Passing Functions
Callbacks as arguments.
Passing Functions is a free C Academy lesson on CoddyKit. This is 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, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.
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
typedefto name callback types. - Guard against
NULLcallbacks with a default.
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Frequently Asked Questions
Is the “Passing Functions” lesson free?
Yes — the full text of “Passing Functions” is free to read here on the web. 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. The C Academy course includes 4 lessons in total.
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, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.
How long does the “Passing Functions” lesson take?
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