Princeton University
COS 217: Introduction to Programming Systems

Purpose

The purpose of this assignment is to help you learn or review (1) the fundamentals of the C programming language, (2) the details of the "de-commenting" task of the C preprocessor, and (3) how to use the GNU/Unix programming tools, especially bash, emacs, and gcc217.

Rules

Make sure you study the course Policies web page before doing this assignment or any of the COS 217 assignments. In particular, note that you may consult with the course instructors, lab TAs, listserv, etc. while doing assignments, as prescribed by that web page. However, there is one exception...

Throughout the semester, each assignment will have an "on your own" part. You must do that part of the assignment completely on your own, without consulting with the course instructors, lab TAs, listserv, etc., except for clarification of requirements. You might think of the "on your own" part of each assignment as a small take-home exam.

For this assignment, "detecting and reporting unterminated comments" (as described below) is the "on your own" part. That part is worth 10% of this assignment.

Background

The C preprocessor is an important part of the C programming system. Given a C source code file, the C preprocessor performs three jobs:

• Merge "physical" lines of source code into "logical" lines. That is, when the preprocessor detects a line that ends with the backslash character, it merges that physical line with the next physical line to form one logical line.
• Remove comments from ("de-comment") the source code.
• Handle preprocessor directives (#define, #include, etc.) that reside in the source code.

The de-comment job is substantial. For example, the C preprocessor must be sensitive to:

• The fact that a comment is a token delimiter. After removing a comment, the C preprocessor must make sure that a white space character is in its place.
• Line numbers. After removing a comment, the C preprocessor sometimes must insert blank lines in its place to preserve the original line numbering.
• String and character constant boundaries. The preprocessor must not consider the character sequence (/*...*/) to be a comment if it occurs inside a string constant ("...") or character constant ('...').

The Task

Your task is to compose a C program named decomment that performs a subset of the de-comment job of the C preprocessor, as defined below.

Functionality

Your program should be a Unix filter. A filter is a program that reads characters from the standard input stream, and writes characters to the standard output stream and possibly to the standard error stream. Specifically, your program should (1) read text, presumably a C program, from the standard input stream, (2) write that same text to the standard output stream with each comment replaced by a space, and (3) write error and warning messages as appropriate to the standard error stream. A typical execution of your program from the shell might look like this:

decomment < somefile.c > somefilewithoutcomments.c 2> errorandwarningmessages

In the following examples a space character is shown as "_s" and a newline character as "_n".

Your program should replace each comment with a space. Examples:

Your program should define "comment" as in the C90 standard. In particular, your program should consider text of the form (/*...*/) to be a comment. It should not consider text of the form (//...) to be a comment. Example:

Your program should allow a comment to span multiple lines. That is, your program should allow a comment to contain newline characters. Your program should add blank lines as necessary to preserve the original line numbering. Examples:

Your program should not recognize nested comments. Example:

Your program should handle C string constants. In particular, your program should not consider text of the form (/*...*/) that occurs within a string constant ("...") to be a comment. Examples:

Similarly, your program should handle C character constants. In particular, your program should not consider text of the form (/*...*/) that occurs within a character constant ('...') to be a comment. Examples:

Note that the C compiler would consider the first of those examples to be erroneous (multiple characters in a character constant). But many C preprocessors would not, and your program should not.

Your program should handle escaped characters within string constants. That is, when your program reads a backslash (\) while processing a string constant, your program should consider the next character to be an ordinary character that is devoid of any special meaning. In particular, your program should consider text of the form ("...\"...") to be a valid string constant which happens to contain the double quote character. Examples:

Similarly, your program should handle escaped characters within character constants. That is, when your program reads a backslash (\) while processing a character constant, your program should consider the next character to be an ordinary character that is devoid of any special meaning. In particular, your program should consider text of the form ('...\'...') to be a valid character constant which happens to contain the quote character. Examples:

Note that the C compiler would consider both of those examples to be erroneous (multiple characters in a character constant). But many C preprocessors would not, and your program should not.

Your program should handle newline characters in C string constants without generating errors or warnings. Examples:

Note that a C compiler would consider those examples to be erroneous (newline character in a string constant). But many C preprocessors would not, and your program should not.

Similarly, your program should handle newline characters in C character constants without generating errors or warnings. Examples:

Note that a C compiler would consider those examples to be erroneous (multiple characters in a character constant, newline character in a character constant). But many C preprocessors would not, and your program should not.

