CHow-ToBeginner · 3 min read

How to Use Callback Function in C: Simple Guide with Example

In C, a callback function is a function passed as an argument to another function to be called later. You use function pointers to pass and invoke callbacks, allowing flexible and reusable code.

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Syntax

A callback function in C is implemented using a function pointer. You declare a function pointer parameter in a function, then pass the address of a function matching that pointer's signature.

return_type (*pointer_name)(parameter_types): declares a function pointer.
function_name: the callback function you want to call.
function_call(pointer_name(...)): calls the callback inside another function.

#include <stdio.h>

void callbackExample(void (*callback)(int)) {
    callback(5);  // Call the passed function with argument 5
}

void myCallback(int num) {
    // This function matches the pointer signature
    printf("Callback called with %d\n", num);
}

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Example

This example shows how to define a callback function and pass it to another function that calls it.

#include <stdio.h>

// Function that takes a callback
void processNumber(int num, void (*callback)(int)) {
    printf("Processing number: %d\n", num);
    callback(num);  // Call the callback with the number
}

// Callback function
void printSquare(int n) {
    printf("Square is: %d\n", n * n);
}

int main() {
    processNumber(4, printSquare);  // Pass printSquare as callback
    return 0;
}

Output

Processing number: 4 Square is: 16

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Common Pitfalls

Common mistakes when using callbacks in C include:

Passing a function with the wrong signature (parameters or return type).
Forgetting to use the * when declaring function pointers.
Calling the callback without checking if it is NULL, which can cause crashes.
Confusing function pointers with regular pointers.

Always ensure the callback matches the expected signature and check for NULL if callbacks are optional.

/* Wrong: callback signature mismatch */
void wrongCallback(float x) {
    printf("Wrong callback\n");
}

/* Correct: matching signature */
void correctCallback(int x) {
    printf("Correct callback with %d\n", x);
}

/* Usage */
void callFunc(void (*cb)(int)) {
    if (cb != NULL) {
        cb(10);
    }
}

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Quick Reference

Concept	Description	Example
Function Pointer Declaration	Declare a pointer to a function with specific parameters and return type	void (*cb)(int)
Passing Callback	Pass function name without parentheses to another function	processNumber(5, printSquare)
Calling Callback	Invoke callback inside function using pointer syntax	callback(num)
Check NULL	Always check if callback is NULL before calling	if (callback) callback(num);

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Key Takeaways

Use function pointers to pass callback functions in C.

Ensure the callback function signature matches the pointer declaration.

Always check if the callback pointer is not NULL before calling it.

Callbacks allow flexible and reusable code by deferring function calls.

Passing the function name without parentheses passes the pointer.