C Academy · Lesson

Declaring Function Pointers

Pointers to functions.

Lesson 1 of 413 steps

Declaring Function Pointers is a free C Academy lesson on CoddyKit. This is lesson 1 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 Have Addresses

Just like data, every function lives at an address in memory. A function pointer stores that address so you can call the function indirectly.

#include <stdio.h>

void greet(void) { printf("hello\n"); }

int main(void) {
    void (*fp)(void) = greet;
    fp();
    return 0;
}

Reading the Declaration

The declaration int (*fp)(int) means: fp is a pointer to a function taking an int and returning an int.

The parentheses around *fp are required.

#include <stdio.h>

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

int main(void) {
    int (*fp)(int) = square;
    printf("%d\n", fp(5));
    return 0;
}

Assigning a Function

A function name decays to its address, so fp = square and fp = &square are equivalent.

#include <stdio.h>

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

int main(void) {
    int (*fp)(int);
    fp = &dbl;
    printf("%d\n", fp(7));
    return 0;
}

Calling Through the Pointer

You can call through a function pointer with fp(args) or the explicit (*fp)(args). Both do the same thing.

#include <stdio.h>

int add1(int x) { return x + 1; }

int main(void) {
    int (*fp)(int) = add1;
    printf("%d\n", fp(10));
    printf("%d\n", (*fp)(10));
    return 0;
}

The Signature Must Match

A function pointer's type includes its return type and parameter types. You can only assign a function whose signature matches exactly.

#include <stdio.h>

double half(double x) { return x / 2.0; }

int main(void) {
    double (*fp)(double) = half;
    printf("%f\n", fp(9.0));
    return 0;
}

Reassigning at Runtime

A function pointer is a variable, so you can point it at different functions while the program runs.

#include <stdio.h>

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

int main(void) {
    int (*op)(int) = inc;
    printf("%d\n", op(5));
    op = dec;
    printf("%d\n", op(5));
    return 0;
}

Choosing by Condition

Select which function to call based on a runtime value, then invoke through one pointer.

#include <stdio.h>

int plus(int a) { return a + 10; }
int minus(int a) { return a - 10; }

int main(void) {
    int up = 1;
    int (*op)(int) = up ? plus : minus;
    printf("%d\n", op(100));
    return 0;
}

typedef for Readability

Function pointer syntax is verbose. A typedef gives the type a name, making declarations clearer.

#include <stdio.h>

typedef int (*IntOp)(int);

int triple(int x) { return x * 3; }

int main(void) {
    IntOp op = triple;
    printf("%d\n", op(4));
    return 0;
}

Two-Argument Functions

Function pointers can describe any signature. Here is one for a binary integer operation.

#include <stdio.h>

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

int main(void) {
    int (*op)(int, int) = sum;
    printf("%d\n", op(3, 4));
    return 0;
}

A Null Function Pointer

Like data pointers, a function pointer can be NULL. Always check before calling one that may be unset.

#include <stdio.h>

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

int main(void) {
    int (*op)(int) = NULL;
    if (op == NULL) op = twice;
    if (op) printf("%d\n", op(8));
    return 0;
}

Printing the Address

You can inspect the stored address. Different functions have different addresses.

#include <stdio.h>

int f(int x) { return x; }

int main(void) {
    int (*fp)(int) = f;
    printf("function pointer is %s\n", fp ? "set" : "null");
    return 0;
}

Quick Check

Test your understanding of function pointer declarations.

Answer it in the interactive lesson

Recap

You learned to declare and use function pointers:

Syntax: ret (*name)(params) with required parentheses.
Assign with the function name (it decays to an address).
Call with fp(args) or (*fp)(args).
The signature must match; typedef improves readability.

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Frequently Asked Questions

Is the “Declaring Function Pointers” lesson free?

Yes — the full text of “Declaring Function Pointers” 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 “Declaring Function Pointers”?

Pointers to functions. 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 1 of 4.

How long does the “Declaring Function Pointers” 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.

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