Function pointers

Learn to define and call function pointers.

What is a function pointer?

A function pointer holds the address of a function. In C the syntax int (*fp)(int); declares a pointer to a function returning int and taking one int.

Example: int square(int x) { return x * x; } and int (*fp)(int) = square;. Here fp holds the address of square. To call it you write fp(5) or (*fp)(5); both return 25. A function pointer lets you choose the function to call at runtime.

Calling a function pointer

You can call a function pointer in two ways: fp(arg) or (*fp)(arg). Both are valid and give the same result.

The first form is more common; the second makes the indirection explicit. What matters is that the pointer's declaration matches the function signature exactly. If the return type or parameter list differs, the behaviour is undefined. That is why function pointer types, though long, must be written in full.

Exercise Square via a function pointer

Write a function int square(int x) that returns x * x. Declare a pointer of type int (*fp)(int) and assign it to square. Call fp(7) and print the result with printf.

Show example solution

#include <stdio.h>

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

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

Test yourself

Which is a pointer to a function returning int and taking int?
- int (*fp)(int);
- int *fp(int);
- int fp*(int);
- (int *)fp(int);
Show answer

int *fp(int); — Int (*fp)(int); — the parentheses make fp a pointer.
Match the function pointer parts

int

Return type
(*fp)

Pointer variable
(int)

Parameter list
fp(5)

Call

Call the function pointer

int square(int x) { return x * x; }
int (*fp)(int) = square;
int r = ____(5);

Show answer

int square(int x) { return x * x; }
int (*fp)(int) = square;
int r = fp(5);

Next lesson: Function pointer parameter

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