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.\" Modified 1997-07-30 by Paul Slootman <paul@wurtel.demon.nl>
.\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" FIXME 2.6.29 added ADJ_SETOFFSET
.\"
.TH ADJTIMEX 2 2004-05-27 "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)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.