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/****************************************************************************
NAME
libgps_shm.c - reasder 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 (c) 2010 by the GPSD project
BSD terms apply: see the file COPYING in the distribution root for details.
***************************************************************************/
#include <stddef.h>
#include <string.h>
#include <errno.h>
#include <sys/time.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include "gpsd.h"
#ifdef SHM_EXPORT_ENABLE
int gps_shm_open(/*@out@*/struct gps_data_t *gpsdata)
/* open a shared-memory connection to the daemon */
{
int shmid;
gpsdata->privdata = NULL;
shmid = shmget((key_t)GPSD_KEY, sizeof(struct gps_data_t), 0);
if (shmid == -1) {
/* daemon isn't running or failed to create shared segment */
return -1;
}
gpsdata->privdata = shmat(shmid, 0, 0);
if ((int)(long)gpsdata->privdata == -1) {
/* attach failed for sume unknown reason */
return -2;
}
return 0;
}
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;
struct shmexport_t *shared = (struct shmexport_t *)gpsdata->privdata;
before = shared->bookend1;
/*
* The bookend-consistency technique wants this to be a forward copy.
* But it is not guaranteed, that memcpy() works this way, and some compilers on some platforms
* are known to implement it as a reverse copy. Notably GCC does
* this on x64 Atom.
*
* The safest thing to do here would be to use a naive C implementation
* of forward byte copy, but if we did that we'd be sacrificing the
* superior performance of the asm optimization GCC does for
*/
(void)memcpy((void *)gpsdata,
(void *)&shared->gpsdata,
sizeof(struct gps_data_t));
after = shared->bookend2;
/*@i1@*/gpsdata->privdata = shared;
return (before == after) ? 0 : -1;
}
}
void gps_shm_close(struct gps_data_t *gpsdata)
{
if (gpsdata->privdata != NULL)
(void)shmdt((const void *)gpsdata->privdata);
}
#endif /* SHM_EXPORT_ENABLE */
/* end */
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