Parameter Passing Types

What is parameter passing in C?

In C programming, parameter passing is the process of passing values from a calling function to a called function.

There are two commonly discussed parameter-passing methods:
1. Call by Value
2. Call by Reference (implemented in C using pointers)

Note: Strictly speaking, C uses pass by value for all function arguments. What is commonly called call by reference in C is achieved by passing pointer values.

Call by value

Definition: In call by value, a copy of the actual argument is passed to the function. Changes made to the formal parameter do not affect the original variable.

Example: C Program to swap two numbers using call by value

#include <stdio.h>

void swap(int a, int b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}

int main()
{
    int x = 10, y = 20;

    swap(x, y);

    printf("x = %d\n", x);
    printf("y = %d\n", y);

    return 0;
}

Output:

x = 10
y = 20

Explanation: The function swaps only the copies of x and y. The original variables remain unchanged.

Call by reference using pointers

Definition: In the commonly used C teaching terminology, call by reference means passing the addresses of variables to a function so that it can modify the original variables through pointers.

Example: C Program to swap two numbers using pointers

#include <stdio.h>

void swap(int *a, int *b)
{
    int temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

int main()
{
    int x = 10, y = 20;

    swap(&x, &y);

    printf("x = %d\n", x);
    printf("y = %d\n", y);

    return 0;
}

Output:

x = 20
y = 10

Explanation

&x passes the address of x.
&y passes the address of y.
int *a and int *b receive copies of those addresses.
*a and *b access and modify the original variables.

How does it work?

swap(&x, &y) passes the addresses of x and y.
The pointer parameters a and b store those addresses.
*a accesses x, and *b accesses y.
The function exchanges the values at those addresses.

This is often called call by reference in introductory teaching, but technically C still passes the pointer arguments by value. The function uses those pointers to modify the caller's objects.

Difference between call by value and call by reference

Feature	Call by Value	Call by Reference (using pointers in C)
What is passed?	Copy of the value	Address of the variable
Pointer required?	No	Yes, for this technique
Original variables modified?	No, not through the copied parameters	Yes, through dereferencing
Function call example	swap(x, y);	swap(&x, &y);
Function parameters	int a, int b	int a, int b

C Programming