Functions in C

A function is a self-contained block of code that performs a specific task. Writing functions makes code modular, reusable, and much easier to debug.

1. Declaring Function Prototypes

C compiles code sequentially from top to bottom. If a function is called in main() before it is defined, compilation will fail unless you provide a function prototype (declaration) at the top of the file:

int getSquare(int num); // Function prototype: tells compiler signature

2. Anatomy of a Function

A function definition consists of a return type, a descriptive name, parameter declarations, and the function body:

#include <stdio.h>

// 1. Prototype
double calculateArea(double width, double height);

int main() {
    // 2. Function Call
    double area = calculateArea(5.5, 4.0);
    printf("Area: %.2lf\n", area);
    return 0;
}

// 3. Function Definition
double calculateArea(double width, double height) {
    return width * height; // returns double value to caller
}

3. Pass-by-Value (Parameter Copying)

By default, variables passed to functions are passed by value. This means C copies the value of the variable into a new parameter variable inside the function. Modifying the parameter inside the function does not affect the original variable in the calling function:

#include <stdio.h>

void incrementValue(int x) {
    x = x + 1;
    printf("Inside function: x = %d\n", x);
}

int main() {
    int num = 10;
    incrementValue(num); // Passes copy of num
    printf("Inside main: num = %d\n", num); // num is still 10!
    return 0;
}

If you want a function to modify a variable directly, you must pass its address using pointers (Pass-by-Reference).