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/* ntpshmread.c -- monitor the client side of an ntpshmwrite.connection
*
* This file is Copyright 2010 by the GPSD project
* SPDX-License-Identifier: BSD-2-clause
*
* Some of this was swiped from the NTPD distribution.
*
*/
#include "../include/gpsd_config.h" // must be before all includes
#include <assert.h>
#include <errno.h>
#include <math.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include "../include/ntpshm.h"
#include "../include/compiler.h"
// initialize a SHM segment
struct shmTime *shm_get(const int unit, const bool create, const bool forall)
{
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
if (ENOENT != errno) {
// if ENOENT (2:No such file or directory)
// could be forgot to be root "Permission denied(13)"
// we scan for many uncommon, usually non-exitent, SHMs as well.
(void)fprintf(stderr, "WARNING: could not open SHM(%d): %s(%d)\n",
unit, strerror(errno), errno);
}
return NULL;
}
p = (struct shmTime *)shmat(shmid, 0, 0);
if ((struct shmTime *)-1 == p) { // error
(void)fprintf(stderr, "WARNING: unit %d, shmat(x%x): %s(%d)\n",
unit, shmid, strerror(errno), errno);
return NULL;
}
return p;
}
// return the name of a specified segment
char *ntp_name(const int unit)
{
static char name[5] = "NTP\0";
name[3] = (char)('0' + unit);
return name;
}
/* try to grab a sample from SHM segment *shm_in
* put it in shm_stat
*
* Return: shmstat_t
*/
enum segstat_t ntp_read(struct shmTime *shm_in, struct shm_stat_t *shm_stat,
const bool consume)
{
struct shmTime shmcopy;
volatile int cnt;
unsigned int cns_new, rns_new;
// clear out old junk
memset(shm_stat, 0, sizeof(*shm_stat));
if (NULL == shm_in) {
// no SHM to read from
shm_stat->status = NO_SEGMENT;
return NO_SEGMENT;
}
// relying on word access to be atomic here
if (0 == shm_in->valid) {
// no data to read
shm_stat->status = NOT_READY;
return NOT_READY;
}
// grab shm_in->count, which is volatile, to compare after memcpy()
cnt = shm_in->count;
/*
* This is proof against concurrency issues if either (a) the
* memory_barrier() call works on this host, or (b) memcpy
* 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_in, sizeof(struct shmTime));
/*
* An updated 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_in->valid = 0;
}
memory_barrier();
/*
* Clash detection in case neither (a) nor (b) was true.
* Not supported in mode 0 (why?).
* Word access to the count field * must be atomic for this to work.
* shm_in->count is (volatile) to prevent compiler rearrangement,
* but does not force cache coherence if memory_barrier() is a NOP.
*/
// FIXME: retry on clash?
if (0 < shmcopy.mode &&
cnt != shm_in->count) {
// count changed, possibly bad memcpy()
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 ((1000 > (cns_new - (unsigned)shm_stat->tvt.tv_nsec)) &&
(1000 > (rns_new - (unsigned)shm_stat->tvr.tv_nsec))) {
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 ((1000 > (cns_new - (unsigned)shm_stat->tvt.tv_nsec)) &&
(1000 > (rns_new - (unsigned)shm_stat->tvr.tv_nsec))) {
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;
}
// vim: set expandtab shiftwidth=4
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