在C编程中使用管道

Question

Im using pipe in c to create a programme that determine if the entered integer number by user is even or odd. Also, It should achieve the following specifications : The Parent process should send the integer number to the forked child process. The child process should receive the sent integer number to determine his type (even or odd). Then, the result should be returned back to parent process in order to display it on the shell for the user. I have to Use Pipes IPC to exchange data between parent and child processes.

my code is as shown below

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

void sendNumber(int);
void RecResult();
int f_des[2];

int main(int argc, char *argv[])
{
    static char message[BUFSIZ];
    if ( pipe(f_des) == -1 )
    {
        perror("Pipe");
        exit(2);
    }
    switch ( fork() )
    {

        case -1: perror("Fork");
            exit(3);

        case 0: /* In the child */
        {

            // read a meesage
            close(f_des[1]);
            read(f_des[0], message, BUFSIZ  );

            int num=atoi(message);
            printf("Message received by child: [%d]\n", num);


            if(num % 2 == 0)

                sprintf(message,"%d is Even \0",num);

            else

                sprintf(message,"%d is Odd \0",num);

            close(f_des[0]);
            write(f_des[1], message, BUFSIZ);
            exit(0);
        }

            break;
        default: /* In the parent */
        {
            int num;
            printf("\n Enter an Integer \n");
            scanf("%d",&num);
            char msg[BUFSIZ];
            sprintf(msg,"%d",num);

            // write a message
            close(f_des[0]);
            write(f_des[1], msg, BUFSIZ);

            printf("\n Waiting Child \n");

            wait(NULL); 
            // read a mesaage
            static char message[BUFSIZ];
            close(f_des[1]);
            read(f_des[0], message, BUFSIZ);

            printf("========> Result:  %s . \n",message);
        }
    }
}

the child receive the message successfully but the parent does not receive any result :S any one can help here is my output

rose@ubuntu:~$ gcc -o test_3 test_3.c
rose@ubuntu:~$ ./test_3

 Enter an Integer 
3

 Waiting Child 
Message received by child: [3]
========> Result:   . 
rose@ubuntu:~$ 

thaanks all ;)

Answer 1

A PIPE channel is a uni-directional communication channel. If you want the child process to write back to the parent process, you will need a second PIPE channel, different than the one used for receiving the message from the parent.