initial checkin of 'gpsdate' program

Makefile

@@ -0,0 +1,11 @@
+
+LDFLAGS=-lgps
+
+gpsdate: gpsdate.o
+	$(CC) $(LDFLAGS) -o $@ $^
+
+%.o: %.c
+	$(CC) $(CFLAGS) -o $@ -c $^
+
+clean:
+	@rm -f gpsdate *.o

gpsdate.c

@@ -0,0 +1,80 @@
+/* gpsdate - small utility to set system RTC based on gpsd time 
+ * (C) 2013 by sysmocom - s.f.m.c. GmbH, Author: Harald Welte
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/* The idea of this program is that you run it once at system boot time,
+ * to set the local RTC to the time received by GPS.  Further synchronization
+ * during system runtime is then handled by ntpd, interfacing with gpsd using
+ * the ntp shared memory protocol.
+ *
+ * However, ntpd is unable to accept a GPS time that's off by more than four
+ * hours from the system RTC, so initial synchronization has to be done
+ * externally.  'ntpdate' is the usual option, but doesn't work if you're
+ * offline.  Thus, this gpsdate utilith was created to fill the gap.
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <errno.h>
+#include <sys/time.h>
+
+#include <gps.h>
+
+static struct gps_data_t gpsdata;
+
+static void callback(struct gps_data_t *gpsdata)
+{
+	struct timeval tv;
+	int rc;
+
+	if (!(gpsdata->set & TIME_SET))
+		return;
+
+	tv.tv_sec = gpsdata->fix.time;
+	/* FIXME: use the fractional part for microseconds */
+	tv.tv_usec = 0;
+
+	rc = settimeofday(&tv, NULL);
+	gps_close(gpsdata);
+	if (rc == 0)
+		exit(EXIT_SUCCESS);
+	else
+		exit(EXIT_FAILURE);
+}
+
+int main(int argc, char **argv)
+{
+	char *host = "localhost";
+
+	if (argc > 1)
+		host = argv[1];
+
+	if (gps_open(host, DEFAULT_GPSD_PORT, &gpsdata)) {
+		fprintf(stderr, "no gpsd running or network error: %d, %s\n",
+			errno, gps_errstr(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	gps_stream(&gpsdata, WATCH_ENABLE|WATCH_JSON, NULL);
+
+	gps_mainloop(&gpsdata, 5000000, callback);
+
+	gps_close(&gpsdata);
+
+	exit(EXIT_SUCCESS);
+}