🏗️ 2. Structure of a C Program

A C program contains parts such as declarations, functions, and statements. Each part has a job, and the C language has rules for how these parts fit together.

There is no single mandatory six-section template that every C program must follow. Textbooks often teach a traditional structure such as documentation, header files, global declarations, main() and user-defined functions because it is useful for beginners, but real C programs can be organized in different ways.

A beginner-friendly C source file can look like this:

/* Documentation / comment */

#include <stdio.h>

#define MAX_VALUE 100

int globalValue = 10;

void displayMessage(void);

int main(void)
{
    int number = 20;

    printf("Number = %d\n", number);
    displayMessage();

    return 0;
}

void displayMessage(void)
{
    printf("Hello from another function!\n");
}

C PROGRAM
│
├── Comments
│
├── Preprocessor directives
│      ├── #include
│      └── #define
│
├── Global declarations / definitions
│
├── Function declarations
│
├── main()
│      ├── local declarations
│      ├── statements
│      └── return
│
└── Other function definitions

Comments explain the code to readers. The compiler does not turn the words inside a comment into instructions to execute.

#include <stdio.h>

int main(void)
{
    // Display a message
    printf("Hello, C!\n");

    /*
       The program ends
       after printing the message.
    */

    return 0;
}

// printf("Hello");

Preprocessor directives begin with #. They are processed before the compiler translates the resulting C source.

#include <stdio.h>

#define PI 3.14159

int main(void)
{
    printf("PI = %.2f\n", PI);

    return 0;
}

A header file commonly supplies function declarations, macros, and types that a source file needs. A declaration tells the compiler what a name refers to.

#include <stdio.h>

int main(void)
{
    printf("Hello\n");

    return 0;
}

In a hosted C environment, the program entry function is main. A common standard form for a program that does not use command-line arguments is:

int main(void)
{
    return 0;
}

The braces { and } define a compound statement. For a function definition, they surround the function body.

int main(void)
{
    printf("Hello\n");

    return 0;
}

int main(void)
      │
      ↓
┌───────────────────────┐
│       function body   │
│                       │
│   statements           │
│   declarations         │
│   expressions          │
│                       │
└───────────────────────┘

The statements inside the braces belong to that function body.

A declaration tells the compiler about an identifier and its type or other properties. For example:

int age;
double salary;
char grade;

A statement describes an action or controls which action happens next. Assignment, if, loops, and return are examples you will study in this course.

int x = 10;

x = x + 5;

printf("%d\n", x);

An expression describes a value or calculation. Its operands are the values it works with, and its operators specify actions such as addition or multiplication.

int a = 10;
int b = 20;

int sum = a + b;

Here:

• a and b are operands.
• + is an operator.
• a + b is an expression.
• int sum = a + b; is a declaration with initialization.

A function call runs a function using the arguments given in the call. Arguments supply values the function needs.

printf("Hello\n");

The structure can be understood as:

Later, when you learn functions, you will see how to create your own functions and call them from main().

The statement:

return 0;

returns the value 0 from main. In a hosted environment, returning from main ends the program and reports a status to the environment that ran it. Zero reports success. It does not add a zero to the displayed output.

main()
  │
  │ performs program work
  │
  ↓
return 0;
  │
  ↓
function ends
  │
  ↓
successful termination status

Avoid placing every task in a large program inside main(). Divide the work into functions with clear tasks.

#include <stdio.h>

void greet(void);

int main(void)
{
    greet();

    return 0;
}

void greet(void)
{
    printf("Welcome to CodeBhavya!\n");
}

/* Program description */

#include <stdio.h>

#define MESSAGE "Hello, CodeBhavya!"

int total = 100;

void displayMessage(void);

int main(void)
{
    int number = 10;

    printf("%s\n", MESSAGE);
    printf("Number = %d\n", number);

    displayMessage();

    return 0;
}

void displayMessage(void)
{
    printf("Learning C step by step!\n");
}

You will use this distinction when working with functions, global variables, and programs split across source files.

Consider this program:

#include <stdio.h>

void greet(void);

int main(void)
{
    greet();

    return 0;
}

void greet(void)
{
    printf("Hello!\n");
}

The function definition appears after main(), but the compiler already knows about greet() because of its declaration:

void greet(void);

The same function can be defined before main():

#include <stdio.h>

void greet(void)
{
    printf("Hello!\n");
}

int main(void)
{
    greet();

    return 0;
}

Separate the order in which code is written from the order in which its statements run.

Source code
    │
    ↓
Preprocessing
    │
    ↓
Compilation
    │
    ↓
Object / executable generation
    │
    ↓
Operating system loads program
    │
    ↓
main()
    │
    ├── declarations
    │
    ├── statements
    │
    ├── function calls
    │
    └── return
    │
    ↓
Program termination

The semicolon ; is used to terminate many C statements.

int x = 10;

x = x + 5;

printf("%d\n", x);

return 0;

void greet(void)
{
    printf("Hello");
}

Spaces, tabs and newlines are generally used to make source code readable. C does not require a particular indentation style.

int main(void)
{
    int a = 10;
    int b = 20;

    printf("%d\n", a + b);

    return 0;
}

The indentation does not change the meaning of this simple program. It improves readability for humans.

C distinguishes between uppercase and lowercase letters.

int age = 20;

printf("%d", age);

These identifiers are different:

age
Age
AGE
aGe

Printf("Hello");

printf("Hello");

#include <stdio.h>

int add(int a, int b);

int main(void)
{
    int x;
    int y;
    int result;

    printf("Enter two numbers: ");
    scanf("%d %d", &x, &y);

    result = add(x, y);

    printf("Sum = %d\n", result);

    return 0;
}

int add(int a, int b)
{
    return a + b;
}