Call: Value & Reference in C

In C, when passing arguments to a function, there are two methods: Call by Value and Call by Reference. Understanding these two methods is essential for effective function management in C programming.

Let's understand Call by Value and Call by Reference in C language one by one:

1) Call by Value

In Call by Value, the actual value of the argument is passed to the function. The changes made to the parameter inside the function do not affect the original argument.

• In Call by Value method, the value of the actual parameters is copied into the formal parameters. In other words, the value of the variable is used in the function call.
• The value of the actual parameter cannot be modified by the formal parameter.
• Separate memory is allocated for actual and formal parameters since the value is copied.
• The actual parameter is the argument used in the function call, whereas the formal parameter is used in the function definition.

Example to demonstrate Call by Value:

                        
#include <stdio.h>  
void change(int num)
{    
    printf("Before adding value inside function num=%d\n", num);    
            num = num + 100;    
    printf("After adding value inside function num=%d\n", num);    
}    
        
int main() 
{    
    int x = 100;    
    printf("Before function call x=%d\n", x);    
    change(x); // passing value in function    
    printf("After function call x=%d\n", x);    
    return 0;  
}    
    
                    

2) Call by Reference

In Call by Reference, the address (reference) of the argument is passed to the function. The function can modify the actual value of the argument since it operates directly on the original variable.

• In Call by Reference, the address of the variable is passed into the function call as the actual parameter.
• The value of the actual parameters can be modified by changing the formal parameters.
• The memory allocation is the same for both formal and actual parameters. Changes made in the function reflect in the actual parameters.

Example to demonstrate Call by Reference:

                        
#include  <stdio.h>
// Function to swap two integers
void swap(int *a, int *b) {  
    int temp;   
    temp = *a;  
    *a = *b;  
    *b = temp;  
    printf("After swapping values in function: a = %d, b = %d\n", *a, *b);  
}  
        
int main() 
{  
    int a, b;  
        
    // User input
    printf("Enter the value of a: ");  
    scanf("%d", &a);  
    printf("Enter the value of b: ");  
    scanf("%d", &b);  
    
    printf("Before swapping values in main: a = %d, b = %d\n", a, b);  
    
    // Call the swap function
    swap(&a, &b);   
    
    printf("After swapping values in main: a = %d, b = %d\n", a, b);  
   
    return 0;  
}    
      
                    

Difference between Call by Value and Call by Reference