man-pages/man2/adjtimex.2

.\" Copyright (c) 1995 Michael Chastain (mec@shell.portal.com), 15 April 1995.
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.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1997-07-30 by Paul Slootman <paul@wurtel.demon.nl>
.\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" FIXME 2.6.29 added ADJ_SETOFFSET
.\"
.TH ADJTIMEX 2 2013-08-07 "Linux" "Linux Programmer's Manual"
.SH NAME
adjtimex \- tune kernel clock
.SH SYNOPSIS
.B #include <sys/timex.h>
.sp
.BI "int adjtimex(struct timex *" "buf" );
.SH DESCRIPTION
Linux uses David L. Mills' clock adjustment algorithm (see RFC\ 1305).
The system call
.BR adjtimex ()
reads and optionally sets adjustment parameters for this algorithm.
It takes a pointer to a
.I timex
structure, updates kernel parameters from field values,
and returns the same structure with current kernel values.
This structure is declared as follows:
.PP
.in +4n
.nf
struct timex {
    int modes;           /* mode selector */
    long offset;         /* time offset (usec) */
    long freq;           /* frequency offset (scaled ppm) */
.\" FIXME -- what is the scaling unit?  2^16 ?
    long maxerror;       /* maximum error (usec) */
    long esterror;       /* estimated error (usec) */
    int status;          /* clock command/status */
    long constant;       /* pll time constant */
    long precision;      /* clock precision (usec) (read-only) */
    long tolerance;      /* clock frequency tolerance (ppm)
                            (read-only) */
    struct timeval time; /* current time (read-only) */
    long tick;           /* usecs between clock ticks */
};
.fi
.in
.PP
The
.I modes
field determines which parameters, if any, to set.
It may contain a
.RI bitwise- or
combination of zero or more of the following bits:
.PP
.in +4n
.nf
#define ADJ_OFFSET            0x0001 /* time offset */
#define ADJ_FREQUENCY         0x0002 /* frequency offset */
#define ADJ_MAXERROR          0x0004 /* maximum time error */
#define ADJ_ESTERROR          0x0008 /* estimated time error */
#define ADJ_STATUS            0x0010 /* clock status */
#define ADJ_TIMECONST         0x0020 /* pll time constant */
#define ADJ_TICK              0x4000 /* tick value */
#define ADJ_OFFSET_SINGLESHOT 0x8001 /* old-fashioned adjtime() */
.fi
.in
.PP
Ordinary users are restricted to a zero value for
.IR mode .
Only the superuser may set any parameters.
.br
.ne 12v
.SH RETURN VALUE
On success,
.BR adjtimex ()
returns the clock state:
.PP
.in +4n
.nf
#define TIME_OK   0 /* clock synchronized */
#define TIME_INS  1 /* insert leap second */
#define TIME_DEL  2 /* delete leap second */
#define TIME_OOP  3 /* leap second in progress */
#define TIME_WAIT 4 /* leap second has occurred */
#define TIME_BAD  5 /* clock not synchronized */
.fi
.in
.PP
On failure,
.BR adjtimex ()
returns \-1 and sets
.IR errno .
.SH ERRORS
.TP
.B EFAULT
.I buf
does not point to writable memory.
.TP
.B EINVAL
An attempt is made to set
.I buf.offset
to a value outside the range \-131071 to +131071,
or to set
.I buf.status
to a value other than those listed above,
or to set
.I buf.tick
to a value outside the range
.RB 900000/ HZ
to
.RB 1100000/ HZ ,
where
.B HZ
is the system timer interrupt frequency.
.TP
.B EPERM
.I buf.mode
is nonzero and the caller does not have sufficient privilege.
Under Linux the
.B CAP_SYS_TIME
capability is required.
.SH CONFORMING TO
.BR adjtimex ()
is Linux-specific and should not be used in programs
intended to be portable.
See
.BR adjtime (3)
for a more portable, but less flexible,
method of adjusting the system clock.
.SH SEE ALSO
.BR settimeofday (2),
.BR adjtime (3),
.BR capabilities (7),
.BR time (7),
.BR adjtimex (8)