Function Pointers in C

What is a Function Pointer?

A function pointer is a pointer that stores the address of a function instead of the address of a variable.

Just as a pointer to an int stores the address of an int variable, a function pointer stores the address of a function.

Why Use Function Pointers?

• Pass functions as arguments to other functions (Callbacks)
• Select a function at runtime
• Implement menus and dispatch tables
• Used extensively in operating systems, device drivers, and libraries like qsort()

General Syntax

return_type (*pointer_name)(parameter_list);

Example:

int (*pf)(int, int);

Meaning:

• pf is a pointer.
• It points to a function.
• The function takes two int arguments.
• The function returns an int.

Understanding the Syntax

int (*pf)(int, int);

Read inside-out:

(*pf)        → pf is a pointer
(int, int)   → points to a function taking two int arguments
int          → function returns int

Declaration Examples

void (*p1)();

Pointer to a function returning void.

int (*p2)(int);

Pointer to a function taking one int and returning int.

float (*p3)(float, float);

Pointer to a function taking two floats and returning float.

char (*p4)(char);

Pointer to a function returning char.

Defining a Function

#include<stdio.h>

int add(int a, int b)
{
    return a + b;
}

Assigning a Function to a Pointer

int (*pf)(int, int);

pf = add;

or

pf = &add;

Both are correct because the function name automatically converts to its address.

Calling Through a Function Pointer

Method

int result = pf(10, 20);

Method 2

int result = (*pf)(10, 20);

Both are equivalent.

Complete Example

#include<stdio.h>

int add(int a, int b)
{
    return a + b;
}

int main()
{
    int (*pf)(int, int);

    pf = add;

    printf("%d", pf(10,20));

    return 0;
}

Output

30

Function Name is a Pointer

Suppose

int add(int, int);

Then

add

means

Address of add()

Therefore,

pf = add;

is equivalent to

pf = &add;

Function Pointer vs Normal Pointer

Normal Pointer	Function Pointer
Stores address of a variable	Stores address of a function
int *p;	int (*pf)(int,int);
p = &x;	pf = add;
*p gives value	pf() calls the function

Difference Between These Declarations

1. Pointer to Function

int (*pf)(int, int);

pf is a pointer to a function.

2. Function Returning Pointer

int *pf(int, int);

pf is a function taking two int arguments and returning a pointer to int.

2. Pointer to Integer

int *p;

p points to an integer variable.

Function Returning Function?

int pf(int)(int); // Invalid. ❌

A function cannot return another function. It may return a pointer to a function.

Array of Function Pointers

int (*arr[3])(int, int);

Meaning:

• arr is an array of 3 function pointers
• Each pointer points to a function taking two ints and returning an int.

Example:

arr[0] = add;
arr[1] = sub;
arr[2] = mul;

printf("%d", arr[1](20,10));

Function Pointer as Parameter

void calculate(int a, int b,
               int (*operation)(int,int))
{
    printf("%d", operation(a,b));
}

Usage

calculate(10,20,add);

This is called a callback function.

Returning a Function Pointer

int add(int,int);
int sub(int,int);

int (*select(int choice))(int,int)
{
    if(choice==1)
        return add;
    else
        return sub;
}

Spiral (Inside-Out) Rule

To read complicated declarations:

1. Start from the variable name.
2. Move right if possible.
3. Move left.
4. Resolve parentheses first.

Example

int (*pf)(int,int);

pf
↓
(*pf)        → pointer
↓
(int,int)    → to function
↓
int          → returning int

Key Points

• A function pointer stores the address of a function.

• General syntax:

  return_type (*pointer_name)(parameters);

• add and &add are equivalent when assigning to a function pointer.

• Functions can be called using pf() or (*pf)().

• A function cannot return another function, but it can return a pointer to a function.

• Arrays of function pointers and function pointers as parameters are widely used for callbacks and dispatch tables.

• Parentheses around *pointer_name are mandatory. Without them, the declaration has a completely different meaning.# Function Pointers in C