Cmput 379 Lab Exercise #1

For the period 12-18 January, 2001

Objective (Your TA will review this exercise at the start of the lab period)

Write a C-program to explore the Unix client/server use of:

fork() -- for creating a child process
pipe() -- for communicating between two processes that have a "parent-child" relationship
Some i/o functions like putc, getc, read, write.

This is also an opportunity to gain a working familiarity with Stevens' book, which will be a mainstay reference for the later programming assignments.

Filenames

Hold your source code in a single file called ParentChild.c and your compiled executable code in ParentChild. The stderr output is to appear in a file called ParentChild.err

Program Specifications

Write a program that consists of a simple infinite loop: it reads a line of input from stdin, and writes the line to stdout. This snippet of a program must terminate properly when a Ctrl-D (end-of-file) is entered. Next add a call to fork() before the loop, so that a parent process creates a child (a clone) that executes its own section of code with its own i/o requests, while the parent continues into the infinite loop as before. Since both the parent and the child are initially attached to the user's keyboard and monitor, it may not be clear who is reading/writing what. Check it out. Revise your program so that the parent side is used for reading from the keyboard (stdin) and sending the information entered to the child via a data-pipe. Similarly the child takes the information from the parent through a data-pipe and writes it to stdout (the monitor). This is done by detaching the parent from the monitor (close stdout) and the child from the keyboard (close stdin) in the appropriate places. Note the pipe() must be set up before the fork(), see Stevens for details.

If stderr is attached to an output file before fork() is called, then both parent and child write to the same error file. Thus stderr may be used to check the sequencing of events. Do this, and have the parent write to stderr a copy of whatever is sends over the pipe. Similarly, have the child write to stderr whatever it receives over the pipe. Check it out. Does it work OK? Finally the parent cannot know how long the input line is going to be. There are a couple of ways of addressing this issue. Implement one of them. Equally, the size of the internal buffer allocated to the pipe is fixed (what is it?), and so this too may pose problems for the child process if the data line is longer. Discuss briefly how you address these issues.

Diagrammatically we have:

               parent                     child
               -------------             ------------
Keyboard ---->|stdin    p[0]|----\      |p[0]   stdin|  
              |stdout   p[1]|     \---->|p[1]  stdout|---> Monitor
               -------------    pipe     ------------

Demonstrate that the program works by invoking the program with ParentChild and typing miscellaneous text at the keyboard.

Program Execution

It should be possible to invoke your program in the following four ways:

ParentChild

ParentChild <infile >outfile

<infile ParentChild >outfile

cat infile | ParentChild >outfile

Where, for demonstration purposes, infile can be ParentChild.c

Do all these cases work equally well?
In what way are some of them fundamentally different?
Does the following command also work?

ParentChild 0<infile 1>outfile 2>ParentChild.err

Comment on these points and summarize what you have learnt in a single email to your TA at the end of the laboratory period.

A Requirement (must work for all four invocations above)

Set up your program so that a Ctrl-D typed at the keyboard is enough to terminate both parent and child properly.

Hints

Remember CMPUT 201 basics, your PATH variable must be set correctly to execute programs in the current directory.

A More Interesting Variation

Move the fork() function inside the parent's infinite loop. As each line is read by the parent a new child process is spawned to "process" (display) the input line. The parent and child processes can be said to be in a client/server relationship.
These processes should all disappear after a Ctrl-D or Ctrl-C is entered. You can check by using the "ps" command.
However, if you enter a Crtl-Z and then use "ps" you should see your parent and child processes "waiting". Restart your application using "fg" and terminate normally. Again use "ps" to check that all processes have disappeared.