How are functions called?

Actually, in C, functions are called via function pointer.

Ex:

• printf: function name / designator
• &printf: function pointer

When we call printf, it would be implicitly converted to &printf. i.e.,

printf("Hello");

//is actually
(&printf)("Hello"); //printf -> &printf

//or
(*printf)("Hello"); //*printf -> *&printf -> printf -> &printf

So printf, &printf, and *printf all work, and they are essentially doing the same thing, that is &printf.

Usage

1. Copy function

Functions cannot be directly copied. For instance,

void hello();
void func() = hello;   //(x) compile error

But with function pointers, it is possible to copy the address of the function and call it via pointer.

void hello();
void (*func)() = hello;  //(o) hello->&hello

func();  //call function via pointer

Some points worth notice:

• when a function pointer is declared, it needs to be specified as a pointer. (i.e., need to add * before it)
• When calling a function via pointer, func() = (*func)() = (**func)() = (********func)(). The reason is that func would always be converted to &func implicitly.

2. Pass function pointers as arguments

void A(int (*B)(int))

As pointer B refers to different functions, the function A becomes general and could perform different tasks.

Example:

We would like to define a function that performs calculation, including addition and multiplication of numbers from 1 to 5.

int add(int, int);
int multiply(int, int);

int main(){
    int addition = calculate(add); //int (*op)(int, int) = add  
    int multiplication = calculate(multiply); //int (*op)(int, int) = multiply
}

int calculate(int (*op)(int, int)){
    int result = 1;
    for(int i=2; i<=5; i++){
        result = op(result, i);
    }
    return result;
}

int add(int a, int b){
    return a+b;
}

int multiply(int a, int b){
    return a*b;
}