Callback Functions in C

What You’ll Learn 📚

• What callback functions are and why they’re useful
• Key terminology and concepts
• Simple to complex examples of callback functions
• Common questions and troubleshooting tips

Introduction to Callback Functions

In C, a callback function is a function that is passed as an argument to another function. This allows a function to call another function, which can be useful for creating flexible and reusable code. Think of it like giving someone your phone number so they can call you back later. 📞

Key Terminology

• Function Pointer: A variable that stores the address of a function. This is how you pass a function as an argument.
• Callback: The act of calling a function that has been passed as an argument.

Simple Example: Hello, Callback!

#include <stdio.h>

// A simple function that takes a callback
void greet(void (*callback)()) {
    printf("Hello!\n");
    callback();
}

// A callback function
void sayGoodbye() {
    printf("Goodbye!\n");
}

int main() {
    // Passing the callback function
    greet(sayGoodbye);
    return 0;
}

Hello!
Goodbye!

Progressively Complex Examples

Example 1: Using Callbacks for Sorting

#include <stdio.h>
#include <stdlib.h>

// Comparison function for qsort
int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

int main() {
    int numbers[] = {4, 2, 3, 1, 5};
    int size = sizeof(numbers) / sizeof(numbers[0]);

    // Using qsort with a callback
    qsort(numbers, size, sizeof(int), compare);

    printf("Sorted numbers: ");
    for(int i = 0; i < size; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    return 0;
}

Sorted numbers: 1 2 3 4 5 

Example 2: Callbacks with Custom Operations

#include <stdio.h>

// A function that applies a callback to each element
void applyOperation(int *arr, int size, void (*operation)(int)) {
    for(int i = 0; i < size; i++) {
        operation(arr[i]);
    }
}

// A callback function
void printSquare(int num) {
    printf("%d squared is %d\n", num, num * num);
}

int main() {
    int numbers[] = {1, 2, 3, 4, 5};
    int size = sizeof(numbers) / sizeof(numbers[0]);

    // Applying the callback
    applyOperation(numbers, size, printSquare);
    return 0;
}

1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25

Common Questions and Answers

1. What is a callback function?

   A callback function is a function that is passed as an argument to another function, allowing the called function to execute the callback at a specific point.

2. Why use callback functions?

   They allow for more flexible and reusable code, enabling functions to be customized with different behaviors without modifying the function itself.

3. How do I declare a function pointer?

   Function pointers are declared by specifying the return type, followed by an asterisk and the pointer name in parentheses, and then the parameter list. For example: void (*funcPtr)().

4. Can a callback function have parameters?

   Yes, a callback function can have parameters, and you must ensure the function pointer matches the signature of the callback function.

5. What if I pass the wrong function signature?

   This will lead to undefined behavior, as the function pointer will not match the expected signature, potentially causing crashes or incorrect results.

Troubleshooting Common Issues

Segmentation Faults: Ensure your function pointers are correctly initialized and match the expected signature.

Debugging Tip: Use printf statements to trace the flow of your program and verify that callbacks are being executed as expected.

Practice Exercises

1. Create a callback function that multiplies each element of an array by 2 and prints the result.
2. Modify the sorting example to sort an array of strings alphabetically using a callback.

Remember, practice makes perfect! Keep experimenting with callback functions, and soon you'll be using them like a pro. If you have any questions, don't hesitate to ask. Happy coding! 😊