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/* ntpshmread.c -- monitor the inner end of an ntpshmwrite.connection
*
* This file is Copyright (c) 2010-2018 by the GPSD project
* SPDX-License-Identifier: BSD-2-clause
*
* Some of this was swiped from the NTPD distribution.
*/
#define _XOPEN_SOURCE 600
#include <string.h>
#include <stdbool.h>
#include <math.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include "ntpshm.h"
#include "compiler.h"
struct shmTime *shm_get(const int unit, const bool create, const bool forall)
/* initialize a SHM 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((key_t)(NTPD_BASE + unit), sizeof(struct shmTime),
(create ? IPC_CREAT : 0) | (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 *ntp_name(const int unit)
/* return the name of a specified segment */
{
static char name[5] = "NTP\0";
name[3] = (char)('0' + unit);
return name;
}
enum segstat_t ntp_read(struct shmTime *shm_in, struct shm_stat_t *shm_stat, const bool consume)
/* try to grab a sample from the specified SHM segment */
{
volatile struct shmTime shmcopy, *shm = shm_in;
volatile int cnt;
unsigned int cns_new, rns_new;
if (shm == NULL) {
shm_stat->status = NO_SEGMENT;
return NO_SEGMENT;
}
shm_stat->tvc.tv_sec = shm_stat->tvc.tv_nsec = 0;
/* relying on word access to be atomic here */
if (shm->valid == 0) {
shm_stat->status = NOT_READY;
return NOT_READY;
}
cnt = shm->count;
/*
* This is proof against concurrency issues if either (a) the
* memory_barrier() call works on this host, or (b) memset
* compiles to an uninterruptible single-instruction bitblt (this
* will probably cease to be true if the structure exceeds your VM
* page size).
*/
memory_barrier();
memcpy((void *)&shmcopy, (void *)shm, sizeof(struct shmTime));
/*
* An update consumer such as ntpd should zero the valid flag at this point.
* A program snooping the updates to collect statistics should not, lest
* it make the data unavailable for consumers.
*/
if (consume)
shm->valid = 0;
memory_barrier();
/*
* Clash detection in case neither (a) nor (b) was true.
* Not supported in mode 0, and word access to the count field
* must be atomic for this to work.
*/
if (shmcopy.mode > 0 && cnt != shm->count) {
shm_stat->status = CLASH;
return shm_stat->status;
}
shm_stat->status = OK;
switch (shmcopy.mode) {
case 0:
shm_stat->tvr.tv_sec = shmcopy.receiveTimeStampSec;
shm_stat->tvr.tv_nsec = shmcopy.receiveTimeStampUSec * 1000;
rns_new = shmcopy.receiveTimeStampNSec;
shm_stat->tvt.tv_sec = shmcopy.clockTimeStampSec;
shm_stat->tvt.tv_nsec = shmcopy.clockTimeStampUSec * 1000;
cns_new = shmcopy.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 contain valid ns-level
** timestamps, possibly generated by extending the old
** us-level timestamps
*/
break;
case 1:
shm_stat->tvr.tv_sec = shmcopy.receiveTimeStampSec;
shm_stat->tvr.tv_nsec = shmcopy.receiveTimeStampUSec * 1000;
rns_new = shmcopy.receiveTimeStampNSec;
shm_stat->tvt.tv_sec = shmcopy.clockTimeStampSec;
shm_stat->tvt.tv_nsec = shmcopy.clockTimeStampUSec * 1000;
cns_new = shmcopy.clockTimeStampNSec;
/* 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->status = BAD_MODE;
break;
}
/*
* leap field is not a leap offset but a leap notification code.
* The values are magic numbers used by NTP and set by GPSD, if at all, in
* the subframe code.
*/
shm_stat->leap = shmcopy.leap;
shm_stat->precision = shmcopy.precision;
return shm_stat->status;
}
/* end */
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