Step 1 of 6

Functions have addresses

Functions live in memory too. The compiled machine code of a function sits at some address, and just as you can store the address of an int in a pointer, you can store the address of a function in a function pointer and call the function through it later. This lets a program choose which function to run while it's running, and it's the foundation of callbacks, plugins and dispatch tables.

#include <stdio.h>

double half(double x) { return x / 2; }
double twice(double x) { return x * 2; }

int main(void) {
    double (*f)(double) = half;
    printf("%.1f\n", f(9));
    f = twice;
    printf("%.1f\n", f(9));
    double (*pick[2])(double) = {half, twice};
    printf("%.1f\n", pick[1](pick[0](10)));
    return 0;
}

4.5
18.0
10.0

Reading the declaration

double (*f)(double) reads from the name outward: f is a pointer (*f, in parentheses) to a function taking a double ((double)) and returning a double. The parentheses around *f are essential. Without them, double *f(double) declares a function named f that returns a double *.

Using them

• A function's name used without calling it (half, not half()) gives its address. You can also write &half; it means the same.
• Call through the pointer exactly like a normal function: f(9). (The old style (*f)(9) also works.)
• The pointer's type must match the function's signature: parameter types and return type.
• pick is an array of function pointers, which is the basis of the dispatch tables coming up in step 3.

Your task

Point op at sub and call it.

Challenge 2

An array can hold function pointers too. Fill the table with add and mul, so the loop prints 7 and then 12.

Challenge 3

Write int twice(int (*f)(int), int x) that applies f to x two times, f(f(x)). With the functions below, the program prints 7 (5 plus 1, plus 1) and 81 (3 squared, squared).

Next: typedef and passing callbacks