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Full Discussion: Unix Piping Problem
Homework and Emergencies Homework & Coursework Questions Unix Piping Problem Post 302704887 by Don Cragun on Sunday 23rd of September 2012 11:36:15 PM
Old 09-24-2012
Quote:
Originally Posted by itsjimmy91
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.

... ... ...
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);
  }
}

... ... ...

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

Here are a few hints:
  1. Since you're using, but not declaring pid this code shouldn't even compile let alone hang when you run it.
  2. You need to close fd 1 in the child before calling dup2 (so that the output from ls is redirected into the write end of the pipe. Then you need to close both of the original pipe file descriptors in the child before exec'ing ls.
  3. In the parent, you need to close the write end of the pipe.
  4. If you don't want ls to hang waiting for the pipe to drain, you need to read the data in the parent that is written by the child.
  5. Don't wait in the parent for the child to exit until you've hit EOF on the data being written by the child.
  6. Check the function call return codes to verify that the functions you call return successfully. Especially when learning how to use functions that are new to you, the return codes (and the value of errno [see also perror()]) will frequently tell you what to look for when things go wrong.
 

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LEARN ABOUT CENTOS

macutil

MACUTIL(1)						      General Commands Manual							MACUTIL(1)

NAME

       macutil - A package that deals with MacIntosh files on a Unix system

DESCRIPTION

       macutil	is  a package that contains a number of utilities that deal with MacIntosh files on a Unix system.  It contains the following pro-
       grams:

       binhex Convert files to BinHex 4.0 compatible hexified form.

       frommac
	      Receives files from the MacIntosh on the Unix system.

       hexbin Convert hexified files to their MacIntosh format.

       macsave
	      Save a series of files from a MacBinary stream as individual files.

       macstream
	      Combine a series of files to a MacBinary stream.

       macunpack
	      Unpack a MacIntosh archive into its constituents.

       tomac  Transmits files from the Unix system to a MacIntosh.

BUGS

       This manual page is hopelessly incomplete!

SEE ALSO

       binhex(1), frommac(1), hexbin(1), macsave(1), macstream(1), macunpack(1), tomac(1)

AUTHOR

       Dik T. Winter, CWI, Amsterdam, The Netherlands (dik@cwi.nl)

3rd Berkeley Distribution					 October 22, 1992							MACUTIL(1)
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