Function pointer and Callback advantages

 

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Example: Button Interrupt with Dynamic Callbacks

#include <stdio.h>
#include <stdint.h>

// Define a callback type
typedef void (*ButtonCallback)(void);

// Global variables
volatile uint8_t button_pressed_flag = 0;
ButtonCallback button_callback = NULL;

// ISR: fast, only sets flag
void BUTTON_IRQHandler(void) {
    button_pressed_flag = 1;
}

// Register a callback dynamically
void register_button_callback(ButtonCallback cb) {
    button_callback = cb;
}

// Two possible actions
void turn_on_led(void) {
    printf("LED turned ON!\n");
}

void send_message(void) {
    printf("Message sent!\n");
}

int main() {
    // At runtime, we can decide which action to use
    int user_choice;

    while (1) {
        printf("Choose action: 1=LED, 2=Message: ");
        scanf("%d", &user_choice);

        if (user_choice == 1)
            register_button_callback(turn_on_led);  // set LED action
        else
            register_button_callback(send_message); // set message action

        // Simulate button press
        BUTTON_IRQHandler();

        // Handle button press safely outside ISR
        if (button_pressed_flag) {
            button_pressed_flag = 0;

            // Here is the **callback in action**
            if (button_callback != NULL) {
                button_callback();  // Calls the function dynamically
            }
        }
    }

    return 0;
}

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Key Points Highlighting the Advantage of Callbacks

Aspect	Without Callback	With Callback
ISR behavior	Hardcoded to one action	ISR stays fast; action is dynamic
Changing behavior	Must edit ISR code	Change button_callback at runtime
Flexibility	Low — each ISR tied to one function	High — ISR works for any registered function
Heavy processing	Can block ISR if inside	Run safely outside ISR via callback

✅ Why we use callback here:

• We can change the button behavior at runtime without touching the interrupt code.

• The ISR stays short and safe, even if the actions are complex.

• The system becomes modular and reusable: the same button can do different things in different program contexts.

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