.. _first_prog:

A simple C program
==================

General form of a C program
---------------------------

    ::

        Preprocessor directives

        int main(void)        
        {                    
            declarations    
            statements     
        }                 

We will see examples of each of type of statement shortly.

Hello World Example
-------------------

    ::

        /*
         * The classic 'Hello World' first program.
        */
        #include <stdio.h>
        
        int main(void)
        {
           printf("Hello, World!\n");
           return 0;
        }


Analysis of Hello World
-----------------------

    Comments

        The `/*  */` pair defines a comment block.

    `#include <stdio.h>` 
    
        Definitions and prototypes are pulled from a system header file.
        `#include "my.h"` is for user defined header files. In Unix, system
        header files are in /usr/include with the libraries in /usr/lib or
        /lib. User defined headers should be in the same directory as the
        source code.

    `int main(void)`

      * `main()` is where the program begins. It is in every program. Notice
        that `main()` is a function like other functions.


      * `int` means that it will return an integer value to the operating
        system. Generally `main()` will either be of type int or void.


      * `(void)` means that no arguments are passed to `main()`.


    `{ }`

        Indicates the start and end of each function or block of code.
        We define a block of code as a set of statements that
        are to be executed sequentially as a group.

    `printf( );`

        A function to print to the screen. C was one of the first languages to
        use a function for printing. The argument is a formated character
        string. `\\n` means to start a new line.

    `return 0;`

        Return to the OS with an `int` value. 0 = success, 1 = failure.