libgps C code example

I spoke too soon. After browsing other SO questions, I ran into this completely unrelated question. Here's my very slightly modified version:

#include <gps.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include <errno.h>

int main() {
struct timeval tv;
struct gps_data_t gps_data;

if ((gps_open("localhost", "2947", &gps_data)) == -1) {
    printf("code: %d, reason: %s\n", errno, gps_errstr(errno));
    return EXIT_FAILURE;
}
gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL);

while (1) {
    /* wait for 2 seconds to receive data */
    if (gps_waiting (&gps_data, 2000000)) {
        /* read data */
        if ((gps_read(&gps_data,NULL,0)) == -1) {
            printf("error occured reading gps data. code: %d, reason: %s\n", errno, gps_errstr(errno));
        } else {
            /* Display data from the GPS receiver. */
            if ((gps_data.status == STATUS_FIX) && 
                (gps_data.fix.mode == MODE_2D || gps_data.fix.mode == MODE_3D) &&
                !isnan(gps_data.fix.latitude) && 
                !isnan(gps_data.fix.longitude)) {
                    //gettimeofday(&tv, NULL); EDIT: tv.tv_sec isn't actually the timestamp!
                    printf("latitude: %f, longitude: %f, speed: %f, timestamp: %lf\n", gps_data.fix.latitude, gps_data.fix.longitude, gps_data.fix.speed, gps_data.fix.time); //EDIT: Replaced tv.tv_sec with gps_data.fix.time
            } else {
                printf("no GPS data available\n");
            }
        }
    }

    sleep(3);
}

/* When you are done... */
gps_stream(&gps_data, WATCH_DISABLE, NULL);
gps_close (&gps_data);

return EXIT_SUCCESS;
}

I compile it by running gcc -o gps filename.c -lm -lgps

For the sake of completeness I would like to offer the following edited version of Nathan's program which demonstrates the shared memory interface approach. It took me an afternoon of searching to find the missing key to getting this to work.

EDIT: I corrected the way to properly handle errors from gps_open and gps_read using errno.

#include <gps.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include <errno.h>

int main() {
int rc;
//struct timeval tv;
struct gps_data_t gps_data;

if ((gps_open(GPSD_SHARED_MEMORY, NULL, &gps_data)) == -1) {
    printf("code: %d, reason: %s\n", errno, gps_errstr(errno));
    return EXIT_FAILURE;
}

for(int i=0; i<10; i++) {
        /* read data */
        if ((gps_read(&gps_data,NULL,0)) == -1) {
            printf("error occured reading gps data. code: %d, reason: %s\n", errno, gps_errstr(errno));
        } else {
            /* Display data from the GPS receiver. */
            if ((gps_data.status == STATUS_FIX) && 
                (gps_data.fix.mode == MODE_2D || gps_data.fix.mode == MODE_3D) &&
                !isnan(gps_data.fix.latitude) && 
                !isnan(gps_data.fix.longitude)) {
                    printf("latitude: %f, longitude: %f, speed: %f, timestamp: %lf\n", gps_data.fix.latitude, gps_data.fix.longitude, gps_data.fix.speed, gps_data.fix.time);
            } else {
                printf("no GPS data available\n");
            }
        }

    sleep(3);
}

/* When you are done... */
gps_close (&gps_data);

return EXIT_SUCCESS;
}