In this lab you will complete a set of C programs that will expose you to the set of system calls that are used in shell pipelines, such as process grouping, pipes, and filed descriptor duplication. There are 2 tasks, and you will likely complete neither of the tasks in lab, You will need to finish the remaining asks outside of the lab period.

In the previous lessons, we discussed the following pipeline in detail:

sleep 10 | sleep 20 | sleep 30 | sleep 50

We identified that the entire pipeline runs as single job in the shell, and each part of the pipeline is a seperate process running in parallel within a process group. The process group is important for terminal signaling, and foreground process are identified based on process group.

In this part of the lab, you implement a standard pipeline unrolling, which is the process of parsing a pipeline based on | and forking each of the children in their own process group. The unrolling code will only concern itself with the individual forks, and not inter-process communication; that's the next part.

In this part of the lab, you will focus on the inter-process communication of the pipeline by implementing a pipeline of commands. This part is not focused on parsing the pipeline, and, instead, you are provided with three pre-parsed pipelines to work with that are defined under pre-compiler directives.

To complete this task, you will need to think about how to unroll a pipeline such that there exists a pipe between each of the processes. This requires one call a sequence of calls to pipe() to and calls to dup2() for duplicating the write/read end of the pipes to stdin/stdout of each of the processes appropriately. This will require careful attention to the start, end and middle of the pipeline.

Here is some sample code demonstrating a one step pipeline, child to parent, with an exec. You should be able to adapt this code to complete the lab:

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

int main(){

  // cat sample-db | grep MD
  char * cmd1[] = {"cat", "sample-db.csv", NULL};
  char * cmd2[] = {"grep", "MD", NULL};

  int pipe_fd[2];
  int status;

  pid_t cpid1,cpid2;

  //open the pipe in parent
  if(pipe(pipe_fd) < 0){
    perror("pipe");
    _exit(1);
  }

  cpid1 = fork();
  if( cpid1 ==  0){
    /* CHILD 1*/

    //redirect output to the pipe
    close(pipe_fd[0]); //close read end of pipe    
    close(1);//close stdout
    dup2(pipe_fd[1], 1); //duplicate pipe to stdout

    //execute the comand
    execvp(cmd1[0], cmd1);
    perror("exec");
    _exit(1);
  }else if(cpid1 < 0){
    /* ERROR */
    perror("fork");
    _exit(1);
  }

  cpid2 = fork();
  if(cpid2 == 0){
    /* CHILD 2*/

    //redirect input from the pipe
    close(pipe_fd[1]); //close write end of pipe
    close(0); //close stdin
    dup2(pipe_fd[0],0); //duplicate pipe to stdin

    //execute the comand
    execvp(cmd2[0], cmd2);
    perror("exec");
    _exit(1);
  }else if(cpid2 < 0){
    /* ERROR */
    perror("fork");
    _exit(1);
  }

  /* PARENT */

  //widow pipe by closing write end of pipe
  close(pipe_fd[1]);

  //wait on all children
  while(wait(&status) > 0 );

  //sucess
  return 0;
}

The program above generate a one-step pipe line for the command:

cat sample-db | grep MD

And if you execute it, you get the desired result:

#>./one-step 
Kris,Marrier,King, Christopher A Esq,228 Runamuck Pl #2808,Baltimore,Baltimore City,MD,21224,410-655-8723,410-804-4694,kris@gmail.com,http://www.kingchristopheraesq.com
Ezekiel,Chui,Sider, Donald C Esq,2 Cedar Ave #84,Easton,Talbot,MD,21601,410-669-1642,410-235-8738,ezekiel@chui.com,http://www.siderdonaldcesq.com
Ilene,Eroman,Robinson, William J Esq,2853 S Central Expy,Glen Burnie,Anne Arundel,MD,21061,410-914-9018,410-937-4543,ilene.eroman@hotmail.com,http://www.robinsonwilliamjesq.com
Fernanda,Jillson,Shank, Edward L Esq,60480 Old Us Highway 51,Preston,Caroline,MD,21655,410-387-5260,410-724-6472,fjillson@aol.com,http://www.shankedwardlesq.com
Sylvia,Cousey,Berg, Charles E,287 Youngstown Warren Rd,Hampstead,Carroll,MD,21074,410-209-9545,410-863-8263,sylvia_cousey@cousey.org,http://www.bergcharlese.com
Loreta,Timenez,Kaminski, Katherine Andritsaki,47857 Coney Island Ave,Clinton,Prince Georges,MD,20735,301-696-6420,301-392-6698,loreta.timenez@hotmail.com,http://www.kaminskikatherineandritsaki.com
Kaitlyn,Ogg,Garrison, Paul E Esq,2 S Biscayne Blvd,Baltimore,Baltimore City,MD,21230,410-665-4903,410-773-3862,kaitlyn.ogg@gmail.com,http://www.garrisonpauleesq.com
Izetta,Dewar,Lisatoni, Jean Esq,2 W Scyene Rd #3,Baltimore,Baltimore City,MD,21217,410-473-1708,410-522-7621,idewar@dewar.com,http://www.lisatonijeanesq.com

The key to properly setting up a pipe between process is widow'ing the pipe. This is where one end of the pipe is closed, normally the end that is not in use. For example:

//redirect input from the pipe
 close(pipe_fd[1]); //close write end of pipe
 close(0); //close stdin
 dup2(pipe_fd[0],0); //duplicate pipe to stdin

Here, the write end of the pipe is closed in the second child since it is not in use. This makes the read end of the pipe a widow in the second child, which means, once the write end of the pipe is closed in the first child, EOF is generated. All instances of the pipe must be widowed, this includes the copy that exists in the parent, and if the pipe isn't widowed, the entire pipeline will hang.