microseconds still to be corrected
from the earlier call.
.PP
This call can be used in time servers that synchronize the clocks
of computers in a local area network.
Such time servers would slow down the clocks of some machines
and speed up the clocks of others 
to bring them to the average network time.
.PP
The 
.PN adjtime
call is restricted to the superuser.
.SH Note
Time is incremented 
in 3906-microsecond (us) ticks on RISC and 10000 us ticks on VAX.
When
.PN adjtime
is called with a delta less than 1 second, time is incremented
according to the following table until the time is corrected:
.PP
.TS
tab(@);
l l l l.
_
.sp 4p
@\fBDefault\fP@\fBFast\fP@\fBSlow\fP
\fBSystem\fP@\fBIncrement\fP@\fBIncrement\fP@\fBIncrement\fP
.sp 4p
_
.sp 4p
RISC@3906 us@3921 us@3891 us
.sp 4p
VAX@10000 us@10001 us@9999 us
.sp 4p
_
.TE
.SH Return Values
A return value of 0 indicates that the call succeeded.
A return value of \-1 indicates that an error occurred, and in this
case an error code is stored in the global variable \fIerrno\fP.
.SH Diagnostics
The following error codes may be set in \fIerrno\fP:
.TP 15
[EFAULT]
An argument points outside the process's allocated address space.
.TP 15
[EPERM]
The process's effective user ID is not that of the super-user.
.SH See Also
date(1), gettimeofday(2)