Functions

Introduction

Functions package logic behind a name and parameter list so you can reuse behavior and keep main() readable.

Understanding the topic

Declaration & definition Return type, name, parameter list, body in braces.

Function call Pass arguments; receive return value.

Prototype Declare before main if definition comes later.

• Declaration & definition — Return type, name, parameter list, body in braces.
• Function call — Pass arguments; receive return value.
• Prototype — Declare before main if definition comes later.

Step-by-step explanation

1. Declaration & definition — Return type, name, parameter list, body in braces.
2. Function call — Pass arguments; receive return value.
3. Prototype — Declare before main if definition comes later.

Syntax reference

Syntax reference:

returnType name(type param) { statements; return value; }

Informative example

Example program:

#include <stdio.h>

int multiply(int a, int b) {
    return a * b;
}

int main(void) {
    printf("%d\n", multiply(6, 7));
    return 0;
}

42

Execution workflow

Best practices

• Enable warnings: gcc -Wall -Wextra -std=c11 source.c -o app
• Give every variable a defined value before it is read.
• Stay inside array bounds — C will not stop you from over-running a buffer.

Common mistakes

• Reading uninitialized storage — behavior is undefined.
• Dismissing compiler warnings instead of fixing root causes.
• Ignoring NULL returns from malloc, fopen, and similar APIs.

Hands-on exercise

Practice problems:

• Write a function to find max of two numbers
• Function with no parameters

Summary

Functions in C — Define reusable routines with parameters and return types.