C Academy · Lesson

Passing Functions

Callbacks as arguments.

Passing Functions is a free C Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Functions as Arguments

Because a function pointer is just a value, you can pass a function into another function. The received function is called a callback.

This lets one routine customize part of its behavior.

#include <stdio.h>

int apply(int x, int (*f)(int)) {
    return f(x);
}

int square(int x) { return x * x; }

int main(void) {
    printf("%d\n", apply(6, square));
    return 0;
}

The Callback Parameter

The callback parameter is declared just like a function pointer variable. Inside, you call it like any function.

#include <stdio.h>

void run_twice(void (*action)(void)) {
    action();
    action();
}

void beep(void) { printf("beep\n"); }

int main(void) {
    run_twice(beep);
    return 0;
}

Swapping Behavior

Pass different callbacks to get different results from the same higher-order function.

#include <stdio.h>

int combine(int a, int b, int (*op)(int, int)) {
    return op(a, b);
}

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

int main(void) {
    printf("%d\n", combine(3, 4, add));
    printf("%d\n", combine(3, 4, mul));
    return 0;
}

Mapping Over an Array

A map function applies a callback to every element of an array in place.

#include <stdio.h>

void map(int *a, int n, int (*f)(int)) {
    for (int i = 0; i < n; i++) a[i] = f(a[i]);
}

int inc(int x) { return x + 1; }

int main(void) {
    int a[] = {1, 2, 3};
    map(a, 3, inc);
    for (int i = 0; i < 3; i++) printf("%d ", a[i]);
    printf("\n");
    return 0;
}

Filtering with a Predicate

A predicate is a callback that returns true or false. Use it to count or select matching elements.

#include <stdio.h>

int count_if(int *a, int n, int (*pred)(int)) {
    int c = 0;
    for (int i = 0; i < n; i++) if (pred(a[i])) c++;
    return c;
}

int is_even(int x) { return x % 2 == 0; }

int main(void) {
    int a[] = {1,2,3,4,5,6};
    printf("%d\n", count_if(a, 6, is_even));
    return 0;
}

Reducing to One Value

A reduce function folds an array into a single value using a binary callback and a starting accumulator.

#include <stdio.h>

int reduce(int *a, int n, int init, int (*op)(int,int)) {
    int acc = init;
    for (int i = 0; i < n; i++) acc = op(acc, a[i]);
    return acc;
}

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

int main(void) {
    int a[] = {1,2,3,4};
    printf("%d\n", reduce(a, 4, 0, add));
    return 0;
}

Passing a Comparison

Sorting and searching often take a comparison callback so the same algorithm works for any ordering.

#include <stdio.h>

int max_by(int a, int b, int (*greater)(int,int)) {
    return greater(a, b) ? a : b;
}

int gt(int a, int b) { return a > b; }

int main(void) {
    printf("%d\n", max_by(3, 8, gt));
    return 0;
}

typedef for Callback Types

Naming the callback type with typedef makes function signatures readable.

#include <stdio.h>

typedef int (*IntFn)(int);

int apply(int x, IntFn f) { return f(x); }

int neg(int x) { return -x; }

int main(void) {
    printf("%d\n", apply(5, neg));
    return 0;
}

Default Callback Fallback

If a callback may be NULL, provide a default behavior so the caller can opt out.

#include <stdio.h>

int apply(int x, int (*f)(int)) {
    if (f == NULL) return x;
    return f(x);
}

int dbl(int x) { return x * 2; }

int main(void) {
    printf("%d\n", apply(9, NULL));
    printf("%d\n", apply(9, dbl));
    return 0;
}

Callbacks Decouple Code

Callbacks let a general algorithm stay independent of the specific operation. The library provides the loop; you provide the behavior.

#include <stdio.h>

void for_each(int *a, int n, void (*visit)(int)) {
    for (int i = 0; i < n; i++) visit(a[i]);
}

void show(int x) { printf("[%d]", x); }

int main(void) {
    int a[] = {7, 8, 9};
    for_each(a, 3, show);
    printf("\n");
    return 0;
}

Choosing Callback at Runtime

Combine condition selection with passing: pick a callback then hand it to a higher-order function.

#include <stdio.h>

int apply(int x, int (*f)(int)) { return f(x); }
int sq(int x) { return x * x; }
int cube(int x) { return x * x * x; }

int main(void) {
    int power = 3;
    int (*f)(int) = (power == 2) ? sq : cube;
    printf("%d\n", apply(2, f));
    return 0;
}

Quick Check

Test your understanding of passing functions.

Recap

You learned to pass functions as callbacks:

Declare a callback parameter like a function pointer.
Build higher-order functions such as map, filter, reduce, and for_each.
Use typedef to name callback types.
Guard against NULL callbacks with a default.

Frequently asked questions

Is the “Passing Functions” lesson free?

Yes — the full text of “Passing Functions” is free to read here on the web, and the C Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.

What will I learn in “Passing Functions”?

Callbacks as arguments. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Passing Functions” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C Academy lesson?

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