Your program should handle unterminated string and character constants without generating errors or warnings. Examples:

Note that a C compiler would consider those examples to be erroneous (unterminated string constant, unterminated character constant, multiple characters in a character constant). But many C preprocessors would not, and your program should not.

Your program should detect an unterminated comment. If your program detects end-of-file before a comment is terminated, it should write the message "Error: line X: unterminated comment" to the standard error stream. "X" should be the number of the line on which the unterminated comment begins. Examples:

Your program (more precisely, its main function) should return EXIT_FAILURE if it was unsuccessful, that is, if it detects an unterminated comment and so was unable to remove comments properly. Otherwise it should return EXIT_SUCCESS or, equivalently 0.

Your program should work for standard input lines of any length.

You may assume that the final line of the standard input stream ends with the newline character.

Your program may assume that the backslash-newline character sequence does not occur in the standard input stream. That is, your program may assume that logical lines are identical to physical lines in the standard input stream.

Design

Design your program as a deterministic finite state automaton (DFA, alias FSA). The DFA concept is described in lectures, and in Section 7.3 of the book Introduction to CS (Sedgewick and Wayne). That book section is available through the web at http://www.cs.princeton.edu/introcs/73fsa/.

Generally, a (large) C program should consist of of multiple source code files. For this assignment, you need not split your source code into multiple files. Instead you may place all source code in a single source code file. Subsequent assignments will ask you to write programs consisting of multiple source code files.

We suggest that your program use the standard C getchar function to read characters from the standard input stream.

Logistics

You should create your program on the hats cluster using bash, emacs, and gcc217.

Step 1: Design a DFA

Express your DFA using the traditional "labeled ovals and labeled arrows" notation. More precisely, use the same notation as is used in the examples from Section 7.3 of the Sedgewick and Wayne book. Let each oval represent a state. Give each state a descriptive name. Let each arrow represent a transition from one state to another. Label each arrow with the character, or class of characters, that causes the transition to occur. We encourage (but do not require) you also to label each arrow with action(s) that should occur (e.g. "print the character") when the corresponding transition occurs.

Express as much of the program's logic as you can within your DFA. The more logic you express in your DFA, the better your grade on the DFA will be.

To properly report unterminated comments, your program must contain logic to keep track of the current line number of the standard input stream. You need not show that logic in your DFA.

Step 2: Create Source Code

Use emacs to create source code in a file named decomment.c that implements your DFA.

Step 3: Preprocess, Compile, Assemble, and Link

Use the gcc217 command to preprocess, compile, assemble, and link your program. Perform each step individually, and examine the intermediate results to the extent possible.

Step 4: Execute

Execute your program multiple times on various input files that test all logical paths through your code.

We have provided several files in hats directory /u/cos217/Assignment1:

(1) sampledecomment is an executable version of a correct assignment solution. Your program should write exactly (character for character) the same data to the standard output stream and the standard error stream as sampledecomment does. You should test your program using commands similar to these:

sampledecomment < somefile.c > output1 2> errors1
decomment < somefile.c > output2 2> errors2
diff -c output1 output2
diff -c errors1 errors2
rm output1 errors1 output2 errors2

The Unix diff command finds differences between two given files. The executions of the diff command shown above should produce no output. If the command diff -c output1 output2 produces output, then sampledecomment and your program have written different characters to the standard output stream. Similarly, if the command diff -c errors1 errors2 produces output, then sampledecomment and your program have written different characters to the standard error stream.

(2) Several .txt files (that is, files whose names end with .txt) can serve as input files to your program.

(3) testdecomment and testdecommentdiff are bash scripts that automate the testing process. Comments at the beginning of those files describe how to use them. After copying the scripts to your project directory, you may need to execute the commands chmod 700 testdecomment and chmod 700 testdecommentdiff to give them "executable" permissions.

Copy those files to your project directory, and use them to help you test your program.

