#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static int split(int fd[2]);
static int redir(int fd, const char *file);
static void sig_handler(int sig);

static int parent;

int main(void) {
    ssize_t bytes;
    char buf[256];
    int fd[2];

    // disable buffering for stdout
    setvbuf(stdout, NULL, _IONBF, 0);

    // redirect stdout to write into a file
    if (redir(1, "log.txt") < 0) {
        perror("redir()");
        exit(-1);
    }

    // install the signal handler for Ctrl+C
    if (signal(SIGINT, sig_handler) == SIG_ERR) {
        perror("signal()");
        exit(-1);
    }

    // split processes and assign file descriptors for communication
    switch (split(fd)) {
    case -1: // error
        exit(-1);

    case 0: // child
        while ((bytes = read(fd[0], buf, sizeof(buf))) > 0) {
            printf("[%u]: %s\n", getpid(), buf);
            sleep(1);
            write(fd[1], "pong", 5);
        }
        break;

    case 1: // parent
        parent = 1;
        write(fd[1], "ping", 5);
        while ((bytes = read(fd[0], buf, sizeof(buf))) > 0) {
            printf("[%u]: %s\n", getpid(), buf);
            sleep(1);
            write(fd[1], "ping", 5);
        }
        break;
    }

    // close the file descriptors
    close(fd[0]);
    close(fd[1]);

    return 0;
}

int split(int fd[2]) {
    int pfd[2][2];

    if (pipe(pfd[0]) < 0) {
        perror("pipe()");
        goto err_pipe1;
    }

    if (pipe(pfd[1]) < 0) {
        perror("pipe()");
        goto err_pipe2;
    }

    switch (fork()) {
    case -1:
        perror("fork()");
        goto err_fork;

    case 0:                // child process
        close(pfd[0][1]);  // close write for child
        close(pfd[1][0]);  // close read for parent
        fd[0] = pfd[0][0]; // keep read for child
        fd[1] = pfd[1][1]; // keep write for parent
        return 0;

    default:               // parent process
        close(pfd[1][1]);  // close write for parent
        close(pfd[0][0]);  // close read for child
        fd[0] = pfd[1][0]; // keep read for parent
        fd[1] = pfd[0][1]; // keep write for child
        return 1;
    }

err_fork:
    close(pfd[1][0]);
    close(pfd[1][1]);
err_pipe2:
    close(pfd[0][0]);
    close(pfd[0][1]);
err_pipe1:
    return -1;
}

int redir(int fd, const char *file) {
    int fd2 = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0644);

    if (fd2 < 0)
        return -1;

    dup2(fd2, fd);
    close(fd2);

    return 0;
}

void sig_handler(int sig) {
    if (parent) {
        const char msg[] = "Programmabbruch durch Nutzer\n";
        write(1, msg, sizeof(msg) - 1);
    }
    exit(0);
}