C - Functions

Overview

Functions encapsulate reusable logic. A function has a return type, a name, parameters, and a body.

Learning Objectives

• Write function prototypes and definitions.
• Pass parameters by value and by address (via pointers).
• Return values and handle void functions.

Prerequisites

• C - Datatypes
• C - Variables

Basic Example

#include <stdio.h>

int add(int a, int b) { return a + b; }
void greet(const char *name) { printf("Hello, %s!\n", name); }

int main(void) {
  printf("%d\n", add(2, 3));
  greet("World");
}

Function Prototypes

// Declare before use (typically in headers)
int add(int, int);
void greet(const char*);

// Define later
int add(int a, int b) { return a + b; }
void greet(const char *name) { printf("Hello, %s!\n", name); }

Pass by Value vs Address

#include <stdio.h>

void by_value(int x) { x = 999; }      // doesn't affect caller
void by_address(int *x) { *x = 999; }  // modifies caller's variable

int main(void) {
  int a = 5, b = 5;
  by_value(a);   printf("a = %d\n", a);  // 5
  by_address(&b); printf("b = %d\n", b);  // 999
}

Common Pitfalls

• Forgetting to declare functions before use (or missing prototypes).
• Mismatched parameter types between prototype and definition.
• Expecting pass-by-value to modify the caller's variables.

Checks for Understanding

1. What happens if you call a function without declaring it first?
2. How do you make a function modify a caller's variable?

Practice

1. Write a function that calculates the area of a rectangle.
2. Write a function that swaps two integers using pointers.

Expected Output

5
Hello, World!

For pass-by-value/address example:

a = 5
b = 999