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User-Defined 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)

returnType functionName(parameter list);

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

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2. Function Definition

returnType functionName(parameter list) {
    // function body
}

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

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3. Function Call

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

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()