Question

The program, asks the user to enter the number of processes to be created as an argument so basically you would do something like:

$ ./program 4

It will generate something like this:

This is process 1 with ID 7389 and parent id 6550
This is process 2 with ID 7390 and parent id 7389
This is process 3 with ID 7391 and parent id 7390
This is process 4 with ID 7392 and parent id 7391

need to insert an item now in the given position so basically

$ ./program insert 3

It should add a new process in line 3 and would look something like:

This is process 1 with ID 7389 and parent id 6550
This is process 2 with ID 7390 and parent id 7389
This is process 3 with ID 7399 and parent id 7389  //NEW
This is process 4 with ID 7391 and parent id 7390
This is process 5 with ID 7392 and parent id 7391

I dont know how to do the insertion, I would appreciate any help or suggestions if possible

Thanks

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


int main(int argc,  char *argv[ ]) {
   pid_t childpid;             /* indicates process should spawn another     */
   int error;                  /* return value from dup2 call                */
   int fd[2];                  /* file descriptors returned by pipe          */
   int i;                      /* number of this process (starting with 1)   */
   int nprocs;                 /* total number of processes in ring          */ 
           /* check command line for a valid number of processes to generate */
   if ( (argc != 2) || ((nprocs = atoi (argv[1])) <= 0) ) {
       fprintf (stderr, "Usage: %s nprocs ID\n", argv[0]);
       return 1; 
   }  
   if (pipe (fd) == -1) {      /* connect std input to std output via a pipe */
      perror("Failed to create starting pipe");
      return 1;
   }
   if ((dup2(fd[0], STDIN_FILENO) == -1) ||
       (dup2(fd[1], STDOUT_FILENO) == -1)) {
      perror("Failed to connect pipe");
      return 1;
   }
   if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
      perror("Failed to close extra descriptors");
      return 1; 
   }        
   for (i = 1; i < nprocs;  i++) {         /* create the remaining processes */
      if (pipe (fd) == -1) {
         fprintf(stderr, "[%ld]:failed to create pipe %d: %s\n",
                (long)getpid(), i, strerror(errno));
         return 1; 
      } 
      if ((childpid = fork()) == -1) {
         fprintf(stderr, "[%ld]:failed to create child %d: %s\n",
                 (long)getpid(), i, strerror(errno));
         return 1; 
      } 
      if (childpid > 0)               /* for parent process, reassign stdout */
          error = dup2(fd[1], STDOUT_FILENO);
      else                              /* for child process, reassign stdin */
          error = dup2(fd[0], STDIN_FILENO);
      if (error == -1) {
         fprintf(stderr, "[%ld]:failed to dup pipes for iteration %d: %s\n",
                 (long)getpid(), i, strerror(errno));
         return 1; 
      } 
      if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
         fprintf(stderr, "[%ld]:failed to close extra descriptors %d: %s\n",
                (long)getpid(), i, strerror(errno));
         return 1; 
      } 
      if (childpid)
         break;
   }                                               /* say hello to the world */
   fprintf(stderr, "This is process %d with ID %ld and parent id %ld\n",
           i, (long)getpid(), (long)getppid());


        close(fd[0]);          /* Close unused read end */
        close(fd[1]);          /* Reader will see EOF */
        wait(NULL);             /* Wait for child */
        exit(EXIT_SUCCESS);
   return 0; 
}

Answer 1

Only process 7389 could create a child process with it as the parent.

You would need some way of sending a message or signal to process 7389 instructing it to create a new child process.