retrieve data from gpsd without wait time - c

Question

I am newbie using gpsd with C. I implemented my first client which uses gps_stream function. If I understood correctly, it is like a pub/sub function that you can read gps data using gps_read. I want to retrieve the data as soon as it is available. The only way I found is to decrease the time on the gps_waiting function. I wonder if there is a way to not use the gps_waiting function and retrieve as soon as possible. Here below is my code.
int runGpsStreamClient() {
int rc;
int count = 0;
clock_t t;
struct gps_data_t gps_data;
t = clock();
if ((rc = gps_open("localhost", "2947", &gps_data)) == -1) {
printf("code: %d, reason: %s\n", rc, gps_errstr(rc));
return EXIT_FAILURE;
}
get_metric(t, "gps_open");
t = clock();
gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL);
get_metric(t, "gps_stream");
while (count < 60) {
/* wait for 0.1 second to receive data */
if (gps_waiting(&gps_data, 100000)) {
t = clock();
int rc = gps_read(&gps_data);
get_metric(t, "gps_read");
/* read data */
if (rc == -1) {
printf("error occurred reading gps data. code: %d, reason: %s\n", rc, gps_errstr(rc));
} else {
/* Display data from the GPS receiver. */
double lat = gps_data.fix.latitude;
double lon = gps_data.fix.longitude;
double alt = gps_data.fix.altitude;
double speed = gps_data.fix.speed;
double climb = gps_data.fix.climb;
time_t seconds = (time_t) gps_data.fix.time;
int status = gps_data.status;
int mode = gps_data.fix.mode;
printf("status[%d], ", status);
printf("mode[%d], ", mode);
printf("latitude[%f], ", lat);
printf("longitude[%f], ", lon);
printf("altitude[%f], ", alt);
printf("speed[%f], ", speed);
printf("v speed[%f], ", climb);
printf("Time[%s].", ctime(&seconds));
if ((status == STATUS_FIX)
&& (mode == MODE_2D || mode == MODE_3D)
&& !isnan(lat) && !isnan(lon)) {
printf(" GPS data OK.\n");
} else {
printf(" GPS data NOK.\n");
}
}
} else {
printf("counter[%d]. Timeout to retrieve data from gpsd. Maybe increase gps_waiting.\n", count);
}
count++;
}
/* When you are done... */
gps_stream(&gps_data, WATCH_DISABLE, NULL);
gps_close(&gps_data);
return EXIT_SUCCESS;
}
Thanks,
Felipe

Answer 1

From the gpsd documents (emphasis mine)
gps_waiting() can be used to check whether there is new data from the daemon. The second argument is the maximum amount of time to wait (in microseconds) on input before returning. It returns true if there is input waiting, false on timeout (no data waiting) or error condition. When using the socket export, this function is a convenience wrapper around a select(2) call...
gps_waiting(&gps_data, t) will block up to t microseconds if there is no new data. As soon as new data is received from the GPS, gps_waiting should return. If no new data is received, the function will timeout and return after t microseconds.
Getting a faster data rate will be dependent on how fast your GPS is outputting data. Merely decreasing the second parameter of gps_waiting will give you the illusion of faster data rates, but if you check the function's return value, you'll see that all you've done is cause the function to time out quicker.