#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>

#define MAXSIZE 4096
void handle_child1(int *fd);
void handle_child2(int *fd);

/* A program to illustrate the basic use of select.
 *
 * The parent forks two children with a pipe to read from each of them and then
 * reads first from child 1 followed by a read from child 2.
*/

int main() {
    char line[MAXSIZE];
    int pipe_child1[2], pipe_child2[2];

    // Before we fork, create a pipe for child 1
    if (pipe(pipe_child1) == -1) {
        perror("pipe");
    }

    int r = fork();
    if (r < 0) {
        perror("fork");
        exit(1);
    } else if (r == 0) {
        handle_child1(pipe_child1);
        exit(0);
    } else {
        // This is the parent. Fork another child,
        // but first close the write file descriptor to child 1
        close(pipe_child1[1]);
        // and make a pipe for the second child
        if (pipe(pipe_child2) == -1) {
            perror("pipe");
        }

        // Now fork the second child
        r = fork();
        if (r < 0) {
            perror("fork");
            exit(1);
        } else if (r == 0) {
            close(pipe_child1[0]);  // still open in parent and inherited
            handle_child2(pipe_child2);
            exit(0);
        } else {
            close(pipe_child2[1]);

            // This is now the parent with 2 children -- each with a pipe
            // from which the parent can read.

            fd_set read_fds;
            FD_ZERO(&read_fds);
            FD_SET(pipe_child1[0], &read_fds);
            FD_SET(pipe_child2[0], &read_fds);

            int numfd;
            if (pipe_child1[0] > pipe_child2[0]) {
                numfd = pipe_child1[0] + 1;
            } else {
                numfd = pipe_child2[0] + 1;
            }

            if (select(numfd, &read_fds, NULL, NULL, NULL) == -1) {
                perror("select");
                exit(1);
            }

            // Read first from child 1
            if (FD_ISSET(pipe_child1[0], &read_fds)) {
                if ((r = read(pipe_child1[0], line, MAXSIZE)) < 0) {
                    perror("read");
                } else if (r == 0) {
                    printf("pipe from child 1 is closed\n");
                } else {
                    printf("Read %s from child 1\n", line);
                }
            }

            // Now read from child 2
            if (FD_ISSET(pipe_child2[0], &read_fds)) {
                if ((r = read(pipe_child2[0], line, MAXSIZE)) < 0) {
                    perror("read");
                } else if (r == 0) {
                    printf("pipe from child 2 is closed\n");
                } else {
                    printf("Read %s from child 2\n", line);
                }
            }
        }
        // could close all the pipes but since program is ending we will just let
        // them be closed automatically
    }
    return 0;
}

void handle_child1(int *fd) {
    close(fd[0]);  // we are only writing from child to parent
    printf("[%d] child\n", getpid());
    // Child will write to parent
    char message[10] = "HELLO DAD";
    write(fd[1], message, 10);
    close(fd[1]);
}

void handle_child2(int *fd) {
    close(fd[0]);  // we are only writing from child to parent
    printf("[%d] child\n", getpid());
    // Child will write to parent
    char message[10] = "Hi mom";
    write(fd[1], message, 10);
    close(fd[1]);
}