Explain function pointer in C

Question

How is the most basic function pointer created in C?

Answer 1

Here's a pretty straightforward example:

#include <stdio.h>
#include <string.h>

int add(int x, int y) {
    return x + y;
}

int sub(int x, int y) {
    return x - y;
}

int main() {
    char const * operator = "addition";

    int (*operation)(int, int) = NULL;
    if (strcmp(operator, "subtract") == 0)
        operation = sub;
    if (strcmp(operator, "addition") == 0)
        operation = add;

    if (operation) {
        int x = 3;
        int y = 7;
        int z = operation(x, y);
        printf("%s %d %d == %d\n", operator, x, y, z);
    }
}

Where is this useful?

A common use case that I've seen is to create interfaces. So, for example, we have a type of object, say a struct that represents a network interface. Now, because there are different hardware backend, we might want to implement the functions differently depending on the hardware implementation.

So we might have a function pointer in the interface struct for initializing the hardware, sending packets, and receiving packets.

Answer 2

This code will demonstrate how to create and modify a basic function pointer

#include <stdio.h>

int my_function(int);
int (*function)(int)=&my_function;

int my_function2(int x){
        function=&my_function;  // & is optional
        return (0);
}

int my_function(int x){
        function=my_function2;
        return (x);
}

/* And an example call */

int main(){
        printf ("%d \n",function(10));
        printf ("%d \n",function(10));
        printf ("%d \n",function(8));
        return 0;
}