Thread: Using sscanf to parse/match an GPS (NMEA) string

I've got some sample code that I'm trying to get working / understand.

The sample code has a line:

Code:

if(sscanf(msg, "GPGGA,%f,%f,%c,%f,%c,%d,%d,%f,%f,%c,%f", &time, &latitude, &ns, &longitude, &ew, &lock, &sats, &hdop, &alt, &unit, &geoid) >= 1)

The char array, msg[256], holds something like:

Code:

GPGGA,172814.0,3723.46587704,N,12202.26957864,W,2,6,1.2,18.893,M,-25.669,M,2.0,0031*4F"

What I don't comprehend (even after looking at the online docs) is just how sscanf, used as above, works.

If msg starts with a different string than GPGGA, will the sscanf stop?
and based on docs, it seems it should return a '0' (for no matches), no?
But if it returns a -1, what does that mean?

(BTW, I am passing it the sample char array I provided, and I get returned '-1')

Hope that was clear, and thanks.

Thanks, both posts were quite helpful!

Originally Posted by laserlight

Recall:

Clearly, the number of input items assigned cannot be a negative number, and as we know EOF is negative, you must be observing a return value of EOF. Therefore, it means that an input failure occurred before any conversion.

OK, so... let me get this straight. The '-1' means I'm not really passing it a valid msg ?

OK, I'm not getting a return of '-1' any more. So msg is valid but not a valid "msg" LOL.
Thanks.

But... I am getting a return of '0'.

1) If msg has more fields in it than the format string in sscanf, that's OK, correct?
2) If data is missing between the commas, is that OK?
IOW, msg looks like GPGGA,,,,,,0,00,99.99,,,,,,*48

If I test my sscanf with a test message

Code:

strcpy(msg,"GPGGA,,,,,,0,00,99.99,,,,,,*48");

I get a return of 0

A more complete test message works better -

Code:

strcpy(msg, "GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47");

I get a return of 11

Can you explain this to me?

Thanks

scanf doesn't support optional fields. It fails as soon as it hits the first comma while trying to read a float. You're better off tokenizing the string. Unfortunately (or not) you can't use strtok since it treats multiple separators in a row as one separator. But you can use strchr to look for commas.

Code:

#include <stdio.h>
#include <stdlib.h>  // atoi, atof
#include <string.h>  // strchr

int main() {
    //char msg[] = "GPGGA,172814.0,3723.46587704,N,12202.26957864,"
    //             "W,2,6,1.2,18.893,M,-25.669,M,2.0,0031*4F";
    char msg[] = "GPGGA,,,,,,7,13,99.99,,,,,,*48";

    float time=0, latitude=0, longitude=0, hdop=0, alt=0, geoid=0;
    char ns='?', ew='?', unit='?';
    int lock=0, sats=0;

    char *p = strchr(msg, ',');
    time = atof(p+1); // if field is empty this encounters a comma and returns 0.0

    p = strchr(p+1, ',');
    latitude = atof(p+1);

    p = strchr(p+1, ',');
    ns = p[1] == ',' ? '?' : p[1];

    p = strchr(p+1, ',');
    longitude = atof(p+1);

    p = strchr(p+1, ',');
    ew = p[1] == ',' ? '?' : p[1];

    p = strchr(p+1, ',');
    lock = atoi(p+1);

    p = strchr(p+1, ',');
    sats = atoi(p+1);

    p = strchr(p+1, ',');
    hdop = atof(p+1);

    p = strchr(p+1, ',');
    alt = atof(p+1);

    p = strchr(p+1, ',');
    unit = p[1] == ',' ? '?' : p[1];

    p = strchr(p+1, ',');
    geoid = atof(p+1);

    printf("%f %f %c %f %c %d %d %f %f %c %f\n",
           time, latitude, ns, longitude, ew, lock,
           sats, hdop, alt, unit, geoid);

    return 0;
}

Thanks laser salem and algor, this has been quite helpful. Now it's way more clear. I also greatly appreciate algor's code example to handle this better.

As far as my code goes, I'm not building something for production so I'll just deal with the shortcomings of using sscanf (and its return code) with messages that may appear with all the blank fields.

Cheers.