adjtimex — tune kernel clock
#include <sys/timex.h>
int
adjtimex( |
struct timex * | buf); |
Linux uses David L. Mills' clock adjustment algorithm (see
RFC 1305). The system call adjtimex() reads and optionally sets
adjustment parameters for this algorithm. It takes a pointer
to a timex structure, updates
kernel parameters from field values, and returns the same
structure with current kernel values. This structure is
declared as follows:
struct timex { int modes; /* mode selector */ long offset; /* time offset (usec) */ long freq; /* frequency offset (scaled ppm) */ 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 */ };
The modes field
determines which parameters, if any, to set. It may contain a
bitwise-or
combination of zero or more of the following bits:
#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() */
Ordinary users are restricted to a zero value for
mode. Only the
superuser may set any parameters.
On success, adjtimex()
returns the clock state:
#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 */
On failure, adjtimex()
returns −1 and sets errno.
buf does not
point to writable memory.
An attempt is made to set buf.offset to a value
outside the range −131071 to +131071, or to set
buf.status to
a value other than those listed above, or to set
buf.tick to a
value outside the range 900000/HZ to 1100000/HZ, where HZ is the system timer interrupt
frequency.
buf.mode
is nonzero and the caller does not have sufficient
privilege. Under Linux the CAP_SYS_TIME capability is
required.
adjtimex() is Linux-specific
and should not be used in programs intended to be portable.
See adjtime(3)
for a more portable, but less flexible, method of adjusting
the system clock.