The structure of a C program follows a specific layout that consists of several key components such as headers, the main function, and optional user-defined functions. Here's a breakdown of the basic structure:

1. Preprocessor Directives (Headers)

Preprocessor directives are lines that begin with a # symbol and provide instructions to the compiler before the actual compilation starts. They include necessary libraries and macros for your program.

example

#include <stdio.h>  // Standard input/output functions like printf, scanf
#include <stdlib.h> // Standard library functions like memory allocation
#include <math.h>   // Math functions like sqrt, sin

2. Global Declarations

Global variables and function prototypes are declared here. These are optional but can be useful if certain variables or functions need to be accessed throughout the program.

int globalVar = 5;  // Global variable declaration
void displayMessage();  // Function prototype

3. Main Function (int main())

Every C program must have a main() function. The program starts execution from this function. The main() function can return an integer value, typically 0 or 1, to indicate the success or failure of the program.

Basic Structure of main():

int main() {
    // Declaration of local variables
    int localVar = 10;

    // Statements (function calls, loops, conditionals, etc.)
    printf("Hello, World!\n");

    // Return statement (usually 0 for successful execution)
    return 0;
}

4. User-Defined Functions

These are additional functions created by the programmer to break the program into modular parts. Functions are defined outside the main() function and can be called from main() or other functions.

void displayMessage() {
    printf("This is a user-defined function!\n");
}

5. Comments

Comments are optional but crucial for documenting the code. They help explain the purpose of code blocks and can be single-line or multi-line.

Complete Example of a Basic C Program:

#include <stdio.h>  // Include standard I/O library

// Function prototype
void displayMessage();

// Main function where execution starts
int main() {
    int number = 10;  // Local variable

    // Call to user-defined function
    displayMessage();

    // Print the value of number
    printf("The value of number is: %d\n", number);
    return 0;
}

// Definition of the user-defined function
void displayMessage() {
    printf("Hello from the user-defined function!\n");
}

Breakdown of Key Concepts:

This is the basic structure of any C program, with the ability to scale by adding more user-defined functions, external libraries, and complex logic.