Function pointers

Having function pointers is another super feature of the C programming language. The two previous sections were about variable pointers and functions, and this section is going to combine them and talk about a more interesting topic: pointers to functions.

They have many applications, but splitting a large binary into smaller binaries and loading them again in another small executable is one of the most important applications. This has led to modularization and software design. Function pointers are building blocks for the implementation of polymorphism in C++ and allow us to extend our existing logic. In this section, we are going to cover them and prepare you for more advanced topics we'll cover over the coming chapters.

Like a variable pointer addressing a variable, a function pointer addresses a function and allows you to call that function indirectly. The following example, example 1.19, can be a good starter for this topic:

#include <stdio.h>
int sum(int a, int b) {
  return a + b;
}
int subtract(int a, int b) {
  return a - b;
}
int main() {
  int (*func_ptr)(int, int);
  func_ptr = NULL;
  func_ptr = &sum;
  int result = func_ptr(5, 4);
  printf("Sum: %d\n", result);
  func_ptr = &subtract;
  result = func_ptr(5, 4);
  printf("Subtract: %d\n", result);
  return 0;
}

Code Box 1-31 [ExtremeC_examples_chapter1_19.c]: Using a single function pointer to call different functions

In the preceding code box, func_ptr is a function pointer. It can only point to a specific class of functions that match its signature. The signature limits the pointer to only point to functions that accept two integer arguments and return an integer result.

As you see, we have defined two functions called sum and subtract matching the func_ptr pointer's signature. The preceding example uses the func_ptr function pointer to point to the sum and subtract functions separately, then call them with the same arguments and compare the results. This is the output of the example:

$ gcc ExtremeC_examples_chapter1_19.c
$ ./a.out
Sum: 9
Subtract: 1
$

Shell Box 1-12: Output of example 1.19

As you see in example 1.19, we can call different functions for the same list of arguments using a single function pointer, and this is an important feature. If you are familiar with object-oriented programming, the first thing that comes to mind is polymorphism and virtual functions. In fact, this is the only way to support polymorphism in C and mimic the C++ virtual functions. We will cover OOP as part of the third part of the book, Object Orientation.

Like variable pointers, it is important to initialize function pointers properly. For those function pointers which are not going to be initialized immediately upon declaration, it is mandatory to make them null. The nullification of function pointers is demonstrated in the preceding example, and it is pretty similar to variable pointers.

It is usually advised to define a new type alias for function pointers. The following example, example 1.20, demonstrates the way it should be done:

#include <stdio.h>
typedef int bool_t;
typedef bool_t (*less_than_func_t)(int, int);
bool_t less_than(int a, int b) {
  return a < b ? 1 : 0;
}
bool_t less_than_modular(int a, int b) {
  return (a % 5) < (b % 5) ? 1 : 0;
}
int main(int argc, char** argv) {
  less_than_func_t func_ptr = NULL;
  func_ptr = &less_than;
  bool_t result = func_ptr(3, 7);
  printf("%d\n", result);
  func_ptr = &less_than_modular;
  result = func_ptr(3, 7);
  printf("%d\n", result);
  return 0;
}

Code Box 1-32 [ExtremeC_examples_chapter1_20.c]: Using a single function pointer to call different functions

The typedef keyword allows you to define an alias for an already defined type. There are two new type aliases in the preceding example: bool_t, which is an alias for the int type, and the less_than_func_t type, which is an alias type for the function pointer type, bool_t (*)(int, int). These aliases add readability to the code and let you choose a shorter name for a long and complex type. In C, the name of a new type usually ends with _t by convention, and you can find this convention in many other standard type aliases such as size_t and time_t.