Function Pointer in C

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

Using function pointers, we can call a function indirectly through its memory address. Function pointers are commonly used in callback functions, event-driven programming, and dynamic function execution.

Why Use Function Pointers?

Function pointers provide flexibility in programs by allowing functions to be selected and executed at runtime.

Common uses include:

Callback functions
Event handling
Function tables
Dynamic operation selection
Implementing polymorphic behavior

Example of Function Pointer

#include <stdio.h>

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

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

    fptr = &add;

    printf("%d", fptr(10, 5));

    return 0;
}

Output

In this example, the function pointer fptr stores the address of the add() function and calls it dynamically.

Function Pointer Declaration

A function pointer must match the return type and parameter list of the function it points to.

Syntax

return_type (*pointer_name)(parameter_types);

Example

int (*fptr)(int, int);

This declaration means:

Function returns an integer.
Function accepts two integer parameters.
fptr stores the address of such a function.

Function Pointer Initialization

After declaration, assign the address of a function.

Syntax

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pointer_name = &function_name;

or simply

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pointer_name = function_name;

Example

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fptr = add;

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fptr = &add;

Both are valid.

Calling a Function Using Function Pointer

Once the function address is stored, the function can be called through the pointer.

Example

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printf("%d", fptr(10, 5));

Output:

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Properties of Function Pointers

Store the address of a function.
Must match the function signature exactly.
Can point to different functions with the same signature.
Cannot perform arithmetic operations like normal pointers.
Useful for dynamic function execution.
Can be stored inside arrays and structures.

Function Pointer as Function Argument (Callback Function)

One of the most important uses of function pointers is passing functions as arguments to other functions.

Example

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#include <stdio.h>

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

int subtract(int a, int b)
{
    return a - b;
}

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

int main()
{
    calc(10, 5, add);
    calc(10, 5, subtract);

    return 0;
}

Output

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15
5

How It Works

First call uses add().
Second call uses subtract().
The function to execute is decided at runtime.

This technique is known as a callback function.

Function Pointers in Structures

C structures cannot directly contain functions, but they can contain function pointers.

Example

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#include <stdio.h>

typedef struct
{
    int width;
    int height;
    int (*area)(int, int);
} Rectangle;

int calculateArea(int w, int h)
{
    return w * h;
}

int main()
{
    Rectangle rect;

    rect.width = 10;
    rect.height = 5;
    rect.area = calculateArea;

    printf("Area = %d",
           rect.area(rect.width, rect.height));

    return 0;
}

Output

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Area = 50

This technique is often used to mimic object-oriented behavior in C.

Array of Function Pointers

Multiple function pointers can be stored inside an array.

Example

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#include <stdio.h>

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

int sub(int a, int b)
{
    return a - b;
}

int mul(int a, int b)
{
    return a * b;
}

int main()
{
    int (*operations[])(int, int) =
    {
        add,
        sub,
        mul
    };

    printf("%d\n", operations[0](10, 5));
    printf("%d\n", operations[1](10, 5));
    printf("%d\n", operations[2](10, 5));

    return 0;
}

Output

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15
5
50

Difference Between Normal Pointer and Function Pointer

Normal Pointer

Stores the address of a variable.

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int num = 10;
int *ptr = &num;