How do I stop at a particular point in a file *str

Question

I have a file called test.txt the file contains:

<this is a test = 1>

<more tests = 42 and more "34">

<start>10.213123 41.21231 23.15323</start>

<random stuff = "4">

<blah 234>

When I see <start> I want to scan in the 3 numbers after into a double like this:

x = 10.213123
y = 41.21231
z = 23.15323

I'm sort of confused because here, fgets scans the entire line, how can I scan in the 3 numbers into a double? Because the numbers can be of various lengths? I made this to print out what it reads from a file but I can't wrap my head around it.

void  print_lines(FILE *stream) {
    char line[MAX_LINE_LENGTH];
    while (fgets(line, MAX_LINE_LENGTH, stream) != NULL) {
        fputs(line, stdout);
    }
}

Answer 1

Just when you see <start> then scan 3 numbers into double. You have the line content in line variable, you can use strtod to scan string into double. You can even use sscanf(line, "<start>%lf %lf %lf</start>", &x, &y, &z);, but using strtod is better for error handling.

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stddef.h>
#include <stdlib.h>

#define MAX_LINE_LENGTH 1024

void  print_lines(FILE *stream) {
    double a, b, c;
    char line[MAX_LINE_LENGTH];
    while (fgets(line, MAX_LINE_LENGTH, stream) != NULL) {
        char *pnt;
        // locate substring `<start>` in the line
        if ((pnt = strstr(line, "<start>") != NULL)) {
            // advance pnt to point to the characters after the `<start>`
            pnt = &pnt[sizeof("<start>") - 1];
            char *pntend;

            // scan first number
            a = strtod(pnt, &pntend);
            if (pnt == pntend) {
                fprintf(stderr, "Error converting a value.\n");
                // well, handle error some better way than ignoring.
            }
            pnt = pntend;
            // scan second number
            b = strtod(pnt, &pntend);
            if (pnt == pntend) {
                fprintf(stderr, "Error converting a value.\n");
                // well, handle error some better way than ignoring.
            }
            pnt = pntend;
            // scan third number
            c = strtod(pnt, &pntend);
            if (pnt == pntend) {
                fprintf(stderr, "Error converting a value.\n");
                // well, handle error some better way than ignoring.
            }

            printf("Read values are %lf %lf %lf\n", a, b, c);
        } else {
            // normal line
            //fputs(line, stdout);
        }
    }
}

int main()
{
    print_lines(stdin);
    return 0;
}