5.6. Pointers to functions

A useful technique is the ability to have pointers to functions. Their declaration is easy: write the declaration as it would be for the function, say

int func(int a, float b);

and simply put brackets around the name and a * in front of it: that declares the pointer. Because of precedence, if you don't parenthesize the name, you declare a function returning a pointer:

/* function returning pointer to int */
int *func(int a, float b);

/* pointer to function returning int */
int (*func)(int a, float b);

Once you've got the pointer, you can assign the address of the right sort of function just by using its name: like an array, a function name is turned into an address when it's used in an expression. You can call the function using one of two forms:

(*func)(1,2);
/* or */
func(1,2);

The second form has been newly blessed by the Standard. Here's a simple example.

#include <stdio.h>
#include <stdlib.h>

void func(int);

main(){
      void (*fp)(int);

      fp = func;

      (*fp)(1);
      fp(2);

      exit(EXIT_SUCCESS);
}

void
func(int arg){
      printf("%d\n", arg);
}

If you like writing finite state machines, you might like to know that you can have an array of pointers to functions, with declaration and use like this:

void (*fparr[])(int, float) = {
                              /* initializers */
                      };
/* then call one */

fparr[5](1, 3.4);

But we'll draw a veil over it at this point!