The GNU C Library: High Accuracy Clock

21.4.4 High Accuracy Clock

The ntp_gettime and ntp_adjtime functions provide an interface to monitor and manipulate the system clock to maintain high accuracy time. For example, you can fine tune the speed of the clock or synchronize it with another time source.

A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks.

These functions are declared in sys/timex.h.

Data Type: struct ntptimeval

This structure is used for information about the system clock. It contains the following members:

struct timeval time: This is the current calendar time, expressed as the elapsed time since the epoch. The struct timeval data type is described in Elapsed Time.
long int maxerror: This is the maximum error, measured in microseconds. Unless updated via ntp_adjtime periodically, this value will reach some platform-specific maximum value.
long int esterror: This is the estimated error, measured in microseconds. This value can be set by ntp_adjtime to indicate the estimated offset of the system clock from the true calendar time.

Function: int ntp_gettime (struct ntptimeval *tptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The ntp_gettime function sets the structure pointed to by tptr to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a ntp_adjtime call is necessary.

The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR: The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.

Data Type: struct timex

This structure is used to control and monitor the system clock. It contains the following members:

unsigned int modes: This variable controls whether and which values are set. Several symbolic constants have to be combined with binary or to specify the effective mode. These constants start with MOD_.
long int offset: This value indicates the current offset of the system clock from the true calendar time. The value is given in microseconds. If bit MOD_OFFSET is set in modes, the offset (and possibly other dependent values) can be set. The offset’s absolute value must not exceed MAXPHASE.
long int frequency: This value indicates the difference in frequency between the true calendar time and the system clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is 1 << SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but the absolute value must not exceed MAXFREQ.
long int maxerror: This is the maximum error, measured in microseconds. A new value can be set using bit MOD_MAXERROR. Unless updated via ntp_adjtime periodically, this value will increase steadily and reach some platform-specific maximum value.
long int esterror: This is the estimated error, measured in microseconds. This value can be set using bit MOD_ESTERROR.
int status: This variable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with STA_. Some of these flags can be updated using the MOD_STATUS bit.
long int constant: This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit MOD_TIMECONST.
long int precision: This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds.
long int tolerance: This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the maxerror every second.
struct timeval time: The current calendar time.
long int tick: The elapsed time between clock ticks in microseconds. A clock tick is a periodic timer interrupt on which the system clock is based.
long int ppsfreq: This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal.
long int jitter: This value expresses a median filtered average of the PPS signal’s dispersion in microseconds.
int shift: This value is a binary exponent for the duration of the PPS calibration interval, ranging from PPS_SHIFT to PPS_SHIFTMAX.
long int stabil: This value represents the median filtered dispersion of the PPS frequency in scaled PPM.
long int jitcnt: This counter represents the number of pulses where the jitter exceeded the allowed maximum MAXTIME.
long int calcnt: This counter reflects the number of successful calibration intervals.
long int errcnt: This counter represents the number of calibration errors (caused by large offsets or jitter).
long int stbcnt: This counter denotes the number of calibrations where the stability exceeded the threshold.

Function: int ntp_adjtime (struct timex *tptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The ntp_adjtime function sets the structure specified by tptr to current values.

In addition, ntp_adjtime updates some settings to match what you pass to it in *tptr. Use the modes element of *tptr to select what settings to update. You can set offset, freq, maxerror, esterror, status, constant, and tick.

modes = zero means set nothing.

Only the superuser can update settings.

The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR: The high accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed.
EPERM: The process specified a settings update, but is not superuser.

For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.

Portability note: Early versions of the GNU C Library did not have this function but did have the synonymous adjtimex.