modify a bidirectional ring

Posted on 1998-05-05
Last Modified: 2012-08-13
Hi I am studying unix programming. However, I have difficulties to do one problem on a book(practical unix programming). There is no solutions provided in the book! I need to modify the following program to make it a bidirectinal ring so that the inormation can flow in either direction between neighbors on the ring.  

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
 * Sample C program for generating a unidirectional ring of processes.
 * Invoke this program with a command-line argument indicating the
 * number of processes on the ring.  Communication is done via pipes
 * that connect the standard output of a process to the standard input
 * of its successor on the ring.  After the ring is created, each
 * process identifies itself with its process ID and the process ID
 * of its parent.  Each process then exits.
void main(int argc,  char *argv[ ])
   int   i;             /* number of this process (starting with 1)   */
   int   childpid;      /* indicates process should spawn another     */
   int   nprocs;        /* total number of processes in ring          */
   int   fd[2];         /* file descriptors returned by pipe          */
   int   error;         /* return value from dup2 call                */
   int status;
    /* check command line for a valid number of processes to generate */
   if ( (argc != 2) || ((nprocs = atoi (argv[1])) <= 0) ) {
       fprintf (stderr, "Usage: %s nprocs\n", argv[0]);
                        /* connect std input to std output via a pipe */
   if (pipe (fd) == -1) {
      perror("Could not create pipe");
   if ((dup2(fd[0], STDIN_FILENO) == -1) ||
       (dup2(fd[1], STDOUT_FILENO) == -1)) {
      perror("Could not dup pipes");
   if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
      perror("Could not close extra descriptors");
        /* create the remaining processes with their connecting pipes */
   for (i = 1; i < nprocs;  i++) {
      if (pipe (fd) == -1) {
         fprintf(stderr,"Could not create pipe %d: %s\n",
            i, strerror(errno));
      if ((childpid = fork()) == -1) {
         fprintf(stderr, "Could not create child %d: %s\n",
            i, strerror(errno));
      if (childpid > 0)        /* for parent process, reassign stdout */
          error = dup2(fd[1], STDOUT_FILENO);
          error = dup2(fd[0], STDIN_FILENO);
      if (error == -1) {
         fprintf(stderr, "Could not dup pipes for iteration %d: %s\n",
                 i, strerror(errno));
      if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
         fprintf(stderr, "Could not close extra descriptors %d: %s\n",
                i, strerror(errno));
      if (childpid)

                                  /* say hello to the world */
   fprintf(stderr,"This is process %d with ID %d and parent id %d\n",
      i, (int)getpid(), (int)getppid());
   exit (0);
}     /* end of main program here */
Question by:kelly_host

Expert Comment

Comment Utility
Please see the question "Token Ring(Bidirectional)" under C Language topic area, at:

Accepted Solution

eelko earned 130 total points
Comment Utility
Use two pipes between parent and child. One to send data from parent to child and one to send data from child to parent.
Offcourse you can't dup them to stdout en stdin cause you only have one stdout and one stdin.

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