The Implementation of Pointer to Function

Introduction

All the functions that are defined in C are compiled into a series of instructions in the memory.

As the function is associated with the memory, we can use a pointer with a function that is associated with the first instruction function defined in the memory after the compilation process. This is an indirect method of referring to a function as a pointer by providing indirection. Indirect reference to a function in simple terms means calling the function using a function pointer which is a variable that points to that function.

Syntax

<type-specifier><(*function _ pointer) ()>;

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For example

int (*function_pointer)();

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The above declaration instructs the C compiler to declare a variable function_pointer, which points to a function that returns an integer. It is important to enclose the name of the function pointer in parentheses.

If we do not do this, and declare the function pointer as,

int *function_pointer();

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This declaration would instruct the C compiler that function_pointer is the name of a function that returns a pointer to an integer i.e. a function returning a pointer and not a variable pointing to the first instruction of the function in the memory.

Assigning the address of a function to a function pointer

To assign the address of a function to a function pointer, it is necessary to declare the function before the assignment is done.

<function_pointer>=<function_pointer>;

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Function pointer is the name of a pointer that would point to the first instruction of the function in the memory. The function name is the name of the function which has already been defined.

Write a program to display the address of a function.

Source Code

#include <stdio.h>
#include<conio.h>
void main()
{
int showname();
clrscr();
printf("\n the address of function showname is %u\n",showname);
showname();  //calling function//
getch();
}
int showname()
{
printf("welcome to shikha tiwari college of management");
return 0;
}

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Output

Calling function using function pointer
 

Syntax

(*<function_pointer>)();

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Source Code

Write a program to illustrate the use of a function pointer to call a function.

#include <stdio.h>
#include<conio.h>
void main()
{
int showname();
int (*function_pointer)();
clrscr();
function_pointer=showname;  //storing the base address of the function //
printf("\n the address of function showname is %u\n",function_pointer);
(*function_pointer)();
getch();
}
showname()
{
printf("\n welcome to shikha tiwari  college of management");
}

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Output

The address of the function is runtime-dependent. A function can also be passed parameter.

To pass a parameter to a function pointer, we include the list of parameters after the function pointer name like.

(*function_pointer)(100,200);

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In this declaration, we assume that function_pointer is the pointer to a function that calculates the maximum of two numbers, the values 100 and 200 are passed as parameters to the function.

Function returning a Pointer

How to pass the address of a variable to a function so that the calling function can also change the values of the parameter passed.

Write a program that illustrates a function returning a pointer.

Source Code

#include <stdio.h>
#include <conio.h>
// Function prototype
int* function();
// Global variable
int global_var;
int main() {
    int* pointer;
    // Calling the function and assigning the returned pointer to 'pointer'
    pointer = function();
    // Printing the address returned by the function
    printf("\nThe address returned by the function is %p", (void*)pointer);
    // Printing the value of the variable in the called function
    printf("\nThe value of the variable in the called function is %d\n", *pointer);
    return 0;
}
// Function definition
int* function() {
    // Assigning a value to the global variable
    global_var = 200;
    // Returning the address of the global variable
    return &global_var;
}

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Output

Summary

In C, functions are compiled into a series of instructions stored in memory. By using pointers, we can reference and call these functions indirectly. A function pointer, which is a variable pointing to the function's memory location, allows for this indirect method of calling functions. This process of using a pointer to call a function is known as providing indirection.