Held: Thursday, 23 January 2014

On to Outline 02 - Some Basic Tasks and Corresponding Tools.

Summary

We begin the course with an overview of some important principles and a few basic exploratory exercises.

Related Pages

• GitHub repo: https://github.com/Grinnell-CSC282/hw1
• EBoard 1 (Source 1) (HTML 1) (PDF 1)

Overview

• About the course.
• Some principles and practices.
• Example: C from K and R.
• Detour: A C memory problem.
• Exercises: Some simple tasks.
• Some important *nix tools.
• Looking ahead.

Administrivia

• Welcome to "Thinking in C and *nix" (or whatever the course is called in the catalog).
• I'll take attendance, mostly to help me make sure that I know everyone's name. (I have 120 students this semester, so knowing everyone is hard.)

About the course

Motivation

This course is an extension of a seminar I ran in spring 2012 and that students asked me to repeat in spring 2013. That seminar was an outgrowth of things I observed and was told in CSC 362 2011F. In particular, the students in that course had much less familiarity with Makefiles than I would have liked and they told me that they generally did not use gdb when debugging their programs. I also observed that many of them were less than competent at dealing with memory leaks in their programs.

I do not blame my students for these flaws in their background. Rather, I expect that it's a consequnce of our decision to teach these issues relatively early in their careers and of different faculty opinions on what is important to teach. (I'm not sure that students who took 161 from me have substantially different backgrounds from those who took it from someone else.)

A few of the students asked me to spend some additional time teaching the things that I consider important (and that they quickly found were important). But we didn't have class sessions to spare. This class is therefore my attempt to address some of the weaknesses that I or the students observed. I hope to offer it every year.

Policies and procedures

• You can find most of the information on this course on the Web.
• I'll admit that many of the "standard" documents (e.g., my notes on disabilities) are simply copied and pasted from previous years. Let me know if anything seems out of place.
• I will focus on keeping the outlines, Eboards, book, and schedule up to date,
• This is an S/D/F course. Show up, be active, do the homework and you pass.
  Miss more than two classes or two assignments and you fail.
• We'll take a quick look at the current version of the schedule so that you can see how I plan to approach the course.

Principles and practices

• From my perspective, this course is about three related issues:
  • Generally good programming practices
  • How to think like a *nix programmer
  • How to think like a C programmer
• We'll look at a few important issues in each

Important programming practices

Since I'm teaching this course, I get to impose my favorite practices on you. (I'll note that most of our students who leave Grinnell and go on to careers in programming find that I'm right about most of these things.)

• Document. Odds are that you will need to reuse some of your stuff later, and you will not remember what you did before.
• Test (preferably automatically). If you plan to have others use your program or library, you should make sure that it works correctly.
• Refactor. If you find yourself programming by copy-paste-change, it's probably time to write a more general function.
• Encapsulate. Your libraries should provide a simple interface, and should stick to that interface. For example, I think it's bad practice to have a library directly issue error messages.
• Format. Your code should be clean and readable. When possible, follow some language formatting standard. (I tend to use the GNU standards in C.)

You can take all of these with a grain of salt (or much more salt) when you're just writing quick-and-dirty code to get stuff done. Keep in mind that your typical goal is efficiency. You don't need to comprehensively test a few-line throwaway program. However, you should document the program, because you may want to do a similar thing later.

Some key *nix principles

Unix and its descendents have been around a long time. They must be doing something right. So, what are the principles that have made nix so successful? Here are a few particularly important principles that many C/nix programmers follow. (We will expand upon these as the seminar continues.)

• Don't do things by hand when you can more efficiently do them with a program.
• Write small programs and libraries that have a very focused purpose.
  • And save them for later.
• Chain together small programs to solve more complex problems.
• Communicate with text files.
• Share!

That first principle may seem a bit daunting. Do you always write programs to do tasks? No, you can certainly use programs. Here are ways you better do things "by program".

• Know the primary tools that Linux provides. (We'll make a list.)
  • It's okay to know generally what they do and to read the documentation when you need more info.
• Be able to write your own small tools in some scripting language. (I tend to script in Perl. Python, Bash, and Ruby can all be useful scripting languages, too.)
• Be able to write your own small tools in C. (Sometimes you need speed.)
• Know enough *nix bash syntax to tie things together.

Some important C practices

In addition to the general programming practices I've mentioned above, there are a few approaches that C programmers should master. (We'll add more as the semester goes on.)

• Know how to use the C preprocessor, particularly #include files and macros.
• Know how to use a debugger.
• Know how to use libraries.
• Understand memory: How pointers are used, how malloc works, etc. It's also useful to trace your programs' memory usage (e.g., using valgrind)

There are also things one should learn about over the longer term, including threads, signals, and some common libraries (such as libraries for processing the command line). Due to limited time, we probably won't cover those in this course.

Thinking in C: A Simple Example

The following code comes from K&R (although it is reformatted). What does it do?

char *
fun (char *t, char *s)
{
  while (*t = *s);
  return t;
} // fun

Detour: A C Memory Problem

Activity/Discussion

To illustrate my point that understanding memory in C is important, let's continue with a problem that a friend gave to me a while ago. He showed me the following fragment of C code.

  x = malloc (...);
  foo ();
  bar ();
  free (x);

The program was crashing on the call to free.

Here are some things he discovered. * If he removed the call to free, the program ran through to completion.
* If he moved the call to free before the call to bar, the program ran through to completion.
* He had no calls to free in bar.

What is likely to be wrong with his code?

Exercises: Some simple tasks

Activity

Here are some tasks that I find it useful to think about. They illustrate some kinds of tasks that people often find they have to do and give you the opportunity to explore different approaches. For each problem, we'll make a list of ways to approach the problem (e.g., "Write a C program") and estimate the time each would take.

• Given a DOS-formatted text file (lines end with \r\n rather than just \n), convert it to a standard text file.
• Given a standard text file, convert all uppercase letters to lowercase.
• Given a standard text file, remove all blank spaces at the end of lines.
• Make a list of all misspelled words in a text file.
• Given a CSV file in which each line has the form
  LastName,FirstName,Assignment,NumericGrade
  find the the five highest grades on homework 2.
• Given an HTML file, find the URLs of all images. In case you don't know HTML, those will typically look like
  <img ... src="*URL*" ...>
  • The img can have any capitalization (img, IMG, Img, iMg, etc.)
  • There can be other text between the img and the src. (That text cannot include a greater than sign.)
  • You may find it easier to start this problem by assuming that there's only one image tag on a line.

Homework

This will appear online some time in the near future (probably over the weekend).

Repository: https://github.com/Grinnell-CSC282/hw1

1. Read chapter 1 of Raymond to get a deeper understanding of the *nix Philosophy.

2. Find good solutions to any of the tasks we did not solve in class. (That is, write a program or command to solve the task.) Put them in a folder with your name in the GitHub repository.

3. Make a list of *nix tools and utilities that you've found useful (or even that you've just heard of). Try to categorize and add a short description. Put them in the tools.md file in the repository.