int socketpair(int domain, int type, int protocol, int sv[]);

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <sys/socket.h>

#include <fcntl.h>

#include <unistd.h>

#define MAXLINE 4096

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

    int fd[];

    pid_t pid;

    char line[MAXLINE];

    int n;

    if (socketpair(AF_UNIX, SOCK_STREAM, , fd) < ) {

        perror("socketpair error");

        exit();

    }

    if ( (pid = fork()) < ) {

        perror("fork error");

        exit();

    } else if (pid > ) {

        close(fd[]);

        write(fd[], "hello world\n", );

        char tmp[MAXLINE];

        read(fd[], tmp, MAXLINE);

        printf("echo is: %s\n", tmp);

    } else {

        close(fd[]);

        n = read(fd[], line, MAXLINE);

        write(STDOUT_FILENO, line, n);

        char tmp[] = "world says hi";

        write(fd[], tmp, strlen(tmp));

    }

    exit();

}