gpsd/libgps_shm.c

/****************************************************************************

NAME
   libgps_shm.c - reader access to shared-memory export

DESCRIPTION
   This is a very lightweight alternative to JSON-over-sockets.  Clients
won't be able to filter by device, and won't get device activation/deactivation
notifications.  But both client and daemon will avoid all the marshalling and
unmarshalling overhead.

PERMISSIONS
   This file is Copyright 2010 by the GPSD project
   SPDX-License-Identifier: BSD-2-clause

***************************************************************************/

#include "gpsd_config.h"

#ifdef SHM_EXPORT_ENABLE

#include <errno.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/time.h>

#include "gpsd.h"
#include "libgps.h"

struct privdata_t
{
    void *shmseg;
    int tick;
};


/* open a shared-memory connection to the daemon */
int gps_shm_open(struct gps_data_t *gpsdata)
{
    int shmid;

    long shmkey = getenv("GPSD_SHM_KEY") ?
                     strtol(getenv("GPSD_SHM_KEY"), NULL, 0) : GPSD_SHM_KEY;

    libgps_debug_trace((DEBUG_CALLS, "gps_shm_open()\n"));

    gpsdata->privdata = NULL;
    shmid = shmget((key_t)shmkey, sizeof(struct gps_data_t), 0);
    if (shmid == -1) {
        /* daemon isn't running or failed to create shared segment */
        return -1;
    }
    gpsdata->privdata = (void *)malloc(sizeof(struct privdata_t));
    if (gpsdata->privdata == NULL)
        return -1;

    PRIVATE(gpsdata)->tick = 0;
    PRIVATE(gpsdata)->shmseg = shmat(shmid, 0, 0);
    if (PRIVATE(gpsdata)->shmseg == (void *) -1) {
        /* attach failed for sume unknown reason */
        free(gpsdata->privdata);
        gpsdata->privdata = NULL;
        return -2;
    }
#ifndef USE_QT
    gpsdata->gps_fd = SHM_PSEUDO_FD;
#else
    gpsdata->gps_fd = (void *)(intptr_t)SHM_PSEUDO_FD;
#endif /* USE_QT */
    return 0;
}

/* check to see if new data has been written */
/* timeout is in uSec */
bool gps_shm_waiting(const struct gps_data_t *gpsdata, int timeout)
{
    volatile struct shmexport_t *shared =
            (struct shmexport_t *)PRIVATE(gpsdata)->shmseg;
    volatile bool newdata = false;
    timespec_t endtime;

    (void)clock_gettime(CLOCK_REALTIME, &endtime);
    endtime.tv_sec += timeout / 1000000;
    endtime.tv_nsec += (timeout % 1000000) * 1000;
    TS_NORM(&endtime);

    /* busy-waiting sucks, but there's not really an alternative */
    for (;;) {
        volatile int bookend1, bookend2;
        timespec_t now;

        memory_barrier();
        bookend1 = shared->bookend1;
        memory_barrier();
        bookend2 = shared->bookend2;
        memory_barrier();
        if (bookend1 == bookend2 && bookend1 > PRIVATE(gpsdata)->tick) {
            newdata = true;
            break;
        }
        (void)clock_gettime(CLOCK_REALTIME, &now);
        if (TS_GT(&now, &endtime)) {
            break;
        }
    }

    return newdata;
}

int gps_shm_read(struct gps_data_t *gpsdata)
/* read an update from the shared-memory segment */
{
    if (gpsdata->privdata == NULL)
        return -1;
    else
    {
        int before, after;
        void *private_save = gpsdata->privdata;
        volatile struct shmexport_t *shared =
            (struct shmexport_t *)PRIVATE(gpsdata)->shmseg;
        struct gps_data_t noclobber;

        /*
         * Following block of instructions must not be reordered,
         * otherwise havoc will ensue.  The memory_barrier() call
         * should prevent reordering of the data accesses.
         *
         * This is a simple optimistic-concurrency technique.  We wrote
         * the second bookend first, then the data, then the first bookend.
         * Reader copies what it sees in normal order; that way, if we
         * start to write the segment during the read, the second bookend will
         * get clobbered first and the data can be detected as bad.
         */
        before = shared->bookend1;
        memory_barrier();
        (void)memcpy((void *)&noclobber,
                     (void *)&shared->gpsdata,
                     sizeof(struct gps_data_t));
        memory_barrier();
        after = shared->bookend2;

        if (before != after)
            return 0;
        else {
            (void)memcpy((void *)gpsdata,
                         (void *)&noclobber,
                         sizeof(struct gps_data_t));
            gpsdata->privdata = private_save;
#ifndef USE_QT
            gpsdata->gps_fd = SHM_PSEUDO_FD;
#else
            gpsdata->gps_fd = (void *)(intptr_t)SHM_PSEUDO_FD;
#endif /* USE_QT */
            PRIVATE(gpsdata)->tick = after;
            if ((gpsdata->set & REPORT_IS)!=0) {
                gpsdata->set = STATUS_SET;
            }
            return (int)sizeof(struct gps_data_t);
        }
    }
}

void gps_shm_close(struct gps_data_t *gpsdata)
{
    if (PRIVATE(gpsdata)) {
        if (PRIVATE(gpsdata)->shmseg != NULL)
            (void)shmdt((const void *)PRIVATE(gpsdata)->shmseg);
        free(PRIVATE(gpsdata));
        gpsdata->privdata = NULL;
    }
}

int gps_shm_mainloop(struct gps_data_t *gpsdata, int timeout,
                         void (*hook)(struct gps_data_t *gpsdata))
/* run a shm main loop with a specified handler */
{
    for (;;) {
        if (!gps_shm_waiting(gpsdata, timeout)) {
            return -1;
        } else {
            int status = gps_shm_read(gpsdata);

            if (status == -1)
                return -1;
            if (status > 0)
                (*hook)(gpsdata);
        }
    }
    //return 0;
}

#endif /* SHM_EXPORT_ENABLE */

/* end */
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