Revision as of 00:48, 27 December 2013

Before moving on, it's important to understand the basic structure of a C program.

Reserved Words

C programs are made of a series of grouped characters and symbols known as tokens. Reserved words are a set of words that have special meaning within your C program. They make up the basic functionality of your program. Reserved words may only be used within their appropriate context - they may be not be used as variable names, or any other type of identifier. We will expand on that later.

Hello World Example

Let's look at the famous Hello World program. It is very important that you are able to create the program text, compile it, run it, and find the output generated by program.

#include <stdio.h>

int main()
{
    puts("Hello, World!");
    return 0;
}

The exact way on how you build and run the program depends on the system and the utilities you are using. For example, if you are using Visual Studio on Windows, all it takes is clicking on the green play button or the Ctrl+F5 shortcut key. If you are using a UNIX or Linux system, you should be able to compile the program using the following command:

gcc hello.c

If everything went alright, an executable output file will be generated. Traditionally, the executable has been named 'a.out'. If you received an error during the compilation process double check your code for omitted characters or misspelling. You should be able to run the file by typing:

./a.out

Other systems might have different rules on how you should compile and run the program. Consult the documentation or an expert familiar with your environment.

Regardless of the compiler and operating system used, the output should be identical for everyone:

Hello, World!

Program Explanation

Regardless of the size and complexity of a C program. C programs consists of functions and variables. A function contains statements that specify the operation to be performed. Variables are a way of naming and storing values for later use by the program.

In our example program, main is a function we defined. Normally you would be allowed to name your functions whatever you like, but "main" happens to be a special function. All C programs begin executing at the beginning of function main. As a corollary, this also means that every program must have a main somewhere.

The first line of the program,

#include <stdio.h>

tells the compiler to include information about the standard I/O library. That line is part of the pre-processor.

One of the ways functions can communicate with one another is for the calling function to provide a list of values to the function it calls. These values are called arguments. The parenthesis after the function name are used to surround the argument list. In our example, "()" is used to indicate an empty list of arguments. A function can return a value back to it's caller. The type of value the function returns is specified in front of the function name. In our example we return an int - this is a requirement when you define function main.

The statements of a function are enclose in a pair of curly brackets { }. The function main contains two statements:

puts("Hello, World!");
return 0;

Generally speaking, every statement in a C program should end with a semicolon.

A function in C is called by simply naming it and adding a list of arguments (if any) in parentheses after it. In our example program,

puts("Hello, World!");

calls the function puts with the argument "Hello, World!". puts is a standard library function that prints output - in this case a string of characters between the double quotes.

In C, a sequence of characters surrounded by double quotes like "Hello, World!" is called a string constant. For the time being their only use is to be printed out by the puts function or as argument to other functions.

The last statement of main,

return 0;

is the return statement. The return statement terminates the execution of a function and returns control to the calling function. Because main is the first function to be executed, the control returns to the host environment. A value of 0 indicates a successful termination of the program.

Formatting Style

The indentations we used in our code is meant to give us easier time reading and grasping the meaning of the code. Characters space, horizontal tab, new-line, vertical tab, and form-feed are collectively called white space. Because the compiler works on a series of tokens and ignores all extra white space, you have a considerable amount of freedom concerning how you format your program. C is a [[[free format]] language - the positioning of characters in the source file is insignificant. There are no rules for where you should place brackets or how you should indent your code. As you will see, this is both a blessing and a curse.

In fact, the following rewrite of the example program we have above is perfectly legal C:

#include <stdio.h>
int main(){puts("Hello, World!");return 0;}

As well as:

#include <stdio.h>
int
main
(
)
{
puts
(
"Hello, World!"
)
;
return
0
;
}

You should be able to compile and run both programs. At this point in time you should not be too worried about the formatting of your program. You will develop a style of your own as you continue to learn the language.

With these two examples in mind we will note that there are many different coding styles in existence. Everyone has an opinion about which is best. Regardless of which one you end up going with, it is extremely important that you keep it consistent.

Comments

Main article: Comments

Even though our hello world example works, it lacks a very important thing: documentation. Every program should be documented enough so that the future you, or more importantly, other programmers know some general things about it like the author, what it does, and perhaps the license associated with it. In C, such information is placed in comments. The characters /* introduce a comment and the characters */ terminate it. For example:

/* This is a comment. */

/*
and
so 
is
this
*/

Comments do not change the meaning of the program - they are simply for the programmer's information. Comments are treated as if they were a single space. They may appear almost anywhere in your code. Comments may not appear within character constant, string literals, or other comments. Therefore, comments also do not nest.

Here is out hello world example with some comments:

/*
    Name:        hello.c
    Description: Prints "Hello, World!"
    Author:      John Doe
*/
#include <stdio.h>

int main()
{
    puts("Hello, World!");
    return 0;
}

Single-line Comments

C99 introduced a second style of comments which begins with // and terminates automatically at the new-line character.

Our Hello Example program could be written as:

// Name:        hello.c
// Description: Prints "Hello, World!"
// Author:      David Schor
#include <stdio.h>

int main()
{
    puts("Hello, World!");
    return 0;
}