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Introduce a leap second table. 

There are several issues surrounding leap seconds that emerge when a clock
takes on the Grand Master role.  One of them is the fact that GPS radios
provide time of day in the UTC time scale and not in TAI, and they do not,
in general, provide any conversion information.  Another issue is the
expectation that the GM provide correct leap second status flags to the
network.  Although both NTP and GPS do, in theory, provide on-line leap
second status, in practice the information is not reliable due to poor
implementations.

In order to provide correct leap second status and TAI - UTC offsets,
this patch introduces a leap second table based on the information
published by the IETF and NIST.  The hard coded default table can be
updated at run time by reading the standard leap seconds file from the
commonly used tzdata package.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
master
Richard Cochran 2020-03-08 08:20:07 -07:00
parent 9c6e0f57b3
commit 43c51cf144
2 changed files with 270 additions and 0 deletions
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206
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/**
* @file lstab.c
* @note Copyright (C) 2012 Richard Cochran <richardcochran@gmail.com>
* @note SPDX-License-Identifier: GPL-2.0+
*/
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include "lstab.h"
/*
* Keep a history of the TAI - UTC offset in a lookup table.
*
* Each entry gives the NTP time when a new TAI offset came into
* effect. This is always the second immediately after a leap second.
*
* The size of the table is the number of entries from the NIST table,
* plus room for two hundred more entries to be added at run time.
* Since there can be at most two leap seconds per year, this allows
* for at least one hundred years.
*
* The table data are available from
*
* https://www.ietf.org/timezones/data/leap-seconds.list
*
* ftp://ftp.nist.gov/pub/time/leap-seconds.list
*
* When updating this table, do not forget to set N_HISTORICAL_LEAPS
* and the expiration date.
*/
#define BASE_TAI_OFFSET 10
#define N_HISTORICAL_LEAPS 28
#define N_LEAPS (N_HISTORICAL_LEAPS + 200)
#define NTP_UTC_OFFSET 2208988800ULL
struct epoch_marker {
int offset; /* TAI - UTC offset of epoch */
uint64_t ntp; /* NTP time of epoch */
uint64_t tai; /* TAI time of epoch */
uint64_t utc; /* UTC time of epoch */
};
struct lstab {
struct epoch_marker lstab[N_LEAPS];
uint64_t expiration_utc;
int length;
};
static const uint64_t expiration_date_ntp = 3802291200ULL; /* 28 June 2020 */
static const uint64_t offset_table[N_LEAPS * 2] = {
2272060800ULL, 10, /* 1 Jan 1972 */
2287785600ULL, 11, /* 1 Jul 1972 */
2303683200ULL, 12, /* 1 Jan 1973 */
2335219200ULL, 13, /* 1 Jan 1974 */
2366755200ULL, 14, /* 1 Jan 1975 */
2398291200ULL, 15, /* 1 Jan 1976 */
2429913600ULL, 16, /* 1 Jan 1977 */
2461449600ULL, 17, /* 1 Jan 1978 */
2492985600ULL, 18, /* 1 Jan 1979 */
2524521600ULL, 19, /* 1 Jan 1980 */
2571782400ULL, 20, /* 1 Jul 1981 */
2603318400ULL, 21, /* 1 Jul 1982 */
2634854400ULL, 22, /* 1 Jul 1983 */
2698012800ULL, 23, /* 1 Jul 1985 */
2776982400ULL, 24, /* 1 Jan 1988 */
2840140800ULL, 25, /* 1 Jan 1990 */
2871676800ULL, 26, /* 1 Jan 1991 */
2918937600ULL, 27, /* 1 Jul 1992 */
2950473600ULL, 28, /* 1 Jul 1993 */
2982009600ULL, 29, /* 1 Jul 1994 */
3029443200ULL, 30, /* 1 Jan 1996 */
3076704000ULL, 31, /* 1 Jul 1997 */
3124137600ULL, 32, /* 1 Jan 1999 */
3345062400ULL, 33, /* 1 Jan 2006 */
3439756800ULL, 34, /* 1 Jan 2009 */
3550089600ULL, 35, /* 1 Jul 2012 */
3644697600ULL, 36, /* 1 Jul 2015 */
3692217600ULL, 37, /* 1 Jan 2017 */
};
static void epoch_marker_init(struct epoch_marker *ls, uint64_t val, int offset)
{
ls->ntp = val;
ls->utc = val - NTP_UTC_OFFSET;
ls->tai = val - NTP_UTC_OFFSET + offset;
ls->offset = offset;
}
static void lstab_init(struct lstab *lstab)
{
struct epoch_marker *ls;
uint64_t offset, val;
int i;
for (i = 0; i < N_HISTORICAL_LEAPS; i++) {
ls = lstab->lstab + i;
val = offset_table[2 * i];
offset = offset_table[2 * i + 1];
epoch_marker_init(ls, val, offset);
}
lstab->expiration_utc = expiration_date_ntp - NTP_UTC_OFFSET;
lstab->length = i;
}
void lstab_print(struct lstab *lstab, FILE *fp)
{
int i, len = lstab->length;
fprintf(fp, "%3s%12s%12s%12s%4s\n", "idx", "NTP", "TAI", "UTC", "OFF");
for (i = 0; i < len; i++) {
fprintf(fp, "%3d" "%12" PRIu64 "%12" PRIu64 "%12" PRIu64 "%4d\n", i,
lstab->lstab[i].ntp, lstab->lstab[i].tai,
lstab->lstab[i].utc, lstab->lstab[i].offset);
}
}
static int lstab_read(struct lstab *lstab, const char *name)
{
uint64_t expiration, val;
struct epoch_marker *ls;
int index = 0, offset;
char buf[1024];
FILE *fp;
fp = fopen(name, "r");
if (!fp) {
fprintf(stderr, "failed to open '%s' for reading: %m\n", name);
return -1;
}
while (1) {
if (!fgets(buf, sizeof(buf), fp)) {
break;
}
if (1 == sscanf(buf, "#@ %" PRIu64, &expiration)) {
lstab->expiration_utc = expiration - NTP_UTC_OFFSET;
continue;
}
if (2 == sscanf(buf, "%" PRIu64 " %d", &val, &offset)) {
ls = lstab->lstab + index;
epoch_marker_init(ls, val, offset);
index++;
}
}
if (!lstab->expiration_utc) {
fprintf(stderr, "missing expiration date in '%s'\n", name);
return -1;
}
lstab->length = index;
return 0;
}
struct lstab *lstab_create(const char *filename)
{
struct lstab *lstab = calloc(1, sizeof(*lstab));
if (!lstab) {
return NULL;
}
if (filename && filename[0]) {
if (lstab_read(lstab, filename)) {
free(lstab);
return NULL;
}
} else {
lstab_init(lstab);
}
return lstab;
}
void lstab_destroy(struct lstab *lstab)
{
free(lstab);
}
enum lstab_result lstab_utc2tai(struct lstab *lstab, uint64_t utctime,
int *tai_offset)
{
int epoch = -1, index, next;
if (utctime > lstab->expiration_utc) {
return LSTAB_UNKNOWN;
}
for (index = lstab->length - 1; index > -1; index--) {
if (utctime >= lstab->lstab[index].utc) {
epoch = index;
break;
}
}
if (epoch == -1) {
return LSTAB_UNKNOWN;
}
*tai_offset = lstab->lstab[epoch].offset;
next = epoch + 1;
if (next < lstab->length && utctime == lstab->lstab[next].utc - 1) {
return LSTAB_AMBIGUOUS;
}
return LSTAB_OK;
}

