#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/wait.h>



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

    int p2c[2], c2p[2];

    if(-1 == pipe(p2c)) {

        perror("Error creating parent to child pipe");

        exit(1);

    }

    if(-1 == pipe(c2p)) {

        perror("Error creating child to parent pipe");

        exit(1);

    }

    int f = fork();

    if(0 > f) {

        perror("Cannot create child process");

        exit(1);

    } else if (0 == f) {

        close(p2c[1]);

        close(c2p[0]);



        int n = 0;

        if(-1 == read(p2c[0], &n, sizeof(int))) {

            perror("Error reading n from parent");

            close(p2c[0]);

            close(c2p[1]);

            exit(1);

        }

        int sum = 0, nr = 0;

        for(int i = 0; i < n; i++) {

            if(-1 == read(p2c[0], &nr, sizeof(int))) {

                perror("Error reading number from parent");

            } else {

                sum += nr;

            }

        }

        if(-1 == write(c2p[1], &sum, sizeof(int))) {

            perror("Error sending sum to parent");

            close(p2c[0]);

            close(c2p[1]);

            exit(1);

        }



        close(p2c[0]);

        close(c2p[1]);

        exit(0);

    } else {

        close(p2c[0]);

        close(c2p[1]);

        srandom(getpid());

        int n;

        printf("n=");

        scanf("%d", &n);

        if(-1 == write(p2c[1], &n, sizeof(int))) {

            perror("Error writing n to child");

            close(p2c[1]);

            close(c2p[0]);

            wait(NULL);

            exit(1);

        }

        if(n > 0) {

            int *arr = malloc(sizeof(int) * n);

            for(int i = 0; i < n; i++) {

                arr[i] = random() % 100;

                printf("a[%d] = %3d\n", i, arr[i]);

            }

            if(-1 == write(p2c[1], arr, sizeof(int) * n)) {

                perror("Error sending array to child");

                close(p2c[1]);

                close(c2p[0]);

                wait(NULL);

                exit(1);

            }

            free(arr);

            int sum = 0;

            if(-1 == read(c2p[0], &sum, sizeof(int))) {

                perror("Error reading sum from child");

                close(p2c[1]);

                close(c2p[0]);

                wait(NULL);

                exit(1);

            }

            printf("sum = %d\n", sum);

        }

        close(p2c[1]);

        close(c2p[0]);

        wait(NULL);

    }

    return 0;

}