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/****************************************************************************
NAME
shmexport.c - shared-memory export from the daemon
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
SPDX-License-Identifier: BSD-2-clause
***************************************************************************/
/* sys/ipc.h needs _XOPEN_SOURCE, 500 means X/Open 1995 */
#define _XOPEN_SOURCE 500
#include "gpsd_config.h"
#ifdef SHM_EXPORT_ENABLE
#include <stddef.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include "gpsd.h"
#include "libgps.h" /* for SHM_PSEUDO_FD */
bool shm_acquire(struct gps_context_t *context)
/* initialize the shared-memory segment to be used for export */
{
long shmkey = getenv("GPSD_SHM_KEY") ? strtol(getenv("GPSD_SHM_KEY"), NULL, 0) : GPSD_SHM_KEY;
int shmid = shmget((key_t)shmkey, sizeof(struct shmexport_t), (int)(IPC_CREAT|0666));
if (shmid == -1) {
gpsd_log(&context->errout, LOG_ERROR,
"shmget(0x%lx, %zd, 0666) for SHM export failed: %s\n",
shmkey,
sizeof(struct shmexport_t),
strerror(errno));
return false;
} else
gpsd_log(&context->errout, LOG_PROG,
"shmget(0x%lx, %zd, 0666) for SHM export succeeded\n",
shmkey,
sizeof(struct shmexport_t));
context->shmexport = (void *)shmat(shmid, 0, 0);
if ((int)(long)context->shmexport == -1) {
gpsd_log(&context->errout, LOG_ERROR,
"shmat failed: %s\n", strerror(errno));
context->shmexport = NULL;
return false;
}
context->shmid = shmid;
gpsd_log(&context->errout, LOG_PROG,
"shmat() for SHM export succeeded, segment %d\n", shmid);
return true;
}
void shm_release(struct gps_context_t *context)
/* release the shared-memory segment used for export */
{
if (context->shmexport == NULL)
return;
/* Mark shmid to go away when no longer used
* Having it linger forever is bad, and when the size enlarges
* it can no longer be opened
*/
if (shmctl(context->shmid, IPC_RMID, NULL) == -1) {
gpsd_log(&context->errout, LOG_WARN,
"shmctl for IPC_RMID failed, errno = %d (%s)\n",
errno, strerror(errno));
}
(void)shmdt((const void *)context->shmexport);
}
void shm_update(struct gps_context_t *context, struct gps_data_t *gpsdata)
/* export an update to all listeners */
{
if (context->shmexport != NULL)
{
static int tick;
volatile struct shmexport_t *shared = (struct shmexport_t *)context->shmexport;
++tick;
/*
* Following block of instructions must not be reordered, otherwise
* havoc will ensue.
*
* This is a simple optimistic-concurrency technique. We write
* 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.
*
* Of course many architectures, like Intel, make no guarantees
* about the actual memory read or write order into RAM, so this
* is partly wishful thinking. Thus the need for the memory_barriers()
* to enforce the required order.
*/
shared->bookend2 = tick;
memory_barrier();
shared->gpsdata = *gpsdata;
memory_barrier();
#ifndef USE_QT
shared->gpsdata.gps_fd = SHM_PSEUDO_FD;
#else
shared->gpsdata.gps_fd = (void *)(intptr_t)SHM_PSEUDO_FD;
#endif /* USE_QT */
memory_barrier();
shared->bookend1 = tick;
}
}
#endif /* SHM_EXPORT_ENABLE */
/* end */
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