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/**
* @file lstab.h
* @note Copyright (C) 2012 Richard Cochran <richardcochran@gmail.com>
* @note SPDX-License-Identifier: GPL-2.0+
*/
#ifndef HAVE_LEAP_SECONDS_H
#define HAVE_LEAP_SECONDS_H
#include <stdint.h>
/** Opaque type */
struct lstab;
/**
* Creates an instance of a leap second table.
* @param filename File from which to initialize the table. If NULL or empty,
* the hard coded default table will be used.
* @return A pointer to a leap second table on success, NULL otherwise.
*/
struct lstab *lstab_create(const char *filename);
/**
* Destroys a leap second table instance.
* @param lstab A pointer obtained via lstab_create().
*/
void lstab_destroy(struct lstab *lstab);
/**
* Enumerates the possible result code for the lstab_utc2tai() method.
*/
enum lstab_result {
/**
* The given UTC value was found in the table, and the
* corresponding TAI time is utctime + tai_offset.
*/
LSTAB_OK,
/**
* The given UTC value is out of the range of the table, and
* the tai_offset return value is not set.
*/
LSTAB_UNKNOWN,
/**
* The given UTC value is ambiguous. The corresponding TAI time is either
*
* utctime + tai_offset
* or
* utctime + tai_offset + 1.
*/
LSTAB_AMBIGUOUS,
};
/**
* Returns the TAI - UTC offset for a given UTC time value.
* @param lstab A pointer obtained via lstab_create().
* @param utctime The UTC time value of interest.
* @param tai_offset Pointer to a buffer to hold the result.
* @return One of the lstab_result enumeration values.
*/
enum lstab_result lstab_utc2tai(struct lstab *lstab, uint64_t utctime,
int *tai_offset);
#endif