Exercise 1: Dust Off That Compiler

2024 Note: I mention a "PDF" in the video but that's not available anymore. Also, your error output might be different as compilers may have improved their error detection.

After you have everything installed, you need to confirm that your compiler works. The easiest way to do that is to write a C program. Since you should already know at least one programming language, I believe you can start with a small but extensive example.

#include <stdio.h>

/* This is a comment. */
int main(int argc, char *argv[])
{
    int distance = 100;

    // this is also a comment
    printf("You are %d miles away.\n", distance);

    return 0;
}

If you have problems getting the code up and running, watch the video for this exercise to see me do it first.

Breaking It Down

There are a few features of the C language in this code that you might or may not have figured out while you were typing it. I'll break this down line by line quickly, and then we can do exercises to understand each part better. Don't worry if you don't understand everything in this breakdown. I am simply giving you a quick dive into C and promise you will learn all of these concepts later in the book.

Here's a line-by-line description of the code:

• ex1.c:1: An include, and it is the way to import the contents of one file into this source file. C has a convention of using .h extensions for header files, which contain lists of functions to use in your program.
• ex1.c:3: This is a multi line comment, and you could put as many lines of text between the opening /* and closing */ characters as you want.
• ex1.c:4: A more complex version of the main function you've been using so far. How C programs work is that the operating system loads your program, and then it runs the function named main. For the function to be totally complete it needs to return an int and take two parameters: an int for the argument count and an array of char * strings for the arguments. Did that just fly over your head? Don't worry, we'll cover this soon.
• ex1.c:5: To start the body of any function, you write a { character that indicates the beginning of a block. In Python, you just did a : and indented. In other languages, you might have a begin or do word to start.
• ex1.c:6: A variable declaration and assignment at the same time. This is how you create a variable, with the syntax type name = value;. In C, statements (except for logic) end in a ';' (semicolon) character.
• ex1.c:8: Another kind of comment. It works like in Python or Ruby where the comment starts at the // and goes until the end of the line.
• ex1.c:9: A call to your old friend printf. Like in many languages, function calls work with the syntax name(arg1, arg2); and can have no arguments or any number of them. The printf function is actually kind of weird in that it can take multiple arguments. You'll see that later.
• ex1.c:11: A return from the main function that gives the operating system (OS) your exit value. You may not be familiar with how UNIX software uses return codes, so we'll cover that as well.
• ex1.c:12: Finally, we end the main function with a closing brace } character, and that's the end of the program.

There's a lot of information in this breakdown, so study it line by line and make sure you at least have a grasp of what's going on. You won't know everything, but you can probably guess before we continue.

What You Should See

You can put this into an ex1.c and then run the commands shown here in this sample shell output. If you're not sure how this works, watch the video that goes with this exercise to see me do it.

$ make ex1
cc -Wall -g    ex1.c   -o ex1
$ ./ex1
You are 100 miles away.
$

The first command make is a tool that knows how to build C programs (and many others). When you run it and give it ex1 you are telling make to look for the ex1.c file, run the compiler to build it, and leave the results in a file named ex1. This ex1 file is an executable that you can run with ./ex1, which outputs your results.

How to Break It

In this book, I'm going to have a small section for each program teaching you how to break the program if it's possible. I'll have you do odd things to the programs, run them in weird ways, or change code so that you can see crashes and compiler errors.

For this program, simply try removing things at random and still get it to compile. Just make a guess at what you can remove, recompile it, and then see what you get for an error.

Extra Credit

• Open the ex1 file in your text editor and change or delete random parts. Try running it and see what happens.

• Print out five more lines of text or something more complex than "hello world."

• Run man 3 printf and read about this function and many others.

• For each line, write out the symbols you don't understand and see if you can guess what they mean. Write a little chart on paper with your guess so you can check it later to see if you got it right.