First cut at an interface for the read side of an NTP SHM connection.

Not yet used or tested.

1 files changed, 141 insertions, 0 deletions

diff --git a/ntpshmread.c b/ntpshmread.c
new file mode 100644
index 00000000..e7362e8c
--- /dev/null
+++ b/ntpshmread.c
@@ -0,0 +1,141 @@
+/* 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.
+ *
+ */
+#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(int unit, 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),
+		 IPC_CREAT | (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;
+
+    if (shm == NULL)
+	return NO_SEGMENT;
+    if (!shm->valid) {
+	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) {
+	    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:
+	return BAD_MODE;
+    }
+    shm->valid = 0;
+
+    shm_stat->leap = shm->leap;
+    return OK;
+}
+
+/* end */