Call by Value and Call by Reference in C

Introduction

C passes function arguments by value — the called function receives a copy of each argument. To modify the caller’s variables from inside a function, you pass pointers (addresses). This pattern is often called call by reference in C textbooks, though technically C always passes the pointer by value.

Call by Value

Changes inside the function do not affect the original variable:

#include <stdio.h>

void increment(int n) {
    n = n + 1;
    printf("Inside function: %d\n", n);
}

int main(void) {
    int x = 10;
    increment(x);
    printf("In main: %d\n", x);
    return 0;
}

Sample Output

Inside function: 11
In main: 10

x in main remains 10 because only a copy was passed.

Call by Reference (Using Pointers)

Pass the address so the function can modify the original:

#include <stdio.h>

void increment(int *n) {
    *n = *n + 1;
    printf("Inside function: %d\n", *n);
}

int main(void) {
    int x = 10;
    increment(&x);
    printf("In main: %d\n", x);
    return 0;
}

Sample Output

Inside function: 11
In main: 11

Swap Example — Why Pointers Matter

#include <stdio.h>

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

int main(void) {
    int x = 5, y = 10;
    printf("Before: x=%d, y=%d\n", x, y);
    swap(&x, &y);
    printf("After:  x=%d, y=%d\n", x, y);
    return 0;
}

Sample Output

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

Without pointers, swap cannot change the caller’s variables. See the full Swap Using Pointers program.

Returning Values vs Using Pointers

Arrays Are Passed by Reference Automatically

When you pass an array to a function, C passes a pointer to the first element — modifications inside the function affect the original array:

void doubleArray(int arr[], int size) {
    for (int i = 0; i < size; i++)
        arr[i] *= 2;
}

Best Practices

• Use pointers when the function must modify caller data.
• Mark read-only pointer parameters as const char *str when not modifying.
• Document which parameters are input-only vs output.
• Check for NULL pointers before dereferencing.

Common Mistakes

• Forgetting & when passing a variable for modification.
• Dereferencing an uninitialized pointer.
• Returning address of a local variable (dangling pointer).

Related Pages

Continue learning with these related tutorials and programs:

• C Tutorials — Browse all C Tutorials.
• C Array — Concept hub — start with arrays before pointers.
• C Strings in C Programming — More in c concepts.
• C Functions — More in c concepts.