Heroic piping in the age of steel

Pipes and Forks Project

by: burt rosenberg
at: university of miami
date: sep 2022

Goals

• To further understand how unix creates processes (except process 1).
• To further understand how unix process communicate through file descriptors.
• To further understand how unix creates pipes between programs.
• To exercise a small number of programming techniques, among them:
  • #ifndef to prevent header imports,
  • ___FILE__ and _LINE__ macros,
  • getopt for a trouble-free life,
  • assert for a less troubled life,
  • why not to use globals and how to document them when you do
  • makefile magic.

  Man Page

  NAME
      my-word-count -- a program that does what wc does, but differently.
      
  SYNOPSIS
      my-word-count [-vclmw] _filename_
  
  DESCRIPTION
      Creates a child process, joined withe a pipe. The parent exec's cat on _filename_
      and the child exec's wc the appropriate flags taken from the command line.
      
      The following options are available:
      
      -v verbose output. This option is not forwarded to the exec'ed processes.
      -clmw see man wc
  
  OUTPUT
  
      See man wc
  
  HISTORY
      Introduced in csc421.231.
      
  BUGS
      
  

  Discussion

  The templates to guide in this are in the class/examples folder of the subversion respository,

  1. [repo]/class/examples/pipe-test (github).
  2. [repo]/class/examples/exec-test (github).

  Create you proj1 directory under your folder, and copy class/proj1 to that folder.

  I suggest cutting and pasting code from pipe-test.c and exec-test.c. Note that the template my-word-count.c correctly references the provided header file my-word-count.h.

  The pipe-test code does a lot of the work.

  1. It creates a pipe
  2. It forks the child
  3. In each of the parent and child, it handles the dup'ing of the pipe ends into the standard streams.
  4. It then sends a fixed message. This step you must change to the appropriate exec's.

  The short exec-test code demonstrates how you will invoke wc and cat. The execlp acts as if the command line received the various arguments to the call. However, we have to review some matters about the argv array.

  1. The first argument to execlp is a command, which will be found using the current path (hence the p in the function name).
  2. The remaining arguments are what will be the elements of argv in the exec'ed program.
  3. execlp is a varargs function, meaning it takes a variable number of arguments. As with arrays in C, the function will not naturally know how any arguments were provided ...
  4. ... execlp looking for arguments (e.g. by walking up the stack) until a null argument is found.
  5. The first argument of argv is the name of the program itself.

  Get the program correct for the simple case of sending no arguments to wc. Then create the code that constructs the option string to be given to wc from the option string given to my-word-count.

  A good basic program works without considering options. However full credit will require properly handling the options. Test targets is given in the makefile for the case of no options, and for a particular combination of options. This is not exhaustive.

  Only the options asked for in the pseudo-manpage (above) are to be implemented.

  References

  • man wc
  • man exec Pipes in C in The Linux Programmer's Guide
  • Source code for unistd.h
  
  This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

  author: burton rosenberg
  created: 4 sep 2023
  update: 4 sep 2023