Function call by reference and call by value

In C, the concepts of “call by value” and “call by reference” are fundamental to understanding how arguments are passed to functions. Here’s a detailed explanation:

Function Call by Value

In call by value, a copy of the actual parameter’s value is passed to the function. Changes made to the parameter inside the function do not affect the original value outside the function. This is the default method of passing arguments in C.

Example of Function Call by Value:

#include <stdio.h>

void modifyValue(int x) {
    x = 10; // Changes the local copy, not the original
}

int main() {
    int a = 5;
    printf("Before function call: %d\n", a); // Output: 5
    modifyValue(a);
    printf("After function call: %d\n", a); // Output: 5
    return 0;
}

In this example, the value of a is passed to the modifyValue function. Inside the function, x is a local copy of a, and changes to x do not affect a.

Functional Call by Reference

In function call by reference, a reference (or pointer) to the actual parameter is passed to the function. This allows the function to modify the original value directly.

Example of Function Call by Reference:

#include <stdio.h>

void modifyValue(int *x) {
    *x = 10; // Changes the value at the address pointed by x
}

int main() {
    int a = 5;
    printf("Before function call: %d\n", a); // Output: 5
    modifyValue(&a); // Pass the address of a
    printf("After function call: %d\n", a); // Output: 10
    return 0;
}

In this example, the address of a is passed to the modifyValue function. Inside the function, x is a pointer to a, and changes to *x directly affect a.

Key Differences of Function call by reference and Function call by value

1. Modification Capability:
   • Function Call by Value: The function cannot modify the actual parameter.
   • Function Call by Reference: The function can modify the actual parameter.
2. Parameter Passing:
   • Call by Value: Passes a copy of the parameter’s value.
   • Call by Reference: Passes the address of the parameter.
3. Safety:
   • Call by Value: Safer, as it prevents unintended modifications to the original variable.
   • Call by Reference: Less safe, as it allows the function to modify the original variable, which can lead to side effects.

Summary

• Function Call by Value: Used when you want to ensure that the original data remains unchanged by the function.
• Function Call by Reference: Used when you need the function to modify the original data.

Understanding these concepts is crucial for effective programming in C, especially when dealing with large data structures or requiring efficient memory management.