Usage of pointers to functions

Posted on Apr 13, 2020

C program to show the usage of pointer to function.

Program 

#include<stdio.h>
#include<conio.h>

int sum_func(int x, int y)
{
    return x+y;
}

void main()
{
	int sum;

    // function pointer
    int (*fptr)(int, int);

    // assign address to function pointer
    fptr = sum_func;

    printf("Function called normally");
    sum = sum_func(10, 20);
    printf("\nSum = %d",sum);

    printf("\n\nFunction called using pointer");
    sum = fptr(20, 30);  // calling a function referring to pointer to a function
    printf("\nSum = %d",sum);

    getch();
}

Output 

Function called normally
Sum = 30

Function called using pointer
Sum = 50

Explanation

It is possible to declare a pointer pointing to a function which can then be used as an argument in another function.
A pointer to a function is declared as follows,
data-type (*pointer-name)(parameter);

Here is an example :
int (*sum_func)(); //legal declaration of pointer to function
int *sum_func(); //This is not a declaration of pointer to function

A function pointer can point to a specific function when it is assigned the name of that function.
int sum_func(int, int); //normal function
int (*fptr)(int, int); //pointer to function
fptr = sum;
Here 'fptr' is a pointer to a function 'sum_func'. Now, 'sum_func' can be called using function pointer 'fptr' along with providing the required argument values.
fptr(20, 30);