CTF

/* telnet philbrick.io 20012 */

/* (credit Patrick Coleman) */

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdlib.h>
#include <error.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <limits.h>

void err_exit(char *e) {
    fprintf(stderr, "%s\n", e);
    _exit(1);
}

char* getln(void) {
    char *buf = NULL;
    size_t len = 0;
    if (getdelim(&buf, &len, '\n', stdin) <= 0) {
        err_exit("getdelim() failed");
    }
    return buf;
}

int main(void) {
    int fd;
    char b[50];

    setbuf(stdin, NULL);
    setbuf(stdout, NULL);
    setbuf(stderr, NULL);

    printf("Quote of the day v0.3. Answer a few questions to get one\n");

    printf("Your double number: ");
    char *buf = getln();

    double d = strtod(buf, NULL);
    if (errno != 0) {
        err_exit("strtold() failed");
    }
    /* Is there any IEEE floating point number which can pass those checks? */
    if (d <= 42.0l) {
        err_exit("d is too low");
    }
    if (d > 42.0l) {
        err_exit("d is too high");
    }

    printf("Your integer number: ");
    buf = getln();

    long l = strtol(buf, NULL, 0);
    if (errno != 0) {
        err_exit("strtol() failed");
    }
    if (l < 0) {
        l = -l;
    }
    l %= 3;
    /* number%3 results in either 0, 1 or 2. So, we're safe here. Most likely :) */
    if (l == 0) {
        err_exit("Your quote: Never miss a chance to keep your mouth shut");
    }
    if (l == 1) {
        err_exit("Your quote: I'm still an atheist, thank God");
    }
    if (l == 2) {
        err_exit("Your quote: Nothing ever goes away");
    }

    printf("Getting to this point means you deserve a flag: ");
    fd = open("./flag.txt", O_RDONLY);
    read(fd, &b, sizeof(b));
    printf("%s\n", b);
    close(fd);

    return 0;
}