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User-Defined Functions in C

A function is a block of code that performs a specific task and can be reused whenever needed. Functions help break a large program into smaller, manageable parts and eliminate code repetition. Types of functions in C:
  1. Predefined Functions — Built-in, e.g. printf(), scanf(), sqrt()
  2. User-Defined Functions — Created by the programmer

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Structure of a Function

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1. Function Declaration (Prototype / Signature)

Tells the compiler the function’s name, return type, and parameter types — without a body. Placed before main(). 
returnType functionName(parameter list);

// Example:
int add(int a, int b);

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2. Function Definition

The actual function body — what the function does. 
returnType functionName(parameter list) {
    // function body
}

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

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3. Function Call

Executes the function from somewhere in the program. 
int result = add(5, 3);

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Complete Example

#include <stdio.h>

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

int main() {
    int result = add(10, 20);
    printf("Sum = %d", result);
    return 0;
}

// Definition
int add(int a, int b) {
    return a + b;
}
// Output: Sum = 30

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Types of Function Based on Arguments and Return

TypeArgumentsReturns
No argument, no returnNoneNone
No argument, with returnNoneYes
With argument, no returnYesNone
With argument, with returnYesYes
Examples: 
void greet();              // no argument, no return
int  getYear();            // no argument, returns a value
void display(int x);       // takes argument, returns nothing
int  add(int a, int b);    // takes arguments, returns a value

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Why Use Functions?

  • Reusability — Write once, use anywhere in the program
  • Modularity — Break complex programs into smaller pieces
  • Readability — Code becomes easier to understand
  • Easy debugging — Isolate and fix bugs in specific functions
  • Avoid repetition — No need to rewrite the same logic

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Key Rules

  • Arguments in C are passed by value by default (changes inside the function don’t affect the original unless pointers are used)
  • A function without a return statement that has a non-void return type will return a garbage value
  • Function prototypes allow defining the function after main()