Challenge 1: Writing Three Lines, Then Reading Them Back — Possible Solution
====================================================================

#include <stdio.h>

int main() {
    FILE *out = fopen("lines.txt", "w");
    if (out == NULL) {
        return 1;
    }
    fprintf(out, "first line\n");
    fprintf(out, "second line\n");
    fprintf(out, "third line\n");
    fclose(out);

    FILE *in = fopen("lines.txt", "r");
    if (in == NULL) {
        return 1;
    }
    char line[256];
    while (fgets(line, sizeof(line), in) != NULL) {
        printf("%s", line);
    }
    fclose(in);

    return 0;
}

Output:

    first line
    second line
    third line

WHY THIS WORKS AS AN ANSWER
------------------------------
The write handle is fully closed with fclose before the file is
reopened for reading -- flushing the buffered writes to disk first, as
the chapter describes -- and fgets is used with an explicit buffer
size (sizeof(line)) rather than an unbounded read, exactly the safe
pattern the chapter contrasts against the removed gets().