Overview

• Preliminaries.
  • Admin.
• About the course.
• Some principles and practices.
• Example: C from K and R.
• Detour: A C memory problem.
• Exercises: Some simple tasks.
• Homework.

Preliminaries

Admin

• Welcome to "Thinking in C and *nix" (or whatever the course is called in the catalog).
  • In the future, it will be CSC 282, Thinking in C and *nix, so I think of it that way.
• 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.)
• Feel free to interrupt me at any time with questions and comments.
• I tend to overplan, so we won't do everything in the outline.

About the course

• Your path to competency in C and *nix.
• 14 classes, Sam here for 12.
• Weekly homework.
• Mixture of lab, discussion, recitation, lecture.
• On the Web
• "Textbook" under development.

Some principles and practices

Three primary approaches

• Good programming practices
• Ways of thinking like a C programmer
• Ways of thinking like a *nix programmer

Good programming practices

• Good programmers document. For clients. For themselves.
• Good programmers follow a formatting standard. We'll use GNU C.
• Good programmers debug with a debugger, not with printf statements (although the occasional printf is useful)
• Good programmers use a source code management system (and use it well)
  • Don't pollute the repo.
  • Don't commit non-working code to the main branch.
• Good programmers test their code (usually with unit tests).
• Good programmers refactor rather than cut/paste/change.

Good C programmers

• Know how to write macros, and how to use them in clever ways
  • And other aspects of the C preprocessor
• Know how to use a debugger, probably gdb. $ gdb program

  run CRASH bt

• Know how to compile with libraries
  • And to use compiler flags
• Understand memory and pointers
• Segment large programs into individual files
• Use Make

Thinking like a *nix programmer

• Do things by code rather than by hand
• Write small programs that they can tie together with other programs.
• Use text files for data and communication.
• Share

Example: C from K and R

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

This function copies s to to. It's mostly strcpy. It returns a pointer to the one after the end-of-string character in the result.

Detour: A C memory problem

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

• Program crashes with the call to free.
• Program does not crash without the call to free (but leaks memory)
• foo and bar use x, but have no calls to free
• Neither foo nor bar reassigns x, although they do include x[i] = ....
• What might be going wrong?

Exercises: Some simple tasks

Given a DOS-formatted text file (lines end with \r\n rather than just \n), convert it to a standard text file.

If you know awk, you could write an awk script to do it. (less than a minute, if you know awk and use it regularly)

If you know flex, you can write a flex program. (five minutes)

If you program regularly in C (which we'll assume, write a C program).
(five to ten minutes).

int ch;
while ((ch = getch()) != EOF)
  {
    if (ch != '\r')
      putchar (ch);
  } // while

Two more tools:

• tr - character based translation (fifteen seconds) tr -d '\r' < dosfile > usablefile
• sed - stream editor (fifteen seconds)

Given a standard text file, convert all uppercase letters to lowercase.

• Find the Python command to do that (use a scripting language).
• Write a C program. (five minutes)
• Copy into LibreOffice, highlight, find appropriate menu item, copy it back, deal with the automatic translation of " to curly quotes, ... (if you're running LibreOffice and it doesn't do extra translation, less than a minute; if you're not running LibreOffice, +2 minutes)

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 HW2

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.
* But, in the end, you should support multiple image tags on the same line.

Detour: Using the Shell

• !! repeat the last command
• !text repeat the last command that starts with the given text
• !123 repeats command 123
• !$ the last thing on the last line

Some important *nix tools

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.

4. Add some notes about what might be wrong with the C code above.