Pointers & Functions in C

What is a Function Pointer?

A function pointer stores the address of a function. Unlike normal pointers that point to data, these point to executable code.

• Can call a function using a pointer
• Can pass a function as an argument
• Used in event-driven and callback systems
• Used in dynamic dispatch and menu systems

Declaration of Function Pointer

return_type (*pointer_name)(parameter_list);

• *pointer_name indicates pointer to a function
• Syntax looks complex but easy with practice

Assigning Function Address

pointer_name = function_name;

A function name without parentheses represents its address.

Calling a Function Using Pointer

pointer_name(arguments); 
(*pointer_name)(arguments);

Both ways are valid in C.

Why Use Function Pointers?

• To pass functions as arguments
• To create callback functions
• To build menu-driven programs
• To replace long switch-case statements
• To implement dynamic behavior

Function Pointer as Argument

Functions can receive another function's address, allowing behavior changes at runtime.

void process(int a, int b, int (*op)(int,int));

This allows selecting different operations dynamically.

Array of Function Pointers

C allows storing multiple function addresses in an array. This is used for menu systems, calculators, and state machines.

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

Function Pointer in Structures

Used in device drivers, GUI frameworks, and plugin systems.

struct Actions {
    void (*start)();
    void (*stop)();
};

Benefits of Using Function Pointers

• Improves flexibility
• Supports modular programming
• Reduces duplicate code
• Allows dynamic behavior selection
• Used in advanced C programming (OS, embedded, drivers)

Common Mistakes

• Forgetting parentheses in pointer declaration
• Passing wrong function signature
• Incorrect number of arguments
• Confusing pointer to function vs. pointer returning function

Pointers & Functions — 10 Examples

1. Passing Pointer to a Function

#include <stdio.h>

void update(int *p){
    *p = *p + 10;
}

int main(){
    int x = 20;
    update(&x);
    printf("Updated: %d", x);
    return 0;
}

2. Function Returning Pointer to Variable

#include <stdio.h>

int* getAddress(int *p){
    return p;
}

int main(){
    int a = 15;
    int *res = getAddress(&a);
    printf("%d", *res);
    return 0;
}

3. Swap Using Pointer Function

#include <stdio.h>

void swap(int *a, int *b){
    int t = *a;
    *a = *b;
    *b = t;
}

int main(){
    int x = 5, y = 9;
    swap(&x, &y);
    printf("%d %d", x, y);
    return 0;
}

4. Passing Array to Function (Pointer Form)

#include <stdio.h>

void show(int *arr){
    for(int i=0; i<5; i++)
        printf("%d ", arr[i]);
}

int main(){
    int a[5] = {1,2,3,4,5};
    show(a);
    return 0;
}

5. Function Pointer Basics

#include <stdio.h>

void hello(){
    printf("Hello World");
}

int main(){
    void (*fp)() = hello;
    fp();
    return 0;
}

6. Function Pointer with Parameters

#include <stdio.h>

void sum(int a, int b){
    printf("%d", a+b);
}

int main(){
    void (*fp)(int,int) = sum;
    fp(5,7);
    return 0;
}

7. Returning Pointer from Function (Local Not Allowed)

#include <stdio.h>

int* wrong(){
    int x = 10;
    return &x;   // invalid: returns address of local variable
}

int main(){
    int *p = wrong(); 
    printf("Undefined Behavior");
    return 0;
}

8. Returning Pointer to Static Variable

#include <stdio.h>

int* value(){
    static int x = 30;
    return &x;
}

int main(){
    int *p = value();
    printf("%d", *p);
    return 0;
}

9. Using Pointer to Change String via Function

#include <stdio.h>

void modify(char *s){
    s[0] = 'Z';
}

int main(){
    char name[] = "Sourav";
    modify(name);
    printf("%s", name);
    return 0;
}

10. Array of Function Pointers

#include <stdio.h>

void add(){ printf("ADD\n"); }
void sub(){ printf("SUB\n"); }
void mul(){ printf("MUL\n"); }

int main(){
    void (*ops[3])() = {add, sub, mul};
    for(int i=0; i<3; i++)
        ops[i]();
    return 0;
}

Practice Questions

1. What is a function pointer? Explain with syntax.
2. How do you assign a function address to a pointer?
3. Difference between calling a function normally and via a pointer?
4. What is the purpose of callbacks?
5. Write the advantages of function pointer arrays.

Practice Task