C Callback Functions

C Callback Functions

C Callback Functions in C language is a function that is passed as an argument to another function, and is called back inside that function.

Callbacks are implemented using function pointers.

Why Callback Functions Are Needed?

Callback functions help in:

Writing generic & reusable code
Implementing event-driven programming
Sorting, searching, filtering
Embedded systems (interrupts)
Operating systems & drivers

Simple Definition

A callback function is a function that is called by another function, not directly by the programmer.

Basic Syntax (Important )

Passing function pointer as argument:

Simple Callback Example (Beginner)

#include <stdio.h>

// Callback function
void greet(int x) {
printf("Hello! Value = %d\n", x);
}

// Function accepting callback
void callFunction(void (*cb)(int)) {
cb(10); // calling callback
}

int main() {
callFunction(greet);
return 0;
}

#include <stdio.h>

// Callback function

void greet(int x) {

printf("Hello! Value = %d\n", x);

}

// Function accepting callback

void callFunction(void (*cb)(int)) {

cb(10); // calling callback

}

int main() {

callFunction(greet);

return 0;

}

greet() is called inside callFunction()

Callback with Multiple Operations (Real Example)

#include <stdio.h>

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

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

int operate(int x, int y, int (*op)(int, int)) {
return op(x, y); // callback call
}

int main() {
printf("Add: %d\n", operate(5, 3, add));
printf("Multiply: %d\n", operate(5, 3, multiply));
return 0;
}

#include <stdio.h>

int add(int a, int b) {

return a + b;

}

int multiply(int a, int b) {

return a * b;

}

int operate(int x, int y, int (*op)(int, int)) {

return op(x, y); // callback call

}

int main() {

printf("Add: %d\n", operate(5, 3, add));

printf("Multiply: %d\n", operate(5, 3, multiply));

return 0;

}

Same function, different behavior
Very common interview example

Callbacks Using `typedef` (Cleaner Code)

Improves readability
Used in professional projects

Callbacks in Sorting (Like `qsort`)

#include <stdio.h>
#include <stdlib.h>int compare(const void *a, const void *b) {
return (*(int *)a - *(int *)b);
}

int main() {
int arr[] = {5, 2, 8, 1};
int n = 4;

qsort(arr, n, sizeof(int), compare);

for (int i = 0; i < n; i++)
printf("%d ", arr[i]);

return 0;
}

#include <stdio.h>

#include <stdlib.h>int compare(const void *a, const void *b) {

return (*(int *)a - *(int *)b);

}

int main() {

int arr[] = {5, 2, 8, 1};

int n = 4;

qsort(arr, n, sizeof(int), compare);

for (int i = 0; i < n; i++)

printf("%d ", arr[i]);

return 0;

}

compare() is a callback
Used internally by qsort()

Callback vs Normal Function

Feature	Normal Function	Callback
Called by	Programmer	Another function
Flexibility	Low	High
Reusability	Limited	Excellent
Used in	Simple logic	Frameworks, OS, Embedded

Callback in Embedded Systems (Concept)

Used for:

Interrupt Service Routines (ISR)
Hardware events
Timers

Common Mistakes

Forgetting function pointer syntax
Mismatch in return type or parameters
Calling function instead of passing pointer

Correct:

Wrong:

Interview Questions (Must Prepare)

What is a callback function?
Why callbacks are use?
How callbacks are implement in C?
Callback vs function pointer
Real-life example of callback
qsort() callback explanation

Summary

Callback functions enable flexible design
Implemented using function pointers
Widely used in OS, embedded, libraries
Very important for TCS, Wipro, Infosys, Capgemini

C Callback Functions