C Hello World & Program Structure

In this lesson, we’ll start with the quintessential “Hello World” program in C and explore the basic structure of a C program.

“Hello World” Program

The “Hello World” program is often the first program you write when learning a new programming language. It’s a simple program that displays the text “Hello, World!” on the screen. Here’s the code for a “Hello World” program in C:

#include <stdio.h>

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

Let’s break down the components of this program:

1. #include <stdio.h>: This line includes the standard input-output library (stdio.h) in your program. This library provides functions like printf and scanf for input and output operations.
2. int main() { ... }: This is the main function of your C program. Every C program must have a main function, which serves as the entry point for the program’s execution.
3. printf("Hello, World!n");: This line uses the printf function to print the text “Hello, World!” to the standard output (usually the console). The n is an escape sequence that represents a newline character, so the program’s output will appear on a new line.
4. return 0;: This line indicates that the program has terminated successfully. The 0 is returned to the operating system to indicate success, while a non-zero value is typically used to indicate an error.

Program Structure in C

A C program typically consists of various elements that work together to perform a specific task. Here’s an overview of the basic structure of a C program:

// Preprocessor Directives
#include <stdio.h>
// Other #include directives as needed

// Function Prototypes (optional)
int add(int a, int b);

// Global Variables (optional)

// Main Function
int main() {
    // Local Variables
    int x, y, result;

    // Statements
    // ...
    
    // Function Calls
    result = add(x, y);

    // Output
    printf("Result: %dn", result);

    // Program Termination
    return 0;
}

// User-Defined Functions (optional)
int add(int a, int b) {
    return a + b;
}

Here’s an explanation of each section:

• Preprocessor Directives: These lines start with #, and they are used to include libraries (e.g., #include <stdio.h>) or define macros.
• Function Prototypes (optional): These declare the function’s signature before the main function. This allows you to call functions before defining them.
• Global Variables (optional): These are variables declared outside of any function and can be accessed from any part of the program. Use them sparingly, as they can lead to unexpected behavior.
• Main Function: The main function is the entry point of your program. It contains local variables, statements, function calls, and the program’s output.
• User-Defined Functions (optional): You can define your own functions to break down your code into manageable pieces, promoting code reusability and readability.

Running Your C Program

To run your C program, follow these steps:

• Write your C code in a text editor and save it with a .c extension (e.g., hello.c).
• Open a terminal or command prompt and navigate to the directory where your C file is located.
• Compile the program using a C compiler like GCC (GNU Compiler Collection). For example:

gcc hello.c -o hello

• This command compiles hello.c into an executable named hello.
• Run the program by typing:

./hello

You should see the “Hello, World!” message printed on the screen.