You also should test your program against its own source code using a command sequence such as this:

sampledecomment < decomment.c > output1 2> errors1
decomment < decomment.c > output2 2> errors2
diff -c output1 output2
diff -c errors1 errors2
rm output1 errors1 output2 errors2

Step 5: Create a readme File

Use emacs to create a text file named readme (not readme.txt, or README, or Readme, etc.) that contains:

• Your name and the assignment number.
• A description of whatever help (if any) you received from others while doing the assignment, and the names of any individuals with whom you collaborated, as prescribed by the course Policies web page.
• (Optionally) An indication of how much time you spent doing the assignment.
• (Optionally) Your assessment of the assignment: Did it help you to learn? What did it help you to learn? Do you have any suggestions for improvement? Etc.
• (Optionally) Any information that will help us to grade your work in the most favorable light. In particular you should describe all known bugs.

Descriptions of your code should not be in the readme file. Instead they should be integrated into your code as comments.

Your readme file should be a plain text file. Don't create your readme file using Microsoft Word or any other word processor.

Step 6: Submit

Submit your work. Submit your decomment.c file, the files that gcc217 generated from it, and your readme file electronically by issuing this command on hats:

submit 1 decomment.c decomment.i decomment.s decomment.o decomment readme

Also submit your DFA. You can do that using either of these two options:

1. Create hard copy of your "labeled ovals and labeled arrows" DFA and give it to your preceptor during precept before the assignment due date. A DFA drawn using drawing software (e.g. Microsoft PowerPoint) would be best. But it is sufficient to submit a neatly hand-drawn DFA.
2. Create a soft copy textual representation of your DFA, and submit it with your other files. For example, the last DFA given in Section 7.3 of the Sedgewick and Wayne book could be expressed as this Textual DFA. Use that textual notation.

If you use option 2, then name the text file dfa (not dfa.txt, DFA, etc.) and submit it by issuing this command on hats:

submit 1 dfa

Grading

We will grade your work on two kinds of quality: quality from the user's point of view, and quality from the programmer's point of view. To encourage good coding practices, we will deduct points if gcc217 generates warning messages.

From the user's point of view, a program has quality if it behaves as it should. The correct behavior of your program is defined by the previous sections of this assignment specification, and by the behavior of the given sampledecomment program.

From the programmer's point of view, a program has quality if it is well styled and thereby easy to maintain. In part, style is defined by the rules given in The Practice of Programming (Kernighan and Pike), as summarized by the Rules of Programming Style document. For this assignment we will pay particular attention to rules 1-24. These additional rules apply:

• Names: You should use a clear and consistent style for variable and function names. One example of such a style is to prefix each variable name with characters that indicate its type. For example, the prefix c might indicate that the variable is of type char, i might indicate int, pc might mean char*, ui might mean unsigned int, etc. But it is fine to use another style -- a style that does not include the type of a variable in its name -- as long as the result is a clear and readable program.
• Comments: Each source code file should begin with a comment that includes your name, the number of the assignment, and the name of the file.
• Comments: Each function -- especially the main function -- should begin with a comment that describes what the function does from the point of view of the caller. (The comment should not describe how the function works.) It should do so by explicitly referring to the function's parameters and return value. The comment also should state what, if anything, the function reads from the standard input stream or any other stream, and what, if anything, the function writes to the standard output stream, the standard error stream, or any other stream. Finally, the function's comment should state which global variables the function uses or affects. In short, a function's comments should describe the flow of data into and out of the function.
• Function modularity: Your program should not consist of one large main function. Instead your program should consist of multiple small functions, each of which performs a single well-defined task. For example, you might create one function to implement each state of your DFA.
• Line lengths: Limit line lengths in your source code to 72 characters. Doing so allows us to print your work in two columns, thus saving paper.

Special Note

As prescribed by Kernighan and Pike style rule 25, generally you should avoid using global variables. Instead all communication of data into and out of a function should occur via the function's parameters and its return value. You should use ordinary call-by-value parameters to communicate data from a calling function to your function. You should use your function's return value to communicate data from your function back to its calling function. You should use call-by-reference parameters to communicate additional data from your function back to its calling function, or as bi-directional channels of communication.

However, call-by-reference involves using pointer variables, which we have not discussed yet. So for this assignment you may use global variables instead of call-by-reference parameters. (But we encourage you to use call-by-reference parameters.)

In short, you should use ordinary call-by-value function parameters and function return values in your program as appropriate. But you need not use call-by-reference parameters; instead you may use global variables. In subsequent assignments you should use global variables only when there is a good reason to do so.

This assignment was written by Robert M. Dondero, Jr. with input from many other faculty members