Functions in C — Basics

A function is a self-contained block of code that performs a specific task. Functions make programs modular, reusable, and easier to debug.

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Types of Functions

1. Built-in (Library) Functions — provided by C standard library (e.g., printf, scanf, sqrt)
2. User-defined Functions — written by the programmer

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Function Components

/* Function Declaration (Prototype) */
return_type function_name(parameter_list);

/* Function Definition */
return_type function_name(parameter_list) {
    /* body */
    return value;
}

/* Function Call */
function_name(arguments);

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Example: Simple Function

#include <stdio.h>

/* Declaration */
int add(int a, int b);

int main() {
    int result = add(5, 3);   /* Call */
    printf("Sum = %d\n", result);
    return 0;
}

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

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Parameter Passing

1. Call by Value

A copy of the argument is passed. Changes inside the function do NOT affect the original.

void increment(int x) {
    x = x + 1;   /* only local copy changes */
}

int main() {
    int a = 5;
    increment(a);
    printf("%d\n", a);   /* still 5 */
}

2. Call by Reference (using pointers)

The address of the argument is passed. Changes inside the function DO affect the original.

void increment(int *x) {
    *x = *x + 1;   /* changes original via pointer */
}

int main() {
    int a = 5;
    increment(&a);
    printf("%d\n", a);   /* now 6 */
}

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void Functions

Functions that do not return a value use the void return type.

void greet(char name[]) {
    printf("Hello, %s!\n", name);
}

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Scope of Variables

int globalVar = 10;   /* global */

void demo() {
    int localVar = 20;   /* local */
    printf("%d %d\n", globalVar, localVar);
}