Function Pointers

# CHAPTER 26

Function Pointers #

1. Introduction #

2. Learning Objectives #

• Declare and initialize a function pointer.

• Understand the complex syntax of function pointers.

• Implement Callback functions.

• Create an array of function pointers.

3. Syntax of Function Pointers #

Syntax: returntype (*pointername)(parameter_types);

#include <stdio.h>

// A normal function
void sayHello(char *name) {
    printf("Hello, %s!\n", name);
}

int main() {
    // 1. Declare a function pointer
    // It points to a function that returns void and takes a char*
    void (*func_ptr)(char*); 

    // 2. Assign the address of the function to the pointer
    func_ptr = &sayHello; // (or just func_ptr = sayHello; works too)

    // 3. Call the function using the pointer
    (*func_ptr)("Alice"); // Prints: Hello, Alice!
    // func_ptr("Alice"); // This shorthand also works!

    return 0;
}

4. Passing Functions as Arguments (Callbacks) #

#include <stdio.h>

// Two simple functions
int add(int a, int b) { return a + b; }
int multiply(int a, int b) { return a * b; }

// A function that TAKES a function pointer as an argument
void executeMath(int x, int y, int (*operation)(int, int)) {
    int result = operation(x, y); // Calling the passed function
    printf("The result is: %d\n", result);
}

int main() {
    printf("Executing Add:\n");
    executeMath(5, 3, add); // Passes the 'add' function

    printf("Executing Multiply:\n");
    executeMath(5, 3, multiply); // Passes the 'multiply' function

    return 0;
}

5. Array of Function Pointers #

#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() {
    // Array of 3 function pointers
    int (*math_operations[3])(int, int) = {add, sub, mul};
    
    int choice = 1; // 0 for add, 1 for sub, 2 for mul
    int a = 10, b = 5;

    if (choice >= 0 && choice < 3) {
        // Calls sub(10, 5)
        int result = math_operations[choice](a, b);
        printf("Result: %d\n", result);
    }

    return 0;
}

6. Using typedef for Cleaner Syntax #

// Creates a new type called "MathFunc"
typedef int (*MathFunc)(int, int);

// Now you can use it like a normal data type!
MathFunc myFunc = add; 

7. Memory-Level Explanation #

8. Common Mistakes #

• Forgetting the parentheses: int *funcptr(int); declares a normal function that returns an int*. You MUST use parentheses: int (*funcptr)(int); to declare a function pointer.

• Signature mismatch: Trying to point a void (*)(int) pointer at a function that returns an int. The compiler will throw a warning or error.

9. Exercises #

1. 1. Write a function sort() that takes an array and a function pointer compare(). Use it to sort an array in both ascending and descending order by passing different compare functions.

1. 2. Use typedef to clean up the function pointer syntax in the callback example above.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: Explain the syntax difference between a function returning a pointer, and a pointer to a function.

• Q: How is the qsort() function in the C standard library implemented? (Answer: It uses function pointers for the comparison logic).

• Q: What is the Text segment, and why are function pointers dangerous if exploited by hackers?

12. Summary #

13. Next Chapter Recommendation #