The way a program is written and its makeup is usually referred to as program structure. Understanding the general flow a C code is crucial to being able to read and understand any piece of C code.

Reserved Words

Main article: Reserved words

C programs are made up 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. The "Hello, World!" program is very important, in that you are able to create the program text, compile it, run it, and find the output generated by the program. Below is the source code for the "Hello, World!" 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 using the Ctrl+F5 shortcut. 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 goes as planned, an executable output file will be generated. Traditionally, the executable has been named 'a.out'. If you receive 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 operating systems might have different rules on how you should compile and run the program. Consult your operating system's documentation, or an expert familiar with your system's environment, for more detailed information.

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

Hello, World!

Program Explanation

C programs consists of functions and variables.

• functions: Contain statements that specify the operation to be performed.
• Variables: A way of naming and storing values for later use by the program.

In our "Hello, World!" example program, main is a function that 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() function somewhere.

The first line of our sample program,

#include <stdio.h>

tells the compiler to include information about the standard I/O (Input/Output) library; that line is part of the preprocessor.

One way that functions communicate with one another is calling a function in order to provide a list of values to the function being called; these values are called arguments. The parenthesis after a function name are used to surround the argument list. In our "Hello, World!" example, "()" is used to indicate an empty list of arguments. A function has the ability to return a value back to its caller. The type of value a 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 enclosed within 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 the parentheses directly after the name. 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, the only use of strings is that they 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, which we used in our code, are meant to provide us a much more pleasurable visual layout for reading, thus allowing for a better grasp on the meaning of the code. Character spaces, horizontal tabs, new-lines, vertical tabs, and form-feeding 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 in C:

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

Another legal, but ugly, example is:

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

You should be able to compile and run both of the above 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.

Keeping 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 coding style you end up relying on, it is extremely important that you keep it consistent.

Comments

Main article: Comments

Even though our "Hello, World!" example works, it lacks one very important thing: documentation. Every program should be documented enough so that in the future you, or, more importantly, other programmers know some general things about, such as: the author, what the program does, and, perhaps, the license associated with the program. 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 a program - they are simply for the programmer's information. Comments are treated as if they were a single space, and they may appear almost anywhere in your code. Comments may not appear within character constants, string literals, or other comments. Therefore, comments can not nest.

Here is our "Hello, World!" example, with some added 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 begin with //, and terminate automatically at the new-line character. These comments are referred to as Single-line Comments.

Our "Hello, World!" example program could be written as:

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

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

Note how we have replaced the previous comment blocks, /* */, with the single-line comments.