Question

Creating a pipe in C language requires simple command which is a s follows.

SYSTEM CALL: pipe();                                                         

PROTOTYPE: int pipe( int fd[2] );                                            

    RETURNS: 0 on success                                                       

             -1 on error: errno = EMFILE (no free descriptors)                 

                                  EMFILE (system file table is full)           

                                  EFAULT (fd array is not valid)               

NOTE: fd[0] is set up for reading, fd[1] is set up for writing

Design a simple C program using ordinary pipes in which a parent and child processes exchange greeting messages. For example, the parent process may send the message “Hello Child Process”, and the child process may return “Hi Parent Process”. Use UNIX pipes to write this program. You need to have bidirectional behavior in your program for which you will need two pipes.

Answer 1

Idea : We need to implement two way communication between Child and Parent process using Pipe

Approach :

1. Initialize Pipe_A for the Child process to write and Parent process to read.
2. Initialize Pipe_B for the Parent process to write and Child process to read.
3. Since, a single pipe is for uni-directional purpose . So, at one time only one functionality of pipe will be used . Thus, close unwanted ends of the pipe from child and parents process.
4. Parent process writes , "Hello Child Process" , and Child process reads this and print on screen.
5. Child process writes , "Hello Parents Process" , and Parent process reads this and print on screen.

CODE:

#include<stdio.h>
#include<unistd.h>

int main()
{   //Initializing Pipe_A and Pipe_B
   int Pipe_A[2], Pipe_B[2];
   //Initializing status flags for both Pipes
   int return_status_pipe_a, return_status_pipe_b;
   int pid;
   //Messages That will be exchanged
   char parent_msg[30] = "Hello Child Process";
   char child_msg[30] = "Hello Parent Process";
   //Initializing Buffer variable
   char read_msg[30];
   //Creating Pipe_A and Checking whether it has been created or not
   return_status_pipe_a=pipe(Pipe_A);
   if(return_status_pipe_a == -1)
   {
       printf("Unable to Create Pipe A\n");
       return 1;
   }
   //Creating Pipe_B and Checking whether it has been created or not
   return_status_pipe_b=pipe(Pipe_B);
   if(return_status_pipe_b == -1)
   {
       printf("Unable to Create Pipe B\n");
       return 1;
   }
   pid = fork();
   //Parent Process
   if(pid !=0)
   {   //Disable Child Write
       close(Pipe_A[1]);
       //Disable Parent Read
       close(Pipe_B[0]);

       printf("Parent (Writing to Child) : %s\n",parent_msg);
       write(Pipe_B[1], parent_msg, sizeof(parent_msg));

       read(Pipe_A[0],read_msg,sizeof(read_msg));
       printf("Parent (Reading from Child) : %s\n",read_msg);
   }
   //Child Process  
   else
   {   //Disable Child Read
       close(Pipe_A[0]);
       //Disabe Parent Write
       close(Pipe_B[1]);

       read(Pipe_B[0],read_msg,sizeof(read_msg));
       printf("Child (Reading from Parent): %s\n", read_msg);
  
       printf("Child (Writing to Parent): %s\n",child_msg);
       write(Pipe_A[1],child_msg,sizeof(child_msg));
      
   }
   return 0;
}

Console Output :