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======Exercise 5: The Structure Of A C Program======

   You know how to use printf and have a couple basic tools at your
   disposal, so let's break down a simple C program line-by-line so you
   know how one is structured. In this program you're going to type in a
   few more things that you're unfamiliar with, and I'm going to lightly
   break them down. Then in the next few exercises we're going to work
   with these concepts.
#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;
}

   Type this code in, make it run, and make sure you get no Valgrind
   errors. You probably won't but get in the habit of checking it.

======What You Should See======

   This has pretty boring output, but the point of this exercise is to
   analyze the code:
$ make ex5
cc -Wall -g    ex5.c   -o ex5
$ ./ex5
======You are 100 miles away.======
$

======Breaking It Down======

   There's a few features of the C language in this code that you might
   have only slightly figured out while you were typing code. Let's break
   this down line-by-line quickly, and then we can do exercises to
   understand each part better:

   ex5.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 then contain lists of functions you
          want to use in your program.

   ex5.c:3
          This is a multi-line comment and you could put as many lines of
          text between the /* and closing */ characters as you want.

   ex5.c:4
          A more complex version of the main function you've been using
          blindly so far. How C programs work is the operating system
          loads your program, and then 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? Do not worry, we'll cover this soon.

   ex5.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.

   ex5.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.

   ex5.c:8
          Another kind of comment, and it works like Python or Ruby
          comments where it starts at the // and goes until the end of the
          line.

   ex5.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. The printf function is
          actually kind of weird and can take multiple arguments. We'll
          see that later.

   ex5.c:11
          A return from the main function, which gives the OS your exit
          value. You may not be familiar with how Unix software uses
          return codes, so we'll cover that as well.

   ex5.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 break-down, so study it
   line-by-line and make sure you at least have a little grasp of what's
   going on. You won't know everything, but you can probably guess before
   we continue.

======Extra Credit======

     * 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 that you can use to check later and see if you get it
       right.
     * Go back to the source code from the previous exercises and do a
       similar break-down to see if you're getting it. Write down what you
       don't know and can't explain to yourself.


   Copyright (C) 2010 Zed. A. Shaw

   Credits