/* 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(const int unit, const bool create, const 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),
		   (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 *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;

    /*
     * 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 = shm->leap;
    shm_stat->precision = shm->precision;
    shm_stat->mode = OK;
    return OK;
}

/* end */