Homework & Coursework Questions

Hey guys. I'm very new to Unix. I'm pretty fluent in Java and C, but I have never actually used Unix for anything. I am in an Operating Systems course now and I have an assignment to write a piece of code that involves forks and piping. I'm stuck.

1. The problem statement, all variables and given/known data:

Write a C program that will create three processes as follows:

Code:

         A
        /   \
       /     \
      /       \
   B            C

Connect process B and C with a pipe. Process C will run ls program and send the output into the pipe. Process B will run sort program and sort the list from process C in reverse order. Process A will print "Welcome, friend!" before the list is displayed and print "Good Bye!” after the list is displayed. Do not use the sleep function. Tell me if your program works.


2. Relevant commands, code, scripts, algorithms:

n/a

3. The attempts at a solution (include all code and scripts):

Here's my crack at it...

Code:

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

int main() {
  int pipefd[2];
  int pid1 = 0; int pid2 = 0;
  char *lsArg[2] = {"ls", 0};
  char *rArg[2] = {"-r", 0};
  pipe(pipefd);
 
  pid1 = fork();
  if(pid == 0) {
    dup2(pipefd[1], 1);
    execv("/bin/ls", lsArg);
  }
  else {
    waitpid(pid1, NULL, 0);
  }
}

4. Complete Name of School (University), City (State), Country, Name of Professor, and Course Number (Link to Course):
Rowan University, NJ USA. Professor Xu.
h ttp ://elvis. rowan .edu/~xu/classes/os/os_f12.htm


Now I realize that I do not have the reverse thing in there, nor do I have the Welcome and goodbye message. I'm just trying to figure out where things go and how they really work.

When I run what I currently have, it lists what my "ls" would list in normal order. It also locks up and doesn't ever seem to really finish, and I can't do anything without disconnecting.

Like I said, new to Unix.. any help would be appreciated. Thanks.

Moderator's Comments:

edit by bakunin: please view this code tag video for how to use code tags when posting code and data.

Please use CODE tags. (I've added them in the text I quoted from your posting.)

Here are a few hints:

Hey man, thanks a lot for the replies. Sorry about the code thing. I have made a few adjustments and I think I am really close. My only problem now is that the program is not actually displaying the information in reverse. Is my reverse command not getting hit?

Code:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
  int pipefd[2];
  int pid1 = 0, pid2 = 0;
  char *lsArg[2] = {"ls", 0};
  char *rArg[2] = {"-r", 0};
  pipe(pipefd);

  printf("Welcome, friend!\n");
  pid1 = fork();
  if(pid1 == 0) {
    close(piped[0]);
    dup2(pipefd[1], 1);
    execv("/bin/ls", lsArg);
  }

  pid2 = fork();
  if(pid2 == 0) {
    close(piped[0]);
    dup2(pipefd[0], 0);
    execv("/bin/ls", rArg);
  }
  else {
    close(pipefd[0]);
    close(pipefd[1]);
    waitpid(pid2, NULL, 0);
  }
}

You are running ls twice. I think you meant to run sort for the second exec.

This line here?

Code:

execv("/bin/ls", rArg);

Maybe I'm not understand this correctly. Won't this line take what is being done in the other side of the pipe (the ls), and then do to it rArg (which is "-r")?

Nope. it runs ls -r. Since ls doesn't read anything you type into it, just lists files, that's pretty pointless.

The assignment told you what command to feed ls's output into -- sort. There's an actual command named that, which does what it says on the label.

You could have just done ls -r in the very first exec to have it list in reverse order from the start, avoiding having to launch a second process. But then you wouldn't be learning how pipes work...

Code:

if(pid2 == 0) {
    close(piped[0]);
    dup2(pipefd[0], 0);
    execv("/bin/ls", rArg);
  }

Is there a reason you're closing the read end and then dup'ing it? Also, in the parent of 2 you're closing the descriptor's and then waiting. Not sure if that's going to have any unintended consequences, but you may want to try closing them after the children are done.

My problem was to do a merge sort with a binary tree of forked processes so it's different as I wasn't using exec() so I'm not sure if I've been much help, but it's what I got.