How can I get data from a GPS module coming with NMEA protocol through a Serial port with c

Question

I need to get data from a GPS module which is coming in NMEA protocol through a Serial port, and the input looks something like this:

$GPRMC,190335.389,V,3754.931,N,08002.496,W,33.6,0.59,110619,,E*47 $GPGGA,190336.389,3754.931,N,08002.496,W,0,00,,,M,,M,,*52 $GPGLL,3754.931,N,08002.496,W,190337.389,V*33 $GPVTG,0.59,T,,M,33.6,N,62.2,K*5C $GPRMC,190339.389,V,3754.932,N,08002.494,W,11.9,0.62,110619,,E*4D $GPGGA,190340.389,3754.932,N,08002.494,W,0,00,,,M,,M,,*52 $GPGLL,3754.932,N,08002.494,W,190341.389,V*33

The thing is, I only need the lines starting with GPRMC. And the problem is the data is coming asynchronous, first comes the first half of a line, for example, then the other half and some of the other line and so on. Now how can I get the input line-by-line and only get the ones starting with GPRMC. The input is coming incessant and I need to get the correct line real-time. How could I do this with C?

I don't really know how to read from a Serial port, I tried something but because the input's coming asynchronous, I couldn't get the correct lines. Oh, and one more thing, the maximum length of a line is 83.

Here's some code I tried, I know it's bad

int a = 0;
int test = 0;
int gprmc_find(char* gps)
{
    while(a < strlen(gps))
    {
        if(gps[a] =='$' && gps[a+1] == 'G' && gps[a+2] == 'P' && gps[a+3] == 'R' )
        {
            test = 1;
            break;
        }
        else
        {
            test = 0;
        }
        a++;
        return test;
    }
}
int main()
{

DWORD  accessdirection =GENERIC_READ | GENERIC_WRITE;
HANDLE hSerial = CreateFile("COM4",
                            accessdirection,
                            0,
                            0,
                            OPEN_EXISTING,
                            0,
                            0);
if (hSerial == INVALID_HANDLE_VALUE) {
    printf("Invalid\n");
}

DCB dcbSerialParams = {0};
dcbSerialParams.DCBlength=sizeof(dcbSerialParams);
if (!GetCommState(hSerial, &dcbSerialParams)) {
     printf("could not get the state of the comport\n");
}
dcbSerialParams.BaudRate=9600;
dcbSerialParams.ByteSize=8;
dcbSerialParams.StopBits=ONESTOPBIT;
dcbSerialParams.Parity=NOPARITY;
if(!SetCommState(hSerial, &dcbSerialParams)){
     printf("Error\n");
}
COMMTIMEOUTS timeouts={0};
timeouts.ReadIntervalTimeout=50;
timeouts.ReadTotalTimeoutConstant=50;
timeouts.ReadTotalTimeoutMultiplier=10;
timeouts.WriteTotalTimeoutConstant=50;
timeouts.WriteTotalTimeoutMultiplier=10;
if(!SetCommTimeouts(hSerial, &timeouts)){
    printf("handle error1");
}
    char buf[83] = {0};
    while(1)
    {


        DWORD dwBytesRead = 0;

       //CloseHandle(hSerial);

        if(!ReadFile(hSerial, buf, 82, &dwBytesRead, NULL)){
            printf("handle error");
        }
        printf(" %d \n", test);
        if(gprmc_find(buf) == 1)
        {
            printf(buf);  
        }
        memset(buf, 0, 83);
        delay(1);
    }
    return 0;

}

Byte- by-byte state machine. Try hard to not do this any other way:) — Martin James, Jun 11 '19 at 19:42
In Linux you can read from a serial terminal a line at a time. See https://stackoverflow.com/questions/53728798/serial-communication-canonical-mode-non-blocking-nl-detection/53739332#53739332 You will have to read every line, and parse for the "$GPRMC" lines that you want to process while tossing the rest. — sawdust, Jun 11 '19 at 21:47

score 0 · Answer 1 · answered Jun 12 '19 at 11:37

GPS NMEA 0183 is a text-based protocol and can be processed with a platform-independent C language standard library.
Many libraries and parsers have already been developed, and you can use them without redevelopment.

For example, you can find this article on the StackOverflow site:
NMEA library - nmeaINFO empty / NMEA Library
C Gps nmea parser from linux serial port does not parse the last line of the read buffer
Parsing code for GPS NMEA string

Other site:
MULTI-PLATFORM SERIAL INTERFACING USING BOOST: A GPS SENSOR AND OPENDDS - PART I / PART II

These are parsers, libraries that parse the read data.
jacketizer/libnmea
kosma/minmea
MaJerle/GPS_NMEA_parser

After analyzing in the above library, you should use only the data you want.

How can I get data from a GPS module coming with NMEA protocol through a Serial port with c

1 Answers1