CSCI 3500: Studio 2

Input and Output

Linux stream redirection is a powerful feature where program inputs and outputs can be hooked together seamlessly by the system to build compound commands. In this studio, you will:

1. Use the read() and write() functions to read input and write output
2. Redirect program inputs and outputs

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers in a text file. When finished, submit your work by sending your text file and source code to the git repository under the appropriate folder.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. One basic Linux tool you might know about is a program called cat. This program concatenates (appends) any provided input to standard output. It's a useful way to print text files to the terminal, or to write bits of data into a text file.

   Go ahead and type the command "cat" and hit enter. Then, write a few lines of text to the console (and hit enter after each line). As the answer to this exercise, describe what happens.

   You can type CTRL-D on a blank line in order to quit cat. Typing CTRL-D sends a special "end of file" character to the terminal.

3. The original purpose of cat is to print text files to the console. Find your favorite text file if you'd like, or you can download one of mine with the wget command:

   wget http://cs.slu.edu/~dferry/courses/csci3500/studios/mars.txt

   In the last exercise, the cat program got its data from standard input (i.e. the keyboard). One neat feature of Linux is that it allows us to redirect streams like standard input and standard output. Using your text file, redirect standard input with the '<' character as such:

   cat < mars.txt

   As the answer to this exercise, describe what happens.

4. We can also redirect output. This time, redirect standard output with the '>' character into a new text file. Use the syntax:

   cat > new.txt

   Enter a few lines of text. Open up your text file to see the results. Remember that you can quit cat with CTRL-D.

   As the answer to this exercise, describe what happens.

5. One last useful tip. Repeat the last exercise, but instead of using a single greater-than symbol, use two. (That is, use '>>' instead of '>'). This appends your input to the end of the file, rather than overwriting the start of the file.

   Leave this answer blank.

6. Now we'll write a short program that mimics the behavior of cat. Start by creating a new file called copy.c and fill it out with the framework for an empty program (see Studio 01 if you'd like to see that framework again).

   Look at the manual pages for the system calls read() and write(), and look at your code from Studio 01 to see how you used write(). What header file do you need to include to use these system calls?

7. Both functions require a buffer (a fixed region of memory) to operate. At the top of your program, define a preprocessor constant with the size of your buffer as such:

   #define bufferSize 200

   Then allocate an array of characters to be your buffer:

   char buffer[ bufferSize ];

   Leave this answer blank.

8. The behavior of cat is to read input forever until it encounters an end of file character. Look up the documentation for the read() system call. What is the return value type for read()? What is the specific return value that indicates the end of a file?

9. Your last job is to translate the following algorithm into C code. Inside an infinite loop:
   1. Read from standard input (STDIN_FILENO) into the buffer.
   2. If the return value indicates end of file, then "break;" out of the loop.
   3. Write the contents of the buffer to standard output (STDOUT_FILENO). Be careful to use the return value from read() so as not to write more than was read.

   When you are satisfied with your code, compile it and test it in all the ways we tested cat above. Does your program's behavior match? Note that you will need to invoke your program with "./", as in:

   ./copy < mars.txt

10. Do you think that modifying bufferSize will affect the correctness of your program? What if bufferSize is very small, or even equal to one? Try a few different values for bufferSize and record the results.

11. What do you think the tradeoff is between having a small bufferSize versus having a large one?

Optional Enrichment Exercises

1. You can write the same program on top of the C standard library as well! Instead of using read() and write(), try using the libc functions fgets() and fprintf().