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/* ntpshmread.c -- monitor the inner end of an ntpshm connection
*
* This file is Copyright (c) 2010 by the GPSD project
* BSD terms apply: see the file COPYING in the distribution root for details.
*
* Some of this was swiped from the NTPD distribution.
*/
#include <string.h>
#include <stdbool.h>
#include <math.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <assert.h>
#include <unistd.h>
#include <stdio.h>
#include <stdint.h>
#include "ntpshm.h"
struct shmTime *shm_get(int unit, bool forall)
/* initialize an initial segment */
{
struct shmTime *p = NULL;
int shmid;
/*
* Big units will give non-ascii but that's OK
* as long as everybody does it the same way.
*/
shmid = shmget(NTPD_BASE + unit, sizeof(struct shmTime),
IPC_CREAT | (forall ? 0666 : 0600));
if (shmid == -1) { /* error */
return NULL;
}
p = (struct shmTime *)shmat (shmid, 0, 0);
if (p == (struct shmTime *)-1) { /* error */
return NULL;
}
return p;
}
char *shm_name(const int unit)
/* return the name of a specified segment */
{
static char name[5] = "NTP\0";
name[3] = '0' + (char)unit;
return name;
}
enum segstat_t shm_query(struct shmTime *shm_in, struct shm_stat_t *shm_stat)
/* try to grab a sample from the specified SHM segment */
{
/* access order is important for lock-free SHM access; we
** enforce order by treating the whole structure volatile.
**
** IMPORTANT NOTE: This does not protect from reordering on CPU
** level, and it does nothing for cache consistency and
** visibility of changes by other cores. We need atomic and/or
** fence instructions for that.
*/
volatile struct shmTime *shm = shm_in;
unsigned int cns_new, rns_new;
int cnt;
shm_stat->tvc.tv_sec = shm_stat->tvc.tv_nsec = 0;
clock_gettime(CLOCK_REALTIME, &shm_stat->tvc);
if (shm == NULL) {
shm_stat->mode = NO_SEGMENT;
return NO_SEGMENT;
}
if (!shm->valid) {
shm_stat->mode = NOT_READY;
return NOT_READY;
}
switch (shm->mode) {
case 0:
shm_stat->tvr.tv_sec = shm->receiveTimeStampSec;
shm_stat->tvr.tv_nsec = shm->receiveTimeStampUSec * 1000;
rns_new = shm->receiveTimeStampNSec;
shm_stat->tvt.tv_sec = shm->clockTimeStampSec;
shm_stat->tvt.tv_nsec = shm->clockTimeStampUSec * 1000;
cns_new = shm->clockTimeStampNSec;
/* Since the following comparisons are between unsigned
** variables they are always well defined, and any
** (signed) underflow will turn into very large unsigned
** values, well above the 1000 cutoff.
**
** Note: The usecs *must* be a *truncated*
** representation of the nsecs. This code will fail for
** *rounded* usecs, and the logic to deal with
** wrap-arounds in the presence of rounded values is
** much more convoluted.
*/
if ( ((cns_new - (unsigned)shm_stat->tvt.tv_nsec) < 1000)
&& ((rns_new - (unsigned)shm_stat->tvr.tv_nsec) < 1000)) {
shm_stat->tvt.tv_nsec = cns_new;
shm_stat->tvr.tv_nsec = rns_new;
}
/* At this point shm_stat->tvr and shm_stat->tvt contains valid ns-level
** timestamps, possibly generated by extending the old
** us-level timestamps
*/
break;
case 1:
cnt = shm->count;
shm_stat->tvr.tv_sec = shm->receiveTimeStampSec;
shm_stat->tvr.tv_nsec = shm->receiveTimeStampUSec * 1000;
rns_new = shm->receiveTimeStampNSec;
shm_stat->tvt.tv_sec = shm->clockTimeStampSec;
shm_stat->tvt.tv_nsec = shm->clockTimeStampUSec * 1000;
cns_new = shm->clockTimeStampNSec;
if (cnt != shm->count) {
shm_stat->mode = CLASH;
return CLASH;
}
/* See the case above for an explanation of the
** following test.
*/
if ( ((cns_new - (unsigned)shm_stat->tvt.tv_nsec) < 1000)
&& ((rns_new - (unsigned)shm_stat->tvr.tv_nsec) < 1000)) {
shm_stat->tvt.tv_nsec = cns_new;
shm_stat->tvr.tv_nsec = rns_new;
}
/* At this point shm_stat->tvr and shm_stat->tvt contains valid ns-level
** timestamps, possibly generated by extending the old
** us-level timestamps
*/
break;
default:
shm_stat->mode = BAD_MODE;
return BAD_MODE;
break;
}
shm->valid = 0;
shm_stat->leap = shm->leap;
shm_stat->precision = shm->precision;
shm_stat->mode = OK;
return OK;
}
/* end */
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