Call by Value and Call by Reference in C

Call by Value and Call by Reference in C: An Overview

Call by Value and Call by Reference in C are the ways to pass arguments to the functions in a programming language. In this C tutorial, we will be looking at call by value and call by reference. Our C training resources will also help you gain hands-on experience and practical knowledge. After reading this article, not only you will have a good understanding of both techniques but also feel confident enough to apply them wherever necessary.

Call by Value and Call by Reference in C

We have already seen functions and their various aspects like formal parameters and actual parameters in the section, Functions in C. In the function call, we used to pass parameters/arguments/data to the function that is being called.

In this section, we will look at the two ways in which parameters are passed in the function call in the C language.

1. Call by Value in C

In this method, the value of the actual parameter is passed to the corresponding formal parameter of the called function. This technique ensures that the original data i.e. actual parameter does not get altered by the called function, as the called function deals only with a copy of that data.

When a programmer opts for the call by value, the memory space for the formal parameters is allocated separately in the function stack, which further reinforces the fact that the original data remains untouched. Thus, formal parameters and actual parameters are allocated to different memory locations.

Example of Call by Value in C

#include <stdio.h>
void swap(int x, int y) {
 int temp = x;
 x = y;
 y = temp;
}
int main() {
 int a = 40, b = 50;
 printf("Before swap, a = %d and b = %d\n", a, b);
 swap(a, b);
 printf("After swap, a = %d and b = %d\n", a, b);
 return 0;
}

The above program in the C Compilerswaps the values of the variables, a and b. In the main() function, the function call passes the actual parameter values i.e. values of a and b to the swap() function. The swap() function swaps the values of a and b.

Due to the call-by-value method, the value of original variables a and b remains unchanged even after swapping by the swap() function.

Output

Before swap, a = 40 and b = 50
After swap, a = 40 and b = 50

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2. Call by Reference in C

In this method, instead of passing the values of the actual parameters to the formal ones, the addresses of the actual parameters are passed. Therefore, the modification by the called function to the formal parameters affects the actual parameters as well.

When a programmer opts for the call by reference, the memory space for the formal parameters and the actual parameters are the same. All the operations in the function are performed on the value stored at the address of the actual parameters, and the modified value gets stored at the same address.

This technique not only conserves memory but also simplifies collaboration, as complex data types like arrays or structures can easily be manipulated without being duplicated needlessly.

Example of Call by Reference in C

#include <stdio.h>
void swap(int *a, int *b) {
 int temp = *a;
 *a = *b;
 *b = temp;
}
int main() {
 int x = 5;
 int y = 10;
 printf("Before swap: x = %d, y = %d\n", x, y);
 swap(&x, &y);
 printf("After swap: x = %d, y = %d\n", x, y);
 return 0;
}

Output

Before swap: x = 5, y = 10
After swap: x = 10, y = 5

Differences between call by value and call by reference in C