Passing pointers to functions in C

Passing pointers to functions

In C, pointers can be passed to functions as parameters.

Passing pointers to functions allows for more efficient memory usage and can modify the values of variables outside of the function.

How to pass pointers to functions

To pass a pointer to a function in C, you can follow these steps:

Declare the function

• Declare the function that will receive the pointer as an argument.

• For example, suppose you want to pass an integer array to a function. You can declare the function as follows:

void my_function(int *arr, int size);

Call the function

• Call the function and pass the pointer as an argument.

• When calling the function, pass the address of the pointer to the function. This can be done using the address-of operator (&).

For example:

int arr[5] = {1, 2, 3, 4, 5};
int size = 5;
my_function(&arr[0], size);

Here, The address of the first element in the array arr is passed to the function my_function.

Define the function to receive the pointer

• In the definition of the function, declare the parameter that receives the pointer using the appropriate pointer type.

For example:

void my_function(int *arr, int size) {
    // Function body goes here
}

Use the pointer in the function

• Inside the function, you can use the pointer to access the values of the array.

For example:

void my_function(int *arr, int size) {
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

Example

#include <stdio.h>

// function prototype
void swap(int *x, int *y);

int main()
{
    int a = 5, b = 10;

    printf("Before swap, a = %d and b = %d\n", a, b);

    // call the swap function and pass the addresses of a and b
    swap(&a, &b);

    printf("After swap, a = %d and b = %d\n", a, b);

    return 0;
}

// function definition
void swap(int *x, int *y)
{
    int temp = *x;
    *x = *y;
    *y = temp;
}

Output:

Explanation

• The swap function takes two pointers to int variables as arguments.
• These pointers are then used to swap the values of the two variables by dereferencing them and using a temporary variable.
• The & operator is used to pass the addresses of a and b to the swap function.
• When the swap function is called with the addresses of a and b, the values of a and b are swapped.
• When the function returns, the values of a and b have been modified to reflect the swap.