How to declare a pointer to a function in C?

In C, a pointer is a variable whose value is the address of another variable or memory block, i.e direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable or block address.

Basic Pointer to a Function

A pointer to a function is simply a variable that stores the address of a function instead of a normal data value.

Syntax

Following is the syntax of basic pointer to a function:

Datatype *variable_name

Algorithm

Following is the algorithm for the basic pointer to a Function:

Begin.
   Define a function show.
      Declare a variable x of the integer datatype.
      Print the value of varisble x.
   Declare a pointer p of the integer datatype.
   Define p as the pointer to the address of show() function.
   Initialize value to p pointer.
End.

Example

This is a simple example in C to understand how the concept of a pointer to a function works. The program creates a function pointer p to store the address of show(), then after it calls indirectly to print the value as the output.

#include<stdio.h>
void show(int x)
{
   printf("Value of x is %d\n", x);
}
int main()
{ 
   // Assign function address
   void (*p)(int); 
   // declaring a pointer
   p = &show; 
   // p is the pointer to the show()
   (*p)(7); 
   //initializing values.
   return 0;
}

Following is the output to the above program:

Value of x is 7.

Function Pointer with Return Value

A function pointer with a return value is used to store the address of a function that takes input, executes, and returns a result dynamically.

Syntax

Following is the syntax is as follows:

return_type (*pointer_name)(parameter_list);  // Declaration
pointer_name = function_name;                 // Assignment

Algorithm

Following is the algorithm for the function pointer with Return Value:

Begin.
   Define a function add that takes two integer parameters.
      Inside add, return the sum of the two integers.
   Declare a function pointer named func_ptr that matches the signature of add.
   Assign the address of the add function to func_ptr.
   Call the function through func_ptr with two integer arguments.
   Store the return value in a variable called result.
   Print the value of result.
End.

Example

In this example, we define a function add() that stores its address in a function pointer (func_ptr) by calling it indirectly with the values and prints the result.

#include<stdio.h>
int add(int a, int b) {
   return a + b;
}
int main() {  
   // Declare function pointer
   int (*func_ptr)(int, int);
   // Assign function address
   func_ptr = add;
   int result = func_ptr(3, 4); 
   // Call function through pointer
   printf("Result = %d\n", result);
   return 0;
}

The above program produces the following result:

Result = 7