What is Callback Function in C: Understanding it in Detail

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Functions play a crucial role in programming in C. Before moving on to callback functions, let’s first understand what functions are.

In simple terms, functions can be described as a piece of code or a group of statements grouped to solve specific problems. When a user inputs a function, the code inside solves it using the logic or algorithm provided and then returns the output.

Callback function

This helps in many ways. Suppose you have to subtract two numbers, and this subtraction is to be performed multiple times in your program. Instead of writing the code repeatedly, you can create a ” SubtractNumbers ” function that takes two inputs (the numbers used for subtraction) and returns the result.

So, when you require subtraction, you can call this function “SubtractNumbers” and use it. Functions are handy as they provide reusability, make the code easy to read and debug.


For example, let’s take a function that divides two numbers:

1. void divideNumbers(int num1, int num2) {

//This line defines a function named divideNumbers that takes two integer parameters, num1, and num2. The function’s return type is void, meaning the function does not return a value.

2. if (num2 == 0) {

//This line starts an if statement that checks if num2 equals zero. If it is, the program executes the code inside the curly braces, which prints an error message using the printf function. If num2 does not equal zero, the program skips the code inside.

printf(“Error: Cannot divide by zero\n”);

} else {

3. printf(“%d\n”, num1 / num2);

//This line prints the result of dividing num1 by num2 using the %d format specifier for integers in the printf function. The result is computed using the / operator, which performs integer division in C.

What is CallBack Function in C?

Callback function in C are those functions that are passed as arguments in other functions. This increases code reusability as you don’t have to write the code repeatedly.

Providing a number as input to a function as an argument is possible. Similarly, giving a function as input to another function as an argument is also possible.

Callback function in C

For instance, suppose you are developing a game where the player can move a character using arrow keys. In this case, you can register a callback function that listens to keystrokes and updates the character’s position on the screen.

Similarly, in web development, callbacks are often find its use to handle asynchronous operations such as fetching data from a server. For example, when a user submits a form, the data is sent to the server, and a callback function is registered to handle the response. Once there is reception of response, there is execution of callback function, and the screen displays the data.

Let us take the following example of a callback function in C:


void calculate(int numbers[], int length, char operation, void (callback)(int)) {
int result = 0;
if (operation == '+') {
for (int i = 0; i < length; i++) {
result += numbers[i];
} else if (operation == '') {
result = 1;
for (int i = 0; i < length; i++) {
result *= numbers[i];
void print_result(int result) {
printf("The result is %d\n", result);
int main() {
int numbers[] = {1, 2, 3, 4};
int length = sizeof(numbers) / sizeof(int);
calculate(numbers, length, '+', print_result);
calculate(numbers, length, '*', print_result);
return 0;

Let’s see how the following code in C works:

1. The line “void calculate(int numbers[], int length, char operation, void (*callback)(int))” declares a function named calculate that takes four parameters:

numbers[]: an array of integers

length: the length of the numbers array

operation: a character representing the operation to perform (+ for addition or * for multiplication)

callback: a function pointer that takes an integer argument and returns void.

2. Line “int result = 0;” declares an integer variable named result and initializes it to 0.

3. The lines “if (operation == ‘+’) {…}” and “else if (operation == ‘*’) {…}” perform the requested calculation based on the operation parameter. If the operation is +, the function adds up all the numbers in the numbers array and stores the result in the result variable. If the operation is *, the function multiplies all the numbers in the numbers array and stores the result in the result variable.

4. The line “callback(result);” calls the function pointed to by the callback function pointer, passing in the result variable as an argument.

5. Line “void print_result(int result) {…}” defines a function named print_result that takes an integer parameter named result and prints a message to the console containing the value of the result.

6.The “int main() {…}” function is the main function of the program. It declares an array of integers named numbers and initializes it with four values. It then calculates the sum and product of the numbers in the numbers array using the calculate function, passing in the print_result function as a callback to print the result. Finally, it returns 0 to indicate successful completion of the program.

Application of Callback Functions in C

Callback functions in C find its use for various purposes, such as event handling, asynchronous processing, and implementing generic algorithms.

Here are some simple use cases for callback functions in C:

Asynchronous processing

Event handling: In graphical user interfaces (GUI), events such as mouse clicks, keyboard inputs, and window resize events needs handling. Callback functions can be used to register event handlers with the GUI framework so that when an event occurs, the framework can call the appropriate callback function to handle the event.

Asynchronous processing: When performing time-consuming operations, such as file I/O or network communication, it’s often necessary to perform these operations asynchronously to avoid blocking the main thread of execution. Callback functions can be used to register completion handlers so that the callback function is called with the results when the process completes.

Generic algorithms: Callback functions can implement generic algorithms that operate on different data types. For example, a sorting algorithm can take a callback function that compares two elements, allowing the algorithm to sort any data.


How does a callback function work in C?

When a function that takes a callback function as an argument is called, it calls it at some point during its execution to perform a specific action.

Can a callback function be called multiple times?

Yes, a callback function can be called multiple times if the function that calls it is executed multiple times.

How do you declare a callback function in C?

To declare a callback function in C, you need to define its signature, which includes its return type and parameter types. You can then pass a function pointer to this function as an argument to another function.

How do you pass a callback function as an argument in C?

To pass a callback function as an argument in C, you need to pass a function pointer to the function that takes the callback function as an argument.

What are the disadvantages of using a callback function in C?

The disadvantages of using a callback function in C include increased complexity and the potential for errors if the callback function is not implemented correctly.

What are some common use cases for callback functions in C?

Common use cases for callback functions in C include event handling, sorting and searching algorithms, and user interface programming.


So this was all about the callback functions in C language. Take a look at the example to a better idea as it might help in many cases.

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