C - Function Pointers in C

1. What is a Function Pointer?

In C, a function pointer is a pointer that stores the address of a function instead of the address of a variable. Just like a normal pointer points to data (such as int or float), a function pointer points to executable code.

This allows a program to call a function indirectly using the pointer. Function pointers are very useful in advanced programming techniques such as callbacks, event handling, and dynamic function selection.

2. Why Do We Need Function Pointers?

Function pointers are mainly used when:

You want to pass a function as an argument to another function.
You want to choose a function at runtime.
You want to implement callbacks.
You want to create flexible and reusable code.

They are commonly used in systems programming, embedded systems, and libraries.

3. Syntax of Function Pointer

The syntax may look complex at first:

return_type (*pointer_name)(parameter_types);

Example:

int (*funcPtr)(int, int);

This means:

funcPtr is a pointer
It points to a function
The function returns an int
The function takes two int parameters

4. Example of Function Pointer

Step 1: Define a function

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

Step 2: Declare a function pointer

int (*funcPtr)(int, int);

Step 3: Assign the function address

funcPtr = add;

Step 4: Call the function using pointer

int result = funcPtr(5, 3);

This will call the add function and store the result (8).

5. Passing Function Pointer as Argument

Function pointers are often passed to other functions.

Example:

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

void calculate(int x, int y, int (*operation)(int, int)) {
    printf("Result = %d", operation(x, y));
}

int main() {
    calculate(4, 5, multiply);
    return 0;
}

Here:

calculate receives a function pointer
It calls the function using the pointer
This makes the program flexible

6. Array of Function Pointers

You can also create an array of function pointers.

Example:

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

int main() {
    int (*operations[2])(int, int) = {add, subtract};
    
    printf("%d\n", operations[0](10, 5)); 
    printf("%d\n", operations[1](10, 5)); 
    
    return 0;
}

This allows selecting functions dynamically using an index.

7. Advantages of Function Pointers

Makes code more flexible.
Helps in implementing callback functions.
Reduces repeated code.
Useful in menu-driven programs.
Used in building libraries and frameworks.

8. Important Points to Remember

The function pointer’s signature must exactly match the function.
Parentheses around *pointer_name are mandatory.
The function name itself represents its address.
You do not need & while assigning the function name.

9. Real-Life Use Case

In real-world C programming:

Sorting functions use function pointers to decide sorting criteria.
Operating systems use function pointers for system calls.
Embedded systems use them for interrupt handling.

Function pointers may look complicated at first, but with practice, they become a powerful tool in writing dynamic and modular C programs.