Call by Value & Call by Reference

In C, function parameter passing can be broadly categorized into two modes: "Call by Value" and "Call by Reference." These two modes determine how the values of arguments are passed to functions.

Call by Value: In "Call by Value," a copy of the actual argument's value is passed to the function. This means that changes made to the parameter inside the function do not affect the original variable outside the function. This mode is often used for simple data types like integers and floats.

Here's an example illustrating "Call by Value":

#include <stdio.h>

// Function to increment a number by 1 (Call by Value)
void incrementByValue(int num) {
    num += 1;
}

int main() {
    int x = 5;
    printf("Before function call: x = %d\n", x);
    
    incrementByValue(x);
    
    printf("After function call: x = %d\n", x);
    
    return 0;
}        

Output:

Before function call: x = 5
After function call: x = 5        

In this example, x remains unchanged after calling the incrementByValue function because it operates on a copy of x.

Call by Reference (using Pointers): In "Call by Reference," a reference to the memory location of the actual argument is passed to the function. This means that changes made to the parameter inside the function will affect the original variable outside the function. This mode is often used when you want to modify the original variable.

Here's an example illustrating "Call by Reference" using pointers:

#include <stdio.h>

// Function to increment a number by 1 (Call by Reference)
void incrementByReference(int *num) {
    (*num) += 1;
}

int main() {
    int x = 5;
    printf("Before function call: x = %d\n", x);
    
    incrementByReference(&x); // Pass a reference to x
    
    printf("After function call: x = %d\n", x);
    
    return 0;
}        

Output:

Before function call: x = 5
After function call: x = 6        

In this example, x is modified inside the incrementByReference function because we passed a pointer to x, allowing the function to access and modify its value.

To summarize:

• "Call by Value" passes a copy of the argument's value, and changes inside the function do not affect the original variable.
• "Call by Reference" passes a reference to the argument's memory location, allowing changes inside the function to affect the original variable. This is typically achieved using pointers.