Lab 7: Pipes in UNIX

Goals: This laboratory exercise provides practice with the use of pipes to allow interprocess communication, and the strategy of problem solving by connecting small, specialized programs.

Preparation: Read through sample programs fork-2.c through fork-6.c in An Introduction to Concurrency in Unix-based [GNU] C Through Annotated Examples.

Collaboration: You will complete this lab in teams of 1 to 3 of your choice  You may, as always, consult with your classmates on issues of design and debugging.

Part A: Experiments with pipe

1. Copy ~walker/c/concurrency/fork-2.c to your account, compile it with gcc and run it a few times -- waiting a few moments between each run.
2. In the statement write (fd[1], ...) change the 23 to 10. Then recompile and rerun. How does this affect the running of the program? Briefly explain what you see.

   Now change the 10 (formerly 23) to 30 and rerun. Again, describe the effect and explain why this happens.

3. In #define change MAX to 15, recompile and rerun. Again describe and explain the result.

4. With MAX set to 15, change the reading/printing section of code for the child to:
   

   read(fd[0], line, MAX);
   printf ("The string received is '%s'\n", line);
   read(fd[0], line, MAX);
   printf ("The string received is '%s'\n", line);
   

   That is, perform the reading and printing twice. Rerun the program.  Describe and explain the result.
5. With MAX set to 40, change the writing section of code for the parent to:
   

   write (fd[1], "ABCDEFGHIJKLMNOPQRSTUVWXYZ", 26);
   write (fd[1], "abcdefghijklmnopqrstuvwxyz", 26);
   

   Rerun the program.  Describe and explan the result.
6. Copy ~walker/c/concurrency/fork-3.c to your account, compile it, and run it. Why do you think the variable value is used in the write statement rather than giving the number 20 directly?
7. Remove the wait statement from the parent. Then recompile and rerun several times. Does the order of the output ever change? Explain why or why not.

Part B: Coordination of Specialized Programs

Consider the following problem, which is based on an exercise by John Stone:

The file /davisjan/csc/213/examples/Iowa-cities.dat contains information about the sixty largest cities and towns in Iowa: their names and populations, as determined by the 2000, 1990, and 1980 censuses. A typical line of the file looks like this:

Grinnell,9105,8902,8868

In this step, you are to write C programs which read data from this file and determine the answers to the following two questions:

• Which of these Iowa cities had the largest percentage decrease in population between 1980 and 2000?

• The 2000 census population of the state of Iowa was 2,926,324. How many Iowans lived outside the sixty largest cities and towns in 2000?

The basic approach for this problem is illustrated in the following diagram:

As this diagram suggests, one parent process will spawn two children; the parent and both children then should collaborate to perform this work. Overall, the parent process will read the file and send a copy of each line through a pipe to each child. Each child will answer one of the above questions. More precisely:

• The parent process should use popen to create one child process. The parent then should send formatted data to that child using the file descriptor returned by popen.
• The one child process (created by popen) should read from standard input and determine the city with the greatest percentage decrease in population.
• The parent process also should use pipe and fork to create a second child process connected to the parent by a pipe. The parent then should bind one end of this pipe to the parent's standard output and send a copy of the formatted file data through this pipe.
• The second child process (created with pipe and fork) should bind its standard input to the pipe. This second child process then should exec a new program that reads from standard in and computes the rural population directly.
  

Work to be Turned In

• Due Friday, 13 October: Commentary for Part A.
• Due Tuesday, 24 October: Programs for Part B.  Follow the instructions for submitting programs.

Janet Davis (davisjan@cs.grinnell.edu)