Print only first and last line that contains a key word in C

Question

I start learning C last week and even the simple tasks can be a challenge for me. Currently I am facing this problem: I have a txt file with many lines, each of them starting with a key word (NMEA format for those who know it):

$GPGGA,***001430.00***,.......

$GPRMC,001430.00,.......

$GPGSV,................    1st time the message arrives

$GPGSA,................


----------


$GPGGA,***005931.00***,...............

$GPRMC,005931.00,............... last time 

$GPGSV,.........................

$GPGSA,.........................

I want to extract the time stamp for the last occurrance of $GPGGA line. Till now I was able just to extract the first and the very last line of the file (unfortunately the last line is not the GPGGA message). I tried to look in the file for the key word $GPGGA and then to sscanf the string from a specific byte and store the value in some variables(hour,min,sec).

Any suggestion would be greatly appreciated. Thank you very much for your time and help.

Here is my code:

entint main(int argc, char **argv) {         
    FILE *opnmea;
    char first[100], last[100];   //first and last time the message arrives
    int hour,min;
    float sec;      // time of observation
    char year[4], month[2], day[2];    // date of obs campaign
    char time[7];

    opnmea = fopen(argv[1], "r");   
    if (opnmea != NULL ) {
        fgets(first, 100, opnmea); // read only first line of the file
        if (strstr(first, "$GPGGA") != NULL)
        {
            sscanf(first + 7, "%2d%2d%f", &hour, &min, &sec);
            printf("First arrival of stream: %d%d%f \n", hour, min, sec);
        }
        fseek(opnmea, 0, SEEK_SET);
        while (!feof(opnmea)) {
            if (strstr(first, "$GPGGA") != NULL) {
                memset(last, 0x00, 100); // clean buffer
                fscanf(opnmea, "%s\n", last); 
            }
        }
        printf("Last arrival of stream: %s\n", last);
        fclose(opnmea);
    }
    else
    {
        printf("\nfile %s not found", argv[1]);
    }
    return 0;
}

Answer 1

The scanf family of functions does not know about newlines, they are treated like regular white space. If you have a line-based format, it is better to read while lines with fgets and then further process these lines, perhaps with sscanf, which scans a string.

There's no need to use feof. The library functions for reading provide special values that indicate the end of the file. For example, fgets will return NULL on error or if the end of the file has been reached.

If you are interested in the timestamps only, you don't have to save the lines. Especially, there's no need for two line buffers. It is enough to extract the information and store it.

Here's an example that shows the basic workings of finding the first and last occurrence of a timestamp. I've conflated the hours, minutes and seconds into a single value for seconds.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>



int main(int argc, char **argv)
{
    FILE *opnmea;
    float first = -1;         // first and ...
    float last = -1;          // ... last timestamp in seconds
    char line[100];

    if (argc != 2) {
        fprintf(stderr, "Usage: prog nmeafile\n");
        exit(1);
    }

    opnmea = fopen(argv[1], "r");

    if (opnmea == NULL) {
        fprintf(stderr, "Can't open '%s'.\n", argv[1]);
        exit(1);
    }

    while (fgets(line, sizeof(line), opnmea)) {
        int hrs, min;
        float sec;

        if (sscanf(line, "$GPGGA,%2d%2d%f", &hrs, &min, &sec) == 3) {
            sec = hrs * 3600.0 + min * 60.0 + sec;

            if (first < 0) first = sec;
            last = sec;
        }
    }

    printf("first: %8.2f sec\n", first);
    printf("last:  %8.2f sec\n", last);
    printf("diff:  %8.2f sec\n", last - first);

    fclose(opnmea);

    return 0